Showing 1–50 of 141 results for author: Kepler, S O

Cataclysmic Variables Photometric Periods from TESS

Authors: S. O. Kepler, Alejandra Daniela Romero, Larissa Luciano Amorim, Marcos Perez Diaz

Abstract: We present a sample of coherent and stable photometric period determinations for cataclysmic variables, based on TESS photometry through sector 102. We analyzed a total of 1557 cataclysmic variable stars and detected periodic variations in 1362 objects, including 286 eclipsing or ellipsoidal-variation systems, 63 polars, and 135 intermediate polars. In particular, we present the first determinatio… ▽ More We present a sample of coherent and stable photometric period determinations for cataclysmic variables, based on TESS photometry through sector 102. We analyzed a total of 1557 cataclysmic variable stars and detected periodic variations in 1362 objects, including 286 eclipsing or ellipsoidal-variation systems, 63 polars, and 135 intermediate polars. In particular, we present the first determination of the optical variability period for 565 cataclysmic variables. Due to the 21x21" pixel size of TESS, we tested whether the variability was coming from the cataclysmic variable and not from a nearby star. For the intermediate polars, we detected spin periods in addition to orbital periods for 83 systems. We detect a clear period gap between ~2 and ~3 h in the eclipsing sample, consistent with previous work. The gap remains for the complete sample of photometric variability periods. There is no apparent gap in the orbital period distribution for intermediate polars. Finally, the median of the photometric period distribution of our complete sample of cataclysmic variables is 3.681 h. Comparing this to the distribution of rotation periods for likely single white dwarfs, we find a similar range, with a median photometric period of 6.803~h. △ Less

Submitted 15 July, 2026; v1 submitted 9 July, 2026; originally announced July 2026.

Comments: 23 pages, 16 figues, 4 tables

White Dwarf Classification of DESI DR1 Spectra1

Authors: Larissa L. Amorim, Weligton. N. Costa Junior, S. O. Kepler, Joao Gabriel Leite Medeiros, Detlev Koester, Alejandra D. Romero

Abstract: We present a new catalog of spectroscopically confirmed white dwarfs from the Dark Energy Spectroscopic Instrument (DESI) Data Release 1. We visually classified 44,417 white dwarf spectra and derived atmospheric parameters for 29,072 DA white dwarfs through spectroscopic model fitting. The resulting mass distribution is non-Gaussian, with a mean mass of $0.677\,M_\odot$, consistent with previous s… ▽ More We present a new catalog of spectroscopically confirmed white dwarfs from the Dark Energy Spectroscopic Instrument (DESI) Data Release 1. We visually classified 44,417 white dwarf spectra and derived atmospheric parameters for 29,072 DA white dwarfs through spectroscopic model fitting. The resulting mass distribution is non-Gaussian, with a mean mass of $0.677\,M_\odot$, consistent with previous studies. We identify 547 magnetic white dwarfs by detecting Zeeman splitting, including 84 new discoveries, and estimate their magnetic field strengths using off-centered, inclined dipole models when possible. We compare our magnetic field determinations with previous measurements and find overall good agreement. Finally, we investigate the relation between stellar properties and magnetism, finding that magnetic white dwarfs are systematically more massive than the general white dwarf population and that intermediate-strength magnetic fields are already present in stars that have not yet entered the crystallization phase. This result suggests that crystallization is unlikely to be the sole mechanism responsible for the origin of magnetic fields in white dwarfs. △ Less

Submitted 1 July, 2026; originally announced July 2026.

Magnetic white dwarfs from DESI

Authors: Larissa L. Amorim, S. O. Kepler, Alejandra D. Romero

Abstract: A significant fraction of white dwarfs, the degenerate remnants of low- and intermediate-mass stars, host strong magnetic fields; yet, the origin and evolution of these magnetic fields remain poorly understood. Building a large, statistically robust sample of these magnetic white dwarfs (MWDs) is crucial for testing competing theories of field generation. We used the white dwarf candidates catalog… ▽ More A significant fraction of white dwarfs, the degenerate remnants of low- and intermediate-mass stars, host strong magnetic fields; yet, the origin and evolution of these magnetic fields remain poorly understood. Building a large, statistically robust sample of these magnetic white dwarfs (MWDs) is crucial for testing competing theories of field generation. We used the white dwarf candidates catalog from Gaia DR3 to select objects with spectra from the first data release of the Dark Energy Spectroscopic Instrument (DESI) survey. We identified candidate MWDs through visual inspection of their spectra, searching for the characteristic Zeeman splitting of absorption lines. After cross-matching with the literature, we present the discovery of 137 new MWDs. Follow-up analysis and atmospheric modeling were used to determine magnetic field strengths ranging from approximately 1 to nearly 500~MG. Our findings demonstrate the exceptional capability of large-scale spectroscopic surveys, such as DESI, to uncover rare stellar populations and advance our understanding of compact-object astrophysics. △ Less

Submitted 20 March, 2026; originally announced March 2026.

Evolution and formation of ultramassive white dwarf stars: The case for a 9Msun progenitor

Authors: Ana S. R. Antonini, Alejandra D. Romero, S. O. Kepler

Abstract: We study the full evolution of a 1.313 Msun white dwarf star that descended from a 9 Msun main-sequence progenitor with an initial metallicity of Z=0.02. Using MESA r24.08.01, we calculate its entire evolution from pre-ZAMS to the WD cooling curve, including both the evolution through 139 thermal pulses and the post-AGB phase. The resulting remnant is an ultramassive H-deficient WD, for which the… ▽ More We study the full evolution of a 1.313 Msun white dwarf star that descended from a 9 Msun main-sequence progenitor with an initial metallicity of Z=0.02. Using MESA r24.08.01, we calculate its entire evolution from pre-ZAMS to the WD cooling curve, including both the evolution through 139 thermal pulses and the post-AGB phase. The resulting remnant is an ultramassive H-deficient WD, for which the composition, in mass fraction, is 47.7% O16, 39.7% Ne20, 4.2% Mg24, 3.3% Na23 and 0.386% C12 -- corresponding to a total mass of 5 x 10^-3 Msun of C --, surrounded by a 1.5 x 10^-5 Msun He layer. We also investigate the effects of fully suppressing the TP-SAGB stage by adopting a high mass-loss rate only after the second dredge-up, and find only minor differences in the final mass and composition. In addition, we calculate models with and without phase separation during the WD stage, estimating a cooling delay of only 16 Myr. This is the first ultramassive white dwarf sequence for which both the TP-SAGB and post-AGB stages are calculated and, to our knowledge, the most massive WD model from complete evolution for which cooling times and detailed abundance profiles are published △ Less

Submitted 11 March, 2026; v1 submitted 9 March, 2026; originally announced March 2026.

Observing bright pulsating white dwarfs with PLATO: A new window into the late stages of stellar evolution

Authors: Murat Uzundag, Alejandro H. Corsico, Nicholas Jannsen, Mukremin Kilic, Pierre Bergeron, Leandro G. Althaus, J. J. Hermes, Ingrid Pelisoli, Keaton J. Bell, Francisco C. De Geronimo, Leila M. Calcaferro, Zsofia Bognar, Valerie Van Grootel, Maria E. Camisassa, Paulina Sowicka, Steven D. Kawaler, S. O. Kepler, Roberto Silvotti, Marcelo M. Miller Bertolami, Margarida Cunha

Abstract: We present the scientific case for exploiting the capabilities of the PLATO mission to study bright pulsating white dwarfs across a wide spectral range, including hydrogen-deficient types (GW Vir and DBV stars) and hydrogen-rich classes (classical DAVs, pulsating extremely low-mass DA white dwarfs, and ultra-massive DA white dwarfs). PLATOs exceptional photometric precision, long-duration continuo… ▽ More We present the scientific case for exploiting the capabilities of the PLATO mission to study bright pulsating white dwarfs across a wide spectral range, including hydrogen-deficient types (GW Vir and DBV stars) and hydrogen-rich classes (classical DAVs, pulsating extremely low-mass DA white dwarfs, and ultra-massive DA white dwarfs). PLATOs exceptional photometric precision, long-duration continuous monitoring, and extensive sky coverage promise transformative advances in white dwarf asteroseismology. Our key objectives include probing the internal structure and chemical stratification of white dwarfs, detecting secular changes in pulsation modes over extended timescales, and discovering rare or previously unknown classes of pulsators. To assess feasibility, we constructed a sample of 650 white dwarf candidates identified within PLATOs Southern LOPS2 field using the PLATO complementary science catalogue combined with Gaia DR3, and derived atmospheric parameters through photometric modeling. This sample comprises 118 DA white dwarfs (including 23 ZZ Ceti candidates), and 41 non-DAs (including 35 DBV candidates). Simulated observations using PlatoSim demonstrate that PLATO will be capable of detecting white dwarf pulsation modes with amplitudes as low as 0.1 mma depending on stellar magnitude, observation duration, pixel location, and the number of contributing cameras. We provide detailed detection limits and visibility forecasts for known pulsators across a representative range of these parameters. Furthermore, we emphasize strong synergies with Gaia astrometry, TESS photometry, and targeted spectroscopic campaigns, which together will enable robust mode identification and detailed stellar modeling. Collectively, these efforts will unlock unprecedented insights into white dwarf origins, evolution and internal physics, and the fate of their planetary systems. △ Less

Submitted 10 June, 2026; v1 submitted 24 November, 2025; originally announced November 2025.

Comments: Accepted for publication in Experimental Astronomy

White Dwarfs with Infrared Excess in the HET Dark Energy Experiment

Authors: Rudy A. Morales, Barbara G. Castanheira, Jayden Blanchard, Detlev Koester, Péter Németh, S. O. Kepler, Erin Mentuch Cooper, Karl Gebhardt

Abstract: White dwarfs with infrared excess emission provide a window into the late stages of stellar evolution and the dynamics of circumstellar environments. Using data from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), we characterized 30 white dwarfs exhibiting infrared excess, including 29 DA and 1 DB stars. While an infrared excess can arise from dusty disks or cool (sub-)stellar compani… ▽ More White dwarfs with infrared excess emission provide a window into the late stages of stellar evolution and the dynamics of circumstellar environments. Using data from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), we characterized 30 white dwarfs exhibiting infrared excess, including 29 DA and 1 DB stars. While an infrared excess can arise from dusty disks or cool (sub-)stellar companions, our sample is limited to stellar companions due to our selection based on SDSS photometry, which is sensitive to excess emission at wavelengths $λ< 1\,μ\mathrm{m}$. Our sample contains 22 newly identified excess sources not previously reported in the literature. Spectroscopic observations are available for 10 sources via SDSS, of which only 8 have prior spectroscopic classifications in the literature. In this paper, we present the determination of the effective temperature and surface gravity of these white dwarfs. We used the Balmer line profiles to compare with current atmospheric models to determine the photospheric parameters of the white dwarfs, minimizing contamination introduced by the infrared source. We used photometric data from the SDSS and the \textit{Gaia} mission to resolve the degeneracies between hot and cold solutions from spectroscopy, constraining the photospheric parameters. These results help refine our understanding of white dwarf evolution in binary systems, focusing on stellar companions that cause the infrared excess. This study contributes to identifying systems with potential substellar companions or unresolved stellar partners, adding to the growing effort to map out the fate of planetary systems after their host stars evolve beyond the main sequence. △ Less

Submitted 15 September, 2025; originally announced September 2025.

A targeted search for binary white dwarf pulsars using Gaia and WISE

Authors: Ingrid Pelisoli, T. R. Marsh, G. Tovmassian, L. A. Amaral, Amornrat Aungwerojwit, M. J. Green, R. P. Ashley, David A. H. Buckley, B. T. Gaensicke, F. -J. Hambsch, K. Inight, S. B. Potter, A. J. Brown, N. Castro Segura, V. S. Dhillon, M. J. Dyer, J. A. Garbutt, D. Jarvis, M. R. Kennedy, S. O. Kepler, P. Kerry, S. P. Littlefair, J. McCormac, J. Munday, S. G. Parsons , et al. (2 additional authors not shown)

Abstract: After its discovery in 2016, the white dwarf binary AR Scorpii (AR Sco) remained for several years the only white dwarf system to show pulsed radio emission associated with a fast-spinning white dwarf. The evolutionary origin and the emission mechanism for AR Sco are not completely understood, with different models proposed. Testing and improving these models requires observational input. Here we… ▽ More After its discovery in 2016, the white dwarf binary AR Scorpii (AR Sco) remained for several years the only white dwarf system to show pulsed radio emission associated with a fast-spinning white dwarf. The evolutionary origin and the emission mechanism for AR Sco are not completely understood, with different models proposed. Testing and improving these models requires observational input. Here we report the results of a targeted search for other binary white dwarf pulsars like AR Sco. Using data from Gaia and WISE, we identified 56 candidate systems with similar properties to AR Sco, of which 26 were previously uncharacterised. These were subject to spectroscopic and photometric follow-up observations. Aside from one new binary white dwarf pulsar found, J191213.72-441045.1, which was reported in a separate work, we find no other systems whose characteristics are akin to AR Sco. The newly characterised systems are primarily young stellar objects (with 10 found) or cataclysmic variables (7 identifications), with the remaining being either blended or non-variable on short timescales. △ Less

Submitted 7 May, 2025; originally announced May 2025.

Comments: 17 pages, 21 figures. Accepted for publication in MNRAS

Searching for new variable white dwarfs: The discovery of the three new pulsating and three new binary systems

Authors: Larissa Antunes Amaral, Maja Vučković, Ingrid Pelisoli, Alina Istrate, S. O. Kepler, Jacob M. Hibbert

Abstract: In recent years, approximately 150 low-mass white dwarfs (WDs), typically with masses below 0.4 solar masses, have been discovered. Observational evidence indicates that most of these low-mass WDs are found in binary systems, supporting binary evolution scenarios as the primary formation pathway. A few extremely low-mass (ELM) WDs in this population have also been found to be pulsationally varia… ▽ More In recent years, approximately 150 low-mass white dwarfs (WDs), typically with masses below 0.4 solar masses, have been discovered. Observational evidence indicates that most of these low-mass WDs are found in binary systems, supporting binary evolution scenarios as the primary formation pathway. A few extremely low-mass (ELM) WDs in this population have also been found to be pulsationally variable. In this work, we present a comprehensive analysis aimed at identifying new variable low-mass WDs. From our candidate selection, we observed 16 objects identified within the ZZ Ceti instability strip. These objects were observed over multiple nights using high-speed photometry from the SOAR/Goodman and SMARTS-1m telescopes. Our analysis led to the discovery of three new pulsating WDs: one pulsating ELM, one low-mass WD, and one ZZ Ceti star. Additionally, we identified three objects in binary systems: two with ellipsoidal variations in their light curves, one of which is likely a pre-ELM star, and a third showing a reflection effect. △ Less

Submitted 15 January, 2025; originally announced January 2025.

Journal ref: A&A 694, A246 (2025)

An analysis of spectroscopic, seismological, astrometric, and photometric masses of pulsating white dwarf stars

Authors: Leila M. Calcaferro, Alejandro H. Córsico, Murat Uzundag, Leandro G. Althaus, S. O. Kepler, Klaus Werner

Abstract: A central challenge in the field of stellar astrophysics lies in accurately determining the mass of isolated stars. However, for pulsating white dwarf (WD) stars, the task becomes more tractable due to the availability of multiple approaches such as spectroscopy, asteroseismology, astrometry, and photometry. The objective of this work is to compare the asteroseismological and spectroscopic mass va… ▽ More A central challenge in the field of stellar astrophysics lies in accurately determining the mass of isolated stars. However, for pulsating white dwarf (WD) stars, the task becomes more tractable due to the availability of multiple approaches such as spectroscopy, asteroseismology, astrometry, and photometry. The objective of this work is to compare the asteroseismological and spectroscopic mass values of WDs in detail and, in turn, to compare them with the masses derived using astrometric parallaxes/distances and photometry. Our analysis encompasses a selection of pulsating WDs with different surface chemical abundances that define the main classes of variable WDs. We calculated their spectroscopic masses, compiled seismological masses, and determined astrometric masses. We also derived photometric masses, when possible. Subsequently, we compared all the sets of stellar masses obtained through these different methods. To ensure consistency and robustness in our comparisons, we used identical WD models and evolutionary tracks across all four methods. The analysis suggests a general consensus among the these methods regarding the masses of pulsating WD with H-rich atmospheres, known as DAV or ZZ Ceti stars, especially for objects with masses below approximately $0.75 M_{\sun}$, although notable disparities emerge for certain massive stars. For pulsating WD stars with He-rich atmospheres, called DBV or V777 Her stars, we find that astrometric masses generally exceed seismological, spectroscopic, and photometric masses. Finally, while there is agreement among the sets of stellar masses for pulsating WDs with C-, O-, and He-rich atmospheres (designated as GW Vir stars), outliers exist where mass determinations by various methods show significant discrepancies. △ Less

Submitted 5 September, 2024; originally announced September 2024.

Comments: 24 pages, 25 figures, 8 tables. To be published in Astronomy and Astrophysics

Thirty Two New Bright ZZ Ceti Stars from TESS: Adding Cycles 4 and 5

Authors: Alejandra D. Romero, S. O. Kepler, Gabriela Oliveira da Rosa, J. J. Hermes

Abstract: Analyzing all 120 s and 20 s light curves obtained by the TESS satellite up to Sector 69 - the end of the fifth year of observations - for all known white dwarfs and white dwarf candidates up to G=17.5 mag, we report the discovery of 32 new pulsating DA white dwarf stars. For all objects, we obtained the period spectra and performed a seismological analysis using full evolutionary models to estima… ▽ More Analyzing all 120 s and 20 s light curves obtained by the TESS satellite up to Sector 69 - the end of the fifth year of observations - for all known white dwarfs and white dwarf candidates up to G=17.5 mag, we report the discovery of 32 new pulsating DA white dwarf stars. For all objects, we obtained the period spectra and performed a seismological analysis using full evolutionary models to estimate the structural parameters, such as effective temperature, stellar mass, and hydrogen envelope mass. The median stellar mass for the pulsators from asteroseismology is 0.609 Msun, in agreement with the median value from photometric and spectroscopic determinations, 0.602 Msun, excluding the low and extremely-low mass objects. Finally, we found rotational-splitting multiplets for 9 stars, which led to rotation periods between 4 h and 1 d. △ Less

Submitted 9 July, 2024; originally announced July 2024.

Comments: Submitted to ApJ. arXiv admin note: text overlap with arXiv:2201.04158

Photometric White Dwarf Rotation

Authors: Gabriela Oliveira da Rosa, S. O. Kepler, L. T. T. Soethe, Alejandra D. Romero, Keaton J. Bell

Abstract: We present a census of photometrically detected rotation periods for white dwarf (WD) stars. We analyzed the light curves of 9285 WD stars observed by the Transiting Exoplanet Survey Satellite up to Sector 69. Using Fourier transform analyses and the TESS_localize software, we detected variability periods for 318 WD stars. The 115 high-probability likely single WDs in our sample have a median rota… ▽ More We present a census of photometrically detected rotation periods for white dwarf (WD) stars. We analyzed the light curves of 9285 WD stars observed by the Transiting Exoplanet Survey Satellite up to Sector 69. Using Fourier transform analyses and the TESS_localize software, we detected variability periods for 318 WD stars. The 115 high-probability likely single WDs in our sample have a median rotational period of 3.9 hr and a median absolute deviation of 3.5 hr. Our distribution is significantly different from the distribution of the rotational period from asteroseismology, which exhibits a longer median period of 24.2 hr and a median absolute deviation of 12.1 hr. In addition, we reported nonpulsating periods for three known pulsating WDs with rotational periods previously determined by asteroseismology: NGC 1501, TIC 7675859, and G226-29. We also calculated evolutionary models including six angular momentum transfer mechanisms from the literature throughout evolution in an attempt to reproduce our findings. Our models indicate that the temperature-period relation of most observational data is best fitted by models with low metallicity, probably indicating problems with the computations of angular momentum loss during the high-mass-loss phase. Our models also generate internal magnetic fields through the Tayler-Spruit dynamo. △ Less

Submitted 8 January, 2025; v1 submitted 6 July, 2024; originally announced July 2024.

Journal ref: The Astrophysical Journal 974, 2024, 314

A 500 pc volume-limited sample of hot subluminous stars I. Space density, scale height, and population properties

Authors: H. Dawson, S. Geier, U. Heber, I. Pelisoli, M. Dorsch, V. Schaffenroth, N. Reindl, R. Culpan, M. Pritzkuleit, J. Vos, A. A. Soemitro, M. M. Roth, D. Schneider, M. Uzundag, M. Vučković, L. Antunes Amaral, A. G. Istrate, S. Justham, R. H. Østensen, J. H. Telting, A. A. Djupvik, R. Raddi, E. M. Green, C. S. Jeffery, S. O. Kepler , et al. (3 additional authors not shown)

Abstract: We present the first volume-limited sample of spectroscopically confirmed hot subluminous stars out to 500 pc, defined using the accurate parallax measurements from the {\em Gaia} space mission data release 3 (DR3). The sample comprises a total of 397 members, with 305 ($\sim 77\%$) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 ($\sim58\%$)… ▽ More We present the first volume-limited sample of spectroscopically confirmed hot subluminous stars out to 500 pc, defined using the accurate parallax measurements from the {\em Gaia} space mission data release 3 (DR3). The sample comprises a total of 397 members, with 305 ($\sim 77\%$) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 ($\sim58\%$) are hydrogen-rich sdBs, 65 are sdOBs ($\sim 21\%$), 32 are sdOs ($\sim 11\%$), and 30 are He-sdO/Bs ($\sim 10\%$). Among them, 48 ($\sim 16\%$) exhibit an infrared excess in their spectral energy distribution fits, suggesting a composite binary system. The hot subdwarf population is estimated to be 90\% complete, assuming that most missing systems are these composite binaries located within the main sequence (MS) in the \emph{Gaia} colour-magnitude diagram (CMD). The remaining sources in the sample include cataclysmic variables (CVs), blue horizontal branch stars (BHBs), hot white dwarfs (WDs), and MS stars. We derived the mid-plane density $ρ_{0}$ and scale height $\mathrm{h}_{z}$ for the non-composite hot subdwarf star population using a hyperbolic sechant profile (sech$^2$). The best-fit values are $ρ_{0}\,=\,5.17\pm 0.33 \times10^{-7}$ stars/pc$^{3}$ and $\mathrm{h}_{z} = 281 \pm 62$ pc. When accounting for the composite-colour hot subdwarfs and their estimated completeness, the mid-plane density increases to $ρ_{0}\,=\,6.15^ {+1.16}_{-0.53} \times10^{-7}$ stars/pc$^{3}$. This corrected space density is an order of magnitude lower than predicted by population synthesis studies, supporting previous observational estimates. △ Less

Submitted 22 March, 2024; originally announced March 2024.

Comments: Accepted for publication with A&A (permission has been granted by A&A to share this article on arXiv), 30 pages, 16 figures, 7 tables (4 + 3 appendices). The full table will be available in electronic form at the CDS

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS VI. Asteroseismology of the GW Vir-type central star of the Planetary Nebula NGC 246

Authors: Leila M. Calcaferro, Paulina Sowicka, Murat Uzundag, Alejandro H. Córsico, S. O. Kepler, Keaton J. Bell, Leandro G. Althaus, Gerald Handler, Steven D. Kawaler, Klaus Werner

Abstract: Significant advances have been achieved through the latest improvements in the photometric observations accomplished by the recent space missions, substantially boosting the study of pulsating stars via asteroseismology. The TESS mission has already proven to be of relevance for pulsating white dwarf and pre-white dwarf stars. We report a detailed asteroseismic analysis of the pulsating PG 1159 st… ▽ More Significant advances have been achieved through the latest improvements in the photometric observations accomplished by the recent space missions, substantially boosting the study of pulsating stars via asteroseismology. The TESS mission has already proven to be of relevance for pulsating white dwarf and pre-white dwarf stars. We report a detailed asteroseismic analysis of the pulsating PG 1159 star NGC 246 (TIC3905338), the central star of the planetary nebula NGC 246, based on high-precision photometric data gathered by the TESS space mission. We reduced TESS observations of NGC 246 and performed a detailed asteroseismic analysis using fully evolutionary PG 1159 models computed accounting for the complete prior evolution of their progenitors. We constrained the mass of this star by comparing the measured mean period spacing with the average of the computed period spacings of the models and also employed the observed individual periods to search for a seismic stellar model. We extracted 17 periodicities from the TESS light curves from the two sectors where NGC246 was observed. All the oscillation frequencies are associated with g-mode pulsations, with periods spanning from ~1460 to ~1823s. We found a constant period spacing of $ΔΠ= 12.9$s, allowing us to deduce that the stellar mass is larger than ~0.87 Mo if the period spacing is assumed to be associated with l= 1 modes, and ~ 0.568 Mo if it is associated with l= 2 modes. The less massive models are more consistent with the distance constraint from Gaia parallax. Although we were not able to find a unique asteroseismic model for this star, the period-to-period fit analyses suggest a high-stellar mass ($\gtrsim$0.74 Mo) when the observed periods are associated with modes with l= 1 only, and both a high ($\gtrsim$ 0.74 Mo) and intermediate (~0.57 Mo) stellar mass when the observed periods are associated with modes with l= 1 and 2. △ Less

Submitted 26 February, 2024; originally announced February 2024.

Comments: 14 pages, 11 figures. Accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 686, A140 (2024)

TIC 378898110: A Bright, Short-Period AM CVn Binary in TESS

Authors: Matthew J. Green, J. J. Hermes, Brad N. Barlow, T. R. Marsh, Ingrid Pelisoli, Boris T. Gänsicke, Ben C. Kaiser, Alejandra Romero, Larissa Antunes Amaral, Kyle Corcoran, Dirk Grupe, Mark R. Kennedy, S. O. Kepler, James Munday, R. P. Ashley, Andrzej S. Baran, Elmé Breedt, Alex J. Brown, V. S. Dhillon, Martin J. Dyer, Paul Kerry, George W. King, S. P. Littlefair, Steven G. Parsons, David I. Sahman

Abstract: AM CVn-type systems are ultracompact, helium-accreting binary systems which are evolutionarily linked to the progenitors of thermonuclear supernovae and are expected to be strong Galactic sources of gravitational waves detectable to upcoming space-based interferometers. AM CVn binaries with orbital periods $\lesssim$ 20--23 min exist in a constant high state with a permanently ionised accretion di… ▽ More AM CVn-type systems are ultracompact, helium-accreting binary systems which are evolutionarily linked to the progenitors of thermonuclear supernovae and are expected to be strong Galactic sources of gravitational waves detectable to upcoming space-based interferometers. AM CVn binaries with orbital periods $\lesssim$ 20--23 min exist in a constant high state with a permanently ionised accretion disc. We present the discovery of TIC 378898110, a bright ($G=14.3$ mag), nearby ($309.3 \pm 1.8$ pc), high-state AM CVn binary discovered in TESS two-minute-cadence photometry. At optical wavelengths this is the third-brightest AM CVn binary known. The photometry of the system shows a 23.07172(6) min periodicity, which is likely to be the `superhump' period and implies an orbital period in the range 22--23 min. There is no detectable spectroscopic variability. The system underwent an unusual, year-long brightening event during which the dominant photometric period changed to a shorter period (constrained to $20.5 \pm 2.0$ min), which we suggest may be evidence for the onset of disc-edge eclipses. The estimated mass transfer rate, $\log (\dot{M} / \mathrm{M_\odot} \mathrm{yr}^{-1}) = -6.8 \pm 1.0$, is unusually high and may suggest a high-mass or thermally inflated donor. The binary is detected as an X-ray source, with a flux of $9.2 ^{+4.2}_{-1.8} \times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$ in the 0.3--10 keV range. TIC 378898110 is the shortest-period binary system discovered with TESS, and its large predicted gravitational-wave amplitude makes it a compelling verification binary for future space-based gravitational wave detectors. △ Less

Submitted 2 November, 2023; originally announced November 2023.

Comments: 15 pages, 14 figures. Accepted to MNRAS

Asteroseismological analysis of the polluted ZZ Ceti star G29-38 with TESS

Authors: Murat Uzundag, Francisco C. De Gerónimo, Alejandro H. Córsico, Roberto Silvotti, Paul A. Bradley, Michael H. Montgomery, Márcio Catelan, Odette Toloza, Keaton J. Bell, S. O. Kepler, Leandro G. Althaus, Scot J. Kleinman, Mukremin Kilic, Susan E. Mullally, Boris T. Gänsicke, Karolina Bąkowska, Sam Barber, Atsuko Nitta

Abstract: G\,29$-$38 (TIC~422526868) is one of the brightest ($V=13.1$) and closest ($d = 17.51$\,pc) pulsating white dwarfs with a hydrogen-rich atmosphere (DAV/ZZ Ceti class). It was observed by the {\sl TESS} spacecraft in sectors 42 and 56. The atmosphere of G~29$-$38 is polluted by heavy elements that are expected to sink out of visible layers on short timescales. The photometric {\sl TESS} data set sp… ▽ More G\,29$-$38 (TIC~422526868) is one of the brightest ($V=13.1$) and closest ($d = 17.51$\,pc) pulsating white dwarfs with a hydrogen-rich atmosphere (DAV/ZZ Ceti class). It was observed by the {\sl TESS} spacecraft in sectors 42 and 56. The atmosphere of G~29$-$38 is polluted by heavy elements that are expected to sink out of visible layers on short timescales. The photometric {\sl TESS} data set spans $\sim 51$ days in total, and from this, we identified 56 significant pulsation frequencies, that include rotational frequency multiplets. In addition, we identified 30 combination frequencies in each sector. The oscillation frequencies that we found are associated with $g$-mode pulsations, with periods spanning from $\sim$ 260 s to $\sim$ 1400 s. We identified %three distinct rotational frequency triplets with a mean separation $δν_{\ell=1}$ of 4.67 $μ$Hz and a quintuplet with a mean separation $δν_{\ell=2}$ of 6.67 $μ$Hz, from which we estimated a rotation period of about $1.35 \pm 0.1$ days. We determined a constant period spacing of 41.20~s for $\ell= 1$ modes and 22.58\,s for $\ell= 2$ modes. We performed period-to-period fit analyses and found an asteroseismological model with $M_{\star}/M_{\odot}=0.632 \pm 0.03$, $T_{\rm eff}=11\, 635\pm 178$ K, and $\log{g}=8.048\pm0.005$ (with a hydrogen envelope mass of $M_{\rm H}\sim 5.6\times 10^{-5}M_{\star}$), in good agreement with the values derived from spectroscopy. We obtained an asteroseismic distance of 17.54 pc, which is in excellent agreement with that provided by {\sl Gaia} (17.51 pc). △ Less

Submitted 9 September, 2023; originally announced September 2023.

Comments: 17 pages, Accepted for publication in MNRAS

An Eclipsing 47 minute Double White Dwarf Binary at 400 pc

Authors: James Munday, P. -E. Tremblay, J. J. Hermes, Brad Barlow, Ingrid Pelisoli, T. R. Marsh, Steven G. Parsons, David Jones, S. O. Kepler, Alex Brown, S. P. Littlefair, R. Hegedus, Andrzej Baran, Elmé Breedt, V. S. Dhillon, Martin J. Dyer, Matthew J. Green, Mark R. Kennedy, Paul Kerry, Isaac D. Lopez, Alejandra D. Romero, Dave Sahman, Hannah L. Worters

Abstract: We present the discovery of the eclipsing double white dwarf (WD) binary WDJ 022558.21-692025.38 that has an orbital period of 47.19 min. Following identification with the Transiting Exoplanet Survey Satellite, we obtained time-series ground based spectroscopy and high-speed multi-band ULTRACAM photometry which indicate a primary DA WD of mass 0.40 +- 0.04 Msol and a 0.28 +- 0.02 Msol mass seconda… ▽ More We present the discovery of the eclipsing double white dwarf (WD) binary WDJ 022558.21-692025.38 that has an orbital period of 47.19 min. Following identification with the Transiting Exoplanet Survey Satellite, we obtained time-series ground based spectroscopy and high-speed multi-band ULTRACAM photometry which indicate a primary DA WD of mass 0.40 +- 0.04 Msol and a 0.28 +- 0.02 Msol mass secondary WD, which is likely of type DA as well. The system becomes the third-closest eclipsing double WD binary discovered with a distance of approximately 400 pc and will be a detectable source for upcoming gravitational wave detectors in the mHz frequency range. Its orbital decay will be measurable photometrically within 10 yrs to a precision of better than 1%. The fate of the binary is to merge in approximately 41 Myr, likely forming a single, more massive WD. △ Less

Submitted 31 July, 2023; originally announced August 2023.

Comments: Accepted for publication in MNRAS, 8 pages + 2 appendix pages, 6 figures

General relativistic pulsations of ultra-massive ZZ Ceti stars

Authors: Alejandro H. Córsico, S. Reece Boston, Leandro G. Althaus, Mukremin Kilic, S. O. Kepler, María E. Camisassa, Santiago Torres

Abstract: Ultra-massive white dwarf stars are currently being discovered at a considerable rate, thanks to surveys such as the {\it Gaia} space mission. These dense and compact stellar remnants likely play a major role in type Ia supernova explosions. It is possible to probe the interiors of ultra-massive white dwarfs through asteroseismology. In the case of the most massive white dwarfs, General Relativity… ▽ More Ultra-massive white dwarf stars are currently being discovered at a considerable rate, thanks to surveys such as the {\it Gaia} space mission. These dense and compact stellar remnants likely play a major role in type Ia supernova explosions. It is possible to probe the interiors of ultra-massive white dwarfs through asteroseismology. In the case of the most massive white dwarfs, General Relativity could affect their structure and pulsations substantially. In this work, we present results of relativistic pulsation calculations employing relativistic ultra-massive ONe-core white dwarf models with hydrogen-rich atmospheres and masses ranging from $1.29$ to $1.369 M_{\odot}$ with the aim of assessing the impact of General Relativity on the adiabatic gravity ($g$)-mode period spectrum of very-high mass ZZ Ceti stars. Employing the relativistic Cowling approximation for the pulsation analysis, we find that the critical buoyancy (Brunt-Väisälä) and acoustic (Lamb) frequencies are larger for the relativistic case, compared to the Newtonian case, due to the relativistic white dwarf models having smaller radii and higher gravities for a fixed stellar mass. In addition, the $g$-mode periods are shorter in the relativistic case than in the Newtonian computations, with relative differences of up to $\sim 50$ \% for the highest-mass models ($1.369 M_{\odot}$) and for effective temperatures typical of the ZZ Ceti instability strip. Hence, the effects of General Relativity on the structure, evolution, and pulsations of white dwarfs with masses larger than $\sim 1.29 M_{\odot}$ cannot be ignored in the asteroseismological analysis of ultra-massive ZZ Ceti stars. △ Less

Submitted 21 July, 2023; originally announced July 2023.

Comments: 15 pages, 21 figures, 2 tables. Accepted for publication in MNRAS

The bright blue side of the night sky: Spectroscopic survey of bright and hot (pre-) white dwarfs

Authors: Nicole Reindl, Ramazan Islami, Klaus Werner, S. O. Kepler, Max Pritzkuleit, Harry Dawson, Matti Dorsch, Alina Istrate, Ingrid Pelisoli, Stephan Geier, Murat Uzundag, Judith Provencal, Stephen Justham

Abstract: We report on the spectroscopic confirmation of 68 new bright ($G=13.5-17.2$ mag) and blue (pre-)white dwarfs (WDs). This finding has allowed us to almost double the number of the hottest ($T_{\mathrm{eff}} \geq 60$kK) known WDs brighter than $G=16$ mag. We increased the number of known ultra-high excitation (UHE) WDs by 20%, found one unambiguous close binary system consisting of one DA WD with an… ▽ More We report on the spectroscopic confirmation of 68 new bright ($G=13.5-17.2$ mag) and blue (pre-)white dwarfs (WDs). This finding has allowed us to almost double the number of the hottest ($T_{\mathrm{eff}} \geq 60$kK) known WDs brighter than $G=16$ mag. We increased the number of known ultra-high excitation (UHE) WDs by 20%, found one unambiguous close binary system consisting of one DA WD with an irradiated low-mass companion, one DAO, and one DOA WD that are likely in their transformation phase of becoming pure DA WDs, one rare, naked O(H) star, two DA and two DAO WDs with $T_{\mathrm{eff}}$ possibly in excess of 100kK, three new DOZ WDs, and three of our targets are central stars of (possible) planetary nebulae. Using non-local thermodynamic equilibrium models, we derived the atmospheric parameters of these stars and by fitting their spectral energy distribution we derived their radii, luminosities, and gravity masses. In addition, we derived their masses in the Kiel and Hertzsprung-Russell diagram (HRD). We find that Kiel, HRD, and gravity mass agree only in half of the cases. This is not unexpected and we attribute this to the neglect of metal opacities, possibly stratified atmospheres, as well as possible uncertainties of the parallax zero point determination. Furthermore, we carried out a search for photometric variability in our targets using archival data, finding that 26% of our targets are variable. This includes 15 new variable stars, with only one of them being clearly an irradiation effect system. Strikingly, the majority of the variable stars exhibit non-sinusoidal light-curve shapes, which are unlikely explained in terms of close binary systems. We propose that a significant fraction of all (not just UHE) WDs develop spots when entering the WD cooling phase. We suggest that this could be related to the on-set of weak magnetic fields and possibly diffusion. △ Less

Submitted 7 July, 2023; originally announced July 2023.

Comments: Accepted for publication in A&A

Journal ref: A&A 677, A29 (2023)

A 5.3-minute-period pulsing white dwarf in a binary detected from radio to X-rays

Authors: Ingrid Pelisoli, T. R. Marsh, David A. H. Buckley, I. Heywood, Stephen. B. Potter, Axel Schwope, Jaco Brink, Annie Standke, P. A. Woudt, S. G. Parsons, M. J. Green, S. O. Kepler, James Munday, A. D. Romero, E. Breedt, A. J. Brown, V. S. Dhillon, M. J. Dyer, P. Kerry, S. P. Littlefair, D. I. Sahman, J. F. Wild

Abstract: White dwarf stars are the most common stellar fossils. When in binaries, they make up the dominant form of compact object binary within the Galaxy and can offer insight into different aspects of binary formation and evolution. One of the most remarkable white dwarf binary systems identified to date is AR Scorpii (henceforth AR Sco). AR Sco is composed of an M-dwarf star and a rapidly-spinning whit… ▽ More White dwarf stars are the most common stellar fossils. When in binaries, they make up the dominant form of compact object binary within the Galaxy and can offer insight into different aspects of binary formation and evolution. One of the most remarkable white dwarf binary systems identified to date is AR Scorpii (henceforth AR Sco). AR Sco is composed of an M-dwarf star and a rapidly-spinning white dwarf in a 3.56-hour orbit. It shows pulsed emission with a period of 1.97 minutes over a broad range of wavelengths, which led to it being known as a white dwarf pulsar. Both the pulse mechanism and the evolutionary origin of AR Sco provide challenges to theoretical models. Here we report the discovery of the first sibling of AR Sco, J191213.72-441045.1 (henceforth J1912-4410), which harbours a white dwarf in a 4.03-hour orbit with an M-dwarf and exhibits pulsed emission with a period of 5.30 minutes. This discovery establishes binary white dwarf pulsars as a class and provides support for proposed formation models for white dwarf pulsars. △ Less

Submitted 15 June, 2023; originally announced June 2023.

Comments: Authors' version of article published in Nature Astronomy (DOI 10.1038/s41550-023-01995-x)

Catalog of magnetic white dwarfs with hydrogen dominated atmospheres

Authors: L. L. Amorim, S. O. Kepler, Baybars Külebi, S. Jordan, A. D. Romero

Abstract: White dwarfs are excellent research laboratories as they reach temperatures, pressures, and magnetic fields that are unattainable on Earth. To better understand how these three physical parameters interact with each other and with other stellar features, we determined the magnetic field strength for a total of 804 hydrogen-rich white dwarfs of which 287 are not in the literature. We fitted the spe… ▽ More White dwarfs are excellent research laboratories as they reach temperatures, pressures, and magnetic fields that are unattainable on Earth. To better understand how these three physical parameters interact with each other and with other stellar features, we determined the magnetic field strength for a total of 804 hydrogen-rich white dwarfs of which 287 are not in the literature. We fitted the spectra observed with the Sloan Digital Sky Survey using atmospheric models that consider the Zeeman effect due to the magnetic field at each point in the stellar disk. Comparing magnetic and non-magnetic WDs, the literature already shows that the magnetic ones have on average higher mass than the non-magnetic. In addition to that, magnetic fields are more common in cooler WDs than in hotter WDs. In consonance, we found that those with higher magnetic field strengths tend to have higher masses, and lower temperatures, for which models indicate the crystallization process has already started. This reinforces the hypothesis that the field is being generated and/or amplified in the cooling process of the white dwarf. Our sample constitutes the largest number of white dwarfs with determined magnetic fields to date. △ Less

Submitted 20 January, 2023; originally announced January 2023.

Comments: Accepted in ApJ on 27th December 2022

Asteroseismology of PG 1541$+$651 and BPM 31594 with TESS

Authors: Alejandra D. Romero, Gabriela Oliveira da Rosa, S. O. Kepler, Paul A. Bradley, Murat Uzundag, Keaton J. Bell, J. J. Hermes, G. R. Lauffer

Abstract: We present the photometric data from TESS for two known ZZ Ceti stars, PG 1541+651 and BPM 31594. Before TESS, both objects only had observations from short runs from ground-based facilities, with three and one period detected, respectively. The TESS data allowed the detection of multiple periodicities, 12 for PG 1541$+$651, and six for BPM 31594, which enables us to perform a detailed asteroseism… ▽ More We present the photometric data from TESS for two known ZZ Ceti stars, PG 1541+651 and BPM 31594. Before TESS, both objects only had observations from short runs from ground-based facilities, with three and one period detected, respectively. The TESS data allowed the detection of multiple periodicities, 12 for PG 1541$+$651, and six for BPM 31594, which enables us to perform a detailed asteroseismological study. For both objects we found a representative asteroseismic model with canonical stellar mass ~ 0.61 Msun and thick hydrogen envelopes, thicker than 10^(-5.3) M_*. The detection of triplets in the Fourier transform also allowed us to estimate mean rotation periods, being ~22 h for PG 1541+651 and 11.6 h for BPM 31594, which is consistent with range of values reported for other ZZ Ceti stars. △ Less

Submitted 27 October, 2022; originally announced October 2022.

Comments: 12 pages, 11 figures. Accepted for publication in MNRAS

Pulsating H-deficient WDs and pre-WDs observed with TESS: V. Discovery of two new DBV pulsators, WD J152738.4-450207.4 and WD 1708-871, and asteroseismology of the already known DBV stars PG 1351+489, EC 20058-5234, and EC 04207-4748

Authors: Alejandro H. Córsico, Murat Uzundag, S. O. Kepler, Leandro G. Althaus, Roberto Silvotti, Paul A. Bradley, Andrzej S. Baran, Detlev Koester, Keaton J. Bell, Alejandra D. Romero, J. J. Hermes, Nicola P. Gentile Fusillo

Abstract: The {\sl TESS} space mission has recently demonstrated its great potential to discover new pulsating white dwarf and pre-white dwarf stars, and to detect periodicities with high precision in already known white-dwarf pulsators. We report the discovery of two new pulsating He-rich atmosphere white dwarfs (DBVs) and present a detailed asteroseismological analysis of three already known DBV stars emp… ▽ More The {\sl TESS} space mission has recently demonstrated its great potential to discover new pulsating white dwarf and pre-white dwarf stars, and to detect periodicities with high precision in already known white-dwarf pulsators. We report the discovery of two new pulsating He-rich atmosphere white dwarfs (DBVs) and present a detailed asteroseismological analysis of three already known DBV stars employing observations collected by the {\sl TESS} mission along with ground-based data. We extracted frequencies from the {\sl TESS} light curves of these DBV stars using a standard pre-whitening procedure to derive the potential pulsation frequencies. All the oscillation frequencies that we found are associated with $g$-mode pulsations with periods spanning from $\sim 190$ s to $\sim 936$ s. We find hints of rotation from frequency triplets in some of the targets, including the two new DBVs. For three targets, we find constant period spacings, which allowed us to infer their stellar masses and constrain the harmonic degree $\ell$ of the modes. We also performed period-to-period fit analyses and found an asteroseismological model for three targets, with stellar masses generally compatible with the spectroscopic masses. Obtaining seismological models allowed us to estimate the seismological distances and compare them with the precise astrometric distances measured with {\it Gaia}. We find a good agreement between the seismic and the astrometric distances for three stars (PG~1351+489, EC~20058$-$5234, and EC~04207$-$4748), although for the other two stars (WD~J152738.4$-$50207 and WD~1708$-$871), the discrepancies are substantial. The high-quality data from the {\sl TESS} mission continue to provide important clues to determine the internal structure of pulsating pre-white dwarf and white dwarf stars through the tools of asteroseismology. △ Less

Submitted 11 October, 2022; originally announced October 2022.

Comments: 22 pages, 27 figures, 21 tables. To be published in Astronomy & Astrophysics

The double white dwarf merger progenitors of SDSS J2211+1136 and ZTF J1901+1458

Authors: M. F. Sousa, J. G. Coelho, J. C. N. de Araujo, S. O. Kepler, J. A. Rueda

Abstract: Double white dwarf (DWD) mergers are possibly the leading formation channel of massive, rapidly rotating, high-field magnetic white dwarfs (HFMWDs). However, the direct link connecting a DWD merger to any observed HFMWD is still missing. We here show that the HFMWDs SDSS J221141.80+113604.4 (hereafter J2211+1136) and ZTF J190132.9+145808.7 (hereafter J1901+1458), might be DWD merger products. J221… ▽ More Double white dwarf (DWD) mergers are possibly the leading formation channel of massive, rapidly rotating, high-field magnetic white dwarfs (HFMWDs). However, the direct link connecting a DWD merger to any observed HFMWD is still missing. We here show that the HFMWDs SDSS J221141.80+113604.4 (hereafter J2211+1136) and ZTF J190132.9+145808.7 (hereafter J1901+1458), might be DWD merger products. J2211+1136 is a $1.27\, M_\odot$ WD with a rotation period of $70.32$ s and a surface magnetic field of $15$ MG. J1901+1458 is a $1.327$--$1.365\, M_\odot$ WD with a rotation period of $416.20$ s, and a surface magnetic field in the range $600$--$900$ MG. With the assumption of single-star evolution, the currently measured WD masses and surface temperatures, the cooling ages of J2211+1136 and J1901+1458 are, respectively, $2.61$--$2.85$ Gyr and $10$--$100$ Myr. We hypothesize that these WDs are DWD merger products and compute the evolution of the post-merged configuration formed by a central WD surrounded by a disk. We show that the post-merger system evolves through three phases depending on whether accretion, mass ejection (propeller), or magnetic braking dominates the torque onto the central WD. We calculate the time the WD spends in each of these phases and obtain the accretion rate and disk mass for which the WD rotational age, i.e., the total time elapsed since the merger to the instant where the WD central remnant reaches the current measured rotation period, agrees with the estimated WD cooling age. We infer the mass values of the primary and secondary WD components of the DWD merger that lead to a post-merger evolution consistent with the observations. △ Less

Submitted 3 November, 2022; v1 submitted 19 August, 2022; originally announced August 2022.

Kepler and TESS Observations of PG 1159-035

Authors: Gabriela Oliveira da Rosa, S. O. Kepler, Alejandro H. Córsico, J. E. S. Costa, J. J. Hermes, S. D. Kawaler, Keaton J. Bell, M. H. Montgomery, J. L. Provencal, D. E. Winget, G. Handler, Bart Dunlap, J. C. Clemens, Murat Uzundag

Abstract: PG 1159-035 is the prototype of the DOV hot pre-white dwarf pulsators. It was observed during the Kepler satellite K2 mission for 69 days in 59 s cadence mode and by the TESS satellite for 25 days in 20 s cadence mode. We present a detailed asteroseismic analysis of those data. We identify a total of 107 frequencies representing 32 l=1 modes, 27 frequencies representing 12 l=2 modes, and 8 combina… ▽ More PG 1159-035 is the prototype of the DOV hot pre-white dwarf pulsators. It was observed during the Kepler satellite K2 mission for 69 days in 59 s cadence mode and by the TESS satellite for 25 days in 20 s cadence mode. We present a detailed asteroseismic analysis of those data. We identify a total of 107 frequencies representing 32 l=1 modes, 27 frequencies representing 12 l=2 modes, and 8 combination frequencies. The combination frequencies and the modes with very high k values represent new detections. The multiplet structure reveals an average splitting of 4.0+/-0.4 muHz for l=1 and 6.8+/-0.2 muHz for l=2, indicating a rotation period of 1.4+/-0.1 days in the region of period formation. In the Fourier transform of the light curve, we find a significant peak at 8.904+/-0.003 muHz suggesting a surface rotation period of 1.299+/-0.002 days. We also present evidence that the observed periods change on timescales shorter than those predicted by current evolutionary models. Our asteroseismic analysis finds an average period spacing for l=1 of 21.28+/-0.02 s. The l=2 modes have a mean spacing of 12.97+/-0.4 s. We performed a detailed asteroseismic fit by comparing the observed periods with those of evolutionary models. The best fit model has Teff=129600+/- 11100 K, mass M*=0.565+/-0.024 Msun, and log g=7.41+0.38-0.54, within the uncertainties of the spectroscopic determinations. We argue for future improvements in the current models, e.g., on the overshooting in the He-burning stage, as the best-fit model does not predict excitation for all the pulsations detected in PG~1159-03. △ Less

Submitted 9 August, 2022; originally announced August 2022.

Comments: 27 pages, 9 tables and 26 figures

The impact of the uncertainties in the 12C(α,γ)16O reaction rate on the evolution of low- to intermediate-mass stars

Authors: Ben T. Pepper, A. G. Istrate, A. D. Romero, S. O. Kepler

Abstract: One of the largest uncertainties in stellar evolutionary computations is the accuracy of the considered reaction rates. The 12C(alpha,gamma)16O reaction is particularly important for the study of low- and intermediate-mass stars as it determines the final C/O ratio in the core which influences the white dwarf cooling evolution. Thus, there is a need for a study of how the computations of white dwa… ▽ More One of the largest uncertainties in stellar evolutionary computations is the accuracy of the considered reaction rates. The 12C(alpha,gamma)16O reaction is particularly important for the study of low- and intermediate-mass stars as it determines the final C/O ratio in the core which influences the white dwarf cooling evolution. Thus, there is a need for a study of how the computations of white dwarfs and their progenitors that are made to date may be affected by the uncertainties of the 12C(alpha,gamma)16O reaction rates. In this work we compute fully evolutionary sequences using the MESA code with initial masses in the range of 0.90 <= Mi/Msun <= 3.05. We consider different adopted reaction rates, obtained from the literature, as well as the extreme limits within their uncertainties. As expected, we find that previous to the core helium burning stage there are no changes to the evolution of the stars. However, the subsequent stages are all affected by the uncertainties of the considered reaction rate. In particular, we find differences to the convective core mass during the core helium burning stage which may affect pulsation properties of subdwarfs, the number of thermal pulses during the asymptotic giant branch and trends between final oxygen abundance in the core and the progenitor masses of the remnant white dwarfs. △ Less

Submitted 6 April, 2022; originally announced April 2022.

Comments: 15 pages, 15 figures, 1 table

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS -- IV. Discovery of two new GW Vir stars: TIC0403800675 and TIC1989122424

Authors: Murat Uzundag, Alejandro H. Corsico, S. O. Kepler, Leandro G. Althaus, Klaus Werner, Nicole Reindl, Maja Vuckovic

Abstract: We present two new GW Vir-type pulsating white dwarf stars, TIC\,0403800675 (WD\,J115727.68-280349.64) and TIC\,1989122424 (WD J211738.38-552801.18) discovered in the Transiting Exoplanet Survey Satellite (TESS) photometric data. For both stars, the TESS light curves reveal the presence of oscillations with periods in a narrow range between 400 and 410\,s, which are associated with typical gravity… ▽ More We present two new GW Vir-type pulsating white dwarf stars, TIC\,0403800675 (WD\,J115727.68-280349.64) and TIC\,1989122424 (WD J211738.38-552801.18) discovered in the Transiting Exoplanet Survey Satellite (TESS) photometric data. For both stars, the TESS light curves reveal the presence of oscillations with periods in a narrow range between 400 and 410\,s, which are associated with typical gravity ($g$)-modes. Follow-up ground-based spectroscopy shows that both stars have similar effective temperature ($T_\mathrm{eff} = 110,000 \pm 10,000$\,K) and surface gravity ($\log g = 7.5 \pm 0.5$), but different He/C composition (mass fractions): He\,=\,0.75 and C\,=\,0.25 for TIC\,0403800675, and He\,=\,0.50 and C\,=\,0.50 for TIC\,1989122424. By performing a fit to their spectral energy distributions, we found for both stars radii and luminosities of $R=0.019\pm0.002\,R_\odot$ and $\log(L/L_\odot)=1.68^{+0.15}_{-0.24}$, respectively. By employing evolutionary tracks of PG~1159 stars, we find the masses of both stars to be $0.56\pm0.18 M_{\odot}$ from the $\log g$-$T_\mathrm{eff}$ diagram and $0.60^{+0.11}_{-0.09} M_{\odot}$ from the Hertzsprung Russell diagram. △ Less

Submitted 5 April, 2022; originally announced April 2022.

Comments: 8 Pages. arXiv admin note: text overlap with arXiv:2108.11093

The Pulsating Helium-Atmosphere White Dwarfs I: New DBVs from the Sloan Digital Sky Survey

Authors: Zachary P. Vanderbosch, J. J. Hermes, Don E. Winget, Michael H. Montgomery, Keaton J. Bell, Atsuko Nitta, S. O. Kepler

Abstract: We present a dedicated search for new pulsating helium-atmosphere (DBV) white dwarfs from the Sloan Digital Sky Survey using the McDonald 2.1m Otto Struve Telescope. In total we observed 55 DB and DBA white dwarfs with spectroscopic temperatures between 19,000 and 35,000K. We find 19 new DBVs and place upper limits on variability for the remaining 36 objects. In combination with previously known D… ▽ More We present a dedicated search for new pulsating helium-atmosphere (DBV) white dwarfs from the Sloan Digital Sky Survey using the McDonald 2.1m Otto Struve Telescope. In total we observed 55 DB and DBA white dwarfs with spectroscopic temperatures between 19,000 and 35,000K. We find 19 new DBVs and place upper limits on variability for the remaining 36 objects. In combination with previously known DBVs, we use these objects to provide an update to the empirical extent of the DB instability strip. With our sample of new DBVs, the red edge is better constrained, as we nearly double the number of DBVs known between 20,000 and 24,000K. We do not find any new DBVs hotter than PG 0112+104, the current hottest DBV at $T_{\mathrm{eff}}\,{\approx}$ 31,000K, but do find pulsations in four DBVs with temperatures between 27,000 and 30,000K, improving empirical constraints on the poorly defined blue edge. We investigate the ensemble pulsation properties of all currently known DBVs, finding that the weighted mean period and total pulsation power exhibit trends with effective temperature that are qualitatively similar to the pulsating hydrogen-atmosphere white dwarfs. △ Less

Submitted 24 January, 2022; originally announced January 2022.

Comments: 19 pages, 8 figures, 5 tables. Accepted to ApJ

Discovery of 74 new bright ZZ Ceti stars in the first three years of TESS

Authors: A. D. Romero, S. O. Kepler, J. J. Hermes, Larissa Antunes Amaral, Murat Uzundag, Zsófia Bognár, Keaton J. Bell, Madison VanWyngarden, Andy Baran, Ingrid Pelisoli, Gabriela da Rosa Oliveira, Detlev Koester, T. S. Klippel, Luciano Fraga, Paul A. Bradley, Maja Vučković, Tyler M. Heintz, Joshua S. Reding, B. C. Kaiser, Stéphane Charpinet

Abstract: We report the discovery of 74 new pulsating DA white dwarf stars, or ZZ Cetis, from the data obtained by the Transiting Exoplanet Survey Satellite (TESS) mission, from Sectors 1 to 39, corresponding to the first 3 cycles. This includes objects from the Southern Hemisphere (Sectors 1-13 and 27-39) and the Northern Hemisphere (Sectors 14-26), observed with 120 s- and 20 s-cadence. Our sample likely… ▽ More We report the discovery of 74 new pulsating DA white dwarf stars, or ZZ Cetis, from the data obtained by the Transiting Exoplanet Survey Satellite (TESS) mission, from Sectors 1 to 39, corresponding to the first 3 cycles. This includes objects from the Southern Hemisphere (Sectors 1-13 and 27-39) and the Northern Hemisphere (Sectors 14-26), observed with 120 s- and 20 s-cadence. Our sample likely includes 13 low-mass and one extremely low-mass white dwarf candidate, considering the mass determinations from fitting Gaia magnitudes and parallax. In addition, we present follow-up time series photometry from ground-based telescopes for 11 objects, which allowed us to detect a larger number of periods. For each object, we analysed the period spectra and performed an asteroseismological analysis, and we estimate the structure parameters of the sample, i.e., stellar mass, effective temperature and hydrogen envelope mass. We estimate a mean asteroseismological mass of <Msis>_~ 0.635 +/-0.015 Msun, excluding the candidate low or extremely-low mass objects. This value is in agreement with the mean mass using estimates from Gaia data, which is <Mphot> ~ 0.631 +/- 0.040 Msun, and with the mean mass of previously known ZZ Cetis of <M*>= 0.644 +/-0.034 Msun. Our sample of 74 new bright ZZ~Cetis increases the number of known ZZ~Cetis by $\sim$20 per cent. △ Less

Submitted 11 January, 2022; originally announced January 2022.

Comments: Accepted for publication in MNRAS

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with {\it TESS}: III. Asteroseismology of the DBV star GD 358

Authors: Alejandro H. Córsico, Murat Uzundag, S. O. Kepler, Roberto Silvotti, Leandro G. Althaus, Detlev Koester, Andrzej S. Baran, Keaton J. Bell, Agnès Bischoff-Kim, J. J. Hermes, Steve D. Kawaler, Judith L. Provencal, Don E. Winget, Michael H. Montgomery, Paul A. Bradley, S. J. Kleinman, Atsuko Nitta

Abstract: The collection of high-quality photometric data by space telescopes is revolutionizing the area of white-dwarf asteroseismology. Among the different kinds of pulsating white dwarfs, there are those that have He-rich atmospheres, and they are called DBVs or V777 Her variable stars. The archetype of these pulsating white dwarfs, GD~358, is the focus of the present paper. We report a thorough asteros… ▽ More The collection of high-quality photometric data by space telescopes is revolutionizing the area of white-dwarf asteroseismology. Among the different kinds of pulsating white dwarfs, there are those that have He-rich atmospheres, and they are called DBVs or V777 Her variable stars. The archetype of these pulsating white dwarfs, GD~358, is the focus of the present paper. We report a thorough asteroseismological analysis of the DBV star GD~358 (TIC~219074038) based on new high-precision photometric data gathered by the {\it TESS} space mission combined with data taken from the Earth. In total, we detected 26 periodicities from the {\it TESS} light curve of this DBV star using a standard pre-whitening. The oscillation frequencies are associated with nonradial $g$(gravity)-mode pulsations with periods from $\sim 422$ s to $\sim 1087$ s. Moreover, we detected 8 combination frequencies between $\sim 543$ s and $\sim 295$ s. We combined these data with a huge amount of observations from the ground. We found a constant period spacing of $39.25\pm0.17$ s, which helped us to infer its mass ($M_{\star}= 0.588\pm0.024 M_{\sun}$) and constrain the harmonic degree $\ell$ of the modes. We carried out a period-fit analysis on GD~358, and we were successful in finding an asteroseismological model with a stellar mass ($M_{\star}= 0.584^{+0.025}_{-0.019} M_{\sun}$), in line with the spectroscopic mass ($M_{\star}= 0.560\pm0.028 M_{\sun}$). We found that the frequency splittings vary according to the radial order of the modes, suggesting differential rotation. Obtaining a seismological made it possible to estimate the seismological distance ($d_{\rm seis}= 42.85\pm 0.73$ pc) of GD~358, which is in very good accordance with the precise astrometric distance measured by {\it GAIA} EDR3 ($π= 23.244\pm 0.024, d_{\rm GAIA}= 43.02\pm 0.04$~pc). △ Less

Submitted 30 November, 2021; originally announced November 2021.

Comments: 15 pages, 10 figures, 5 tables. To be published in Astronomy and Astrophysics. arXiv admin note: text overlap with arXiv:2011.03629

Journal ref: A&A 659, A30 (2022)

Hot Degenerates in the MCT Survey. III. A Sample of White Dwarf Stars in the Southern Hemisphere

Authors: P. Bergeron, F. Wesemael, G. Fontaine, R. Lamontagne, S. Demers, A. Bédard, M. -J. Gingras, S. Blouin, M. J. Irwin, S. O. Kepler

Abstract: We present optical spectra of 144 white dwarfs detected in the Montreal-Cambridge-Tololo (MCT) colorimetric survey, including 120 DA, 12 DB, 4 DO, 1 DQ, and 7 DC stars. We also perform a model atmosphere analysis of all objects in our sample using the so-called spectroscopic technique, or the photometric technique in the case of DC white dwarfs. The main objective of this paper is to contribute to… ▽ More We present optical spectra of 144 white dwarfs detected in the Montreal-Cambridge-Tololo (MCT) colorimetric survey, including 120 DA, 12 DB, 4 DO, 1 DQ, and 7 DC stars. We also perform a model atmosphere analysis of all objects in our sample using the so-called spectroscopic technique, or the photometric technique in the case of DC white dwarfs. The main objective of this paper is to contribute to the ongoing effort of confirming spectroscopically all white dwarf candidates in the Gaia survey, in particular in the southern hemisphere. All our spectra are made available in the Montreal White Dwarf Database. △ Less

Submitted 30 August, 2021; originally announced August 2021.

Comments: Accepted for publication in The Astronomical Journal, 43 pages, 2 tables, 13 figures

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS II. Discovery of two new GW Vir stars: TIC333432673 and TIC095332541

Authors: Murat Uzundag, Alejandro H. Córsico, S. O. Kepler, Leandro G. Althaus, Klaus Werner, Nicole Reindl, Keaton J. Bell, Michael Higgins, Gabriela O. da Rosa, Maja Vučković, Alina Istrate

Abstract: In this paper, we present the observations of two new GW Vir stars from the extended \textit{TESS} mission in both 120\,s short-cadence and 20\,s ultra-short-cadence mode of two pre-white dwarf stars showing hydrogen deficiency. We performed an asteroseismological analysis of these stars on the basis of PG~1159 evolutionary models that take into account the complete evolution of the progenitor sta… ▽ More In this paper, we present the observations of two new GW Vir stars from the extended \textit{TESS} mission in both 120\,s short-cadence and 20\,s ultra-short-cadence mode of two pre-white dwarf stars showing hydrogen deficiency. We performed an asteroseismological analysis of these stars on the basis of PG~1159 evolutionary models that take into account the complete evolution of the progenitor stars. We searched for patterns of uniform period spacings in order to constrain the stellar mass of the stars, and employed the individual observed periods to search for a representative seismological model. The analysis of the {\it TESS} light curves of TIC\,333432673 and TIC\,095332541 reveals the presence of several oscillations with periods ranging from 350 to 500~s associated to typical gravity ($g$)-modes. From follow-up ground-based spectroscopy, we find that both stars have similar effective temperature ($T_\mathrm{eff} = 120,000 \pm 10,000$\,K) and surface gravity ($\log g = 7.5 \pm 0.5$) but a different He/C composition. On the basis of PG~1159 evolutionary tracks, we derived a spectroscopic mass of $M_{\star}$ = $0.58^{+0.16}_{-0.08}\,M_{\odot}$ for both stars. Our asteroseismological analysis of TIC\,333432673 allowed us to find a constant period spacing compatible with a stellar mass $M_{\star}\sim 0.60-0.61\,M_{\odot}$, and an asteroseismological model for this star with a stellar mass $M_{\star}$ = $0.589\pm 0.020$ $M_{\odot}$, and a seismological distance of $d= 459^{+188}_{-156}$ pc. For this star, we find an excellent agreement between the different methods to infer the stellar mass, and also between the seismological distance and that measured with {\it Gaia} ($d_{\rm Gaia}= 389^{+5.6}_{-5.2}$ pc). For TIC\,095332541, we have found a possible period spacing that suggests a stellar mass of $M_{\star}\sim 0.55-0.57\,M_{\odot}$. △ Less

Submitted 25 August, 2021; originally announced August 2021.

Comments: 12 pages, 13 figures, Accepted for publication in A&A. arXiv admin note: text overlap with arXiv:2011.03629

Journal ref: A&A 655, A27 (2021)

White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 16

Authors: S. O. Kepler, Detlev Koester, Ingrid Pelisoli, Alejandra D Romero, Gustavo Ourique

Abstract: White dwarfs are the end state of the evolution of more than 97% of all stars, and therefore carry information on the structure and evolution of the Galaxy through their luminosity function and initial-to-final mass relation. Examining the new spectra of all white or blue stars in the Sloan Digital Sky Survey Data Release 16, we report the spectral classification of 2410 stars, down to our identif… ▽ More White dwarfs are the end state of the evolution of more than 97% of all stars, and therefore carry information on the structure and evolution of the Galaxy through their luminosity function and initial-to-final mass relation. Examining the new spectra of all white or blue stars in the Sloan Digital Sky Survey Data Release 16, we report the spectral classification of 2410 stars, down to our identification cut-off of signal-to-noise ratio equal to three. We newly identify 1404 DAs, 189 DZs, 103 DCs, 12 DBs, and 9 CVs. The remaining objects are a mix of carbon or L stars (dC/L), narrow-lined hydrogen-dominated stars (sdA), dwarf F stars and P Cyg objects. As white dwarf stars were not targeted by SDSS DR16, the number of new discoveries is much smaller than in previous releases. We also report atmospheric parameters and masses for a subset consisting of 555 new DAs, 10 new DBs, and 85 DZs for spectra with signal-to-noise ratio larger than 10. △ Less

Submitted 24 August, 2021; originally announced August 2021.

Comments: arXiv admin note: substantial text overlap with arXiv:1904.01626

Convection and rotation boosted prescription of magnetic braking: application to the formation of extremely low-mass white dwarfs

Authors: L. T. T. Soethe, S. O. Kepler

Abstract: Extremely low-mass white dwarfs (ELM WDs) are the result of binary evolution in which a low-mass donor star is stripped by its companion leaving behind a helium-core white dwarf. We explore the formation of ELM WDs in binary systems considering the Convection And Rotation Boosted magnetic braking treatment. Our evolutionary sequences were calculated using the MESA code, with initial masses of 1.0… ▽ More Extremely low-mass white dwarfs (ELM WDs) are the result of binary evolution in which a low-mass donor star is stripped by its companion leaving behind a helium-core white dwarf. We explore the formation of ELM WDs in binary systems considering the Convection And Rotation Boosted magnetic braking treatment. Our evolutionary sequences were calculated using the MESA code, with initial masses of 1.0 and 1.2 Msun (donor), and 1.4 (accretor), compatible with low mass X-ray binaries (LMXB) systems. We obtain ELM models in the range 0.15 to 0.27 Msun from a broad range of initial orbital periods, 1 to 25 d. The bifurcation period, where the initial period is equal to the final period, ranges from 20 to 25 days. In addition to LMXBs, we show that ultra-compact X-ray binaries (UCXB) and wide-orbit binary millisecond pulsars can also be formed. The relation between mass and orbital period obtained is compatible with the observational data from He white dwarf companions to pulsars. △ Less

Submitted 8 July, 2021; originally announced July 2021.

Comments: 16 pages, 6 figures. Accepted for publication in MNRAS

Mysterious, Variable, and Extremely Hot: White Dwarfs Showing Ultra-High Excitation Lines I. Photometric Variability

Authors: Nicole Reindl, Veronika Schaffenroth, Semih Filiz, Stephan Geier, Ingrid Pelisoli, S. O. Kepler

Abstract: About 10% of all stars exhibit absorption lines of ultra-high excited (UHE) metals (e.g. OVIII) in their optical spectra when entering the white dwarf cooling sequence. The recent discovery of a both spectroscopic and photometric variable UHE white dwarf led to the speculation that the UHE lines might be created in a shock-heated circumstellar magnetosphere. We investigate (multi-band) light curve… ▽ More About 10% of all stars exhibit absorption lines of ultra-high excited (UHE) metals (e.g. OVIII) in their optical spectra when entering the white dwarf cooling sequence. The recent discovery of a both spectroscopic and photometric variable UHE white dwarf led to the speculation that the UHE lines might be created in a shock-heated circumstellar magnetosphere. We investigate (multi-band) light curves from several ground- and space-based surveys of all 16 currently known UHE white dwarfs (including one newly discovered) and eight white dwarfs that show only the HeII line problem, as both phenomena are believed to be connected. We find that $75^{+8}_{-13}$% of the UHE white dwarfs, and $75^{+9}_{-19}$% of the HeII line problem white dwarfs are significantly photometrically variable, with periods ranging from 0.22d to 2.93d and amplitudes from a few tenth to a few hundredth mag. The high variability rate is in stark contrast to the variability rate amongst normal hot white dwarfs (we find $9^{+4}_{-2}$%), marking UHE and HeII line problem white dwarfs as a new class of variable stars. The period distribution of our sample agrees with both the orbital period distribution of post-common envelope binaries and the rotational period distribution of magnetic white dwarfs if we assume that the objects in our sample will spin-up as a consequence of further contraction. The lack of increasing photometric amplitudes towards longer wavelengths, as well as the non-detection of optical emission lines arising from the highly irradiated face of a hypothetical secondary in the optical spectra of our stars, makes it seem unlikely that an irradiated late type companion is the origin of the photometric variability. Instead, we believe that spots on the surfaces of these stars and/or geometrical effects of circumstellar material might be responsible. (abridged) △ Less

Submitted 8 February, 2021; originally announced February 2021.

Comments: 24 pages, accepted for publication in A&A

Journal ref: A&A 647, A184 (2021)

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS: I. Asteroseismology of the GW Vir stars RX J2117+3412, HS 2324+3944, NGC 6905, NGC 1501, NGC 2371, and K 1-16

Authors: Alejandro H. Córsico, Murat Uzundag, S. O. Kepler, Leandro G. Althaus, Roberto Silvotti, Andrzej S. Baran, Maja Vučković, Klaus Werner, Keaton J. Bell, Michael Higgins

Abstract: In this paper, we present a detailed asteroseismological analysis of six GW Vir stars including the observations collected by the TESS mission. We processed and analyzed TESS observations of RX J2117+3412, HS 2324+3944, NGC 6905, NGC 1501, NGC 2371, and K 1-16. We carried out a detailed asteroseismological analysis of these stars on the basis of PG 1159 evolutionary models that take into account t… ▽ More In this paper, we present a detailed asteroseismological analysis of six GW Vir stars including the observations collected by the TESS mission. We processed and analyzed TESS observations of RX J2117+3412, HS 2324+3944, NGC 6905, NGC 1501, NGC 2371, and K 1-16. We carried out a detailed asteroseismological analysis of these stars on the basis of PG 1159 evolutionary models that take into account the complete evolution of the progenitor stars. In total, we extracted 58 periodicities from the TESS light curves using a standard pre-whitening procedure to derive the potential pulsation frequencies. All the oscillation frequencies that we found are associated with g-mode pulsations with periods spanning from $\sim 817$ s to $\sim 2682$ s. We find constant period spacings for all but one star, which allowed us to infer their stellar masses and constrain the harmonic degree $\ell$ of the modes. We performed period-to-period fit analyses on five of the six analyzed stars. For four stars, we were able to find an asteroseismological model with masses in agreement with the stellar-mass values inferred from the period spacings, and generally compatible with the spectroscopic masses. We estimated the seismological distance and compared it with the precise astrometric distance measured with GAIA. Finally, we find that the period spectrum of K 1-16 exhibits dramatic changes in frequency and amplitude. The high-quality data collected by the TESS space mission, considered simultaneously with ground-based observations, are able to provide a very valuable input to the asteroseismology of GW Vir stars, similar to the case of other classes of pulsating white-dwarf stars. The TESS mission, in conjunction with future space missions and upcoming surveys, will make impressive progress in white-dwarf asteroseismology. △ Less

Submitted 6 November, 2020; originally announced November 2020.

Comments: 34 pages, 33 figures, 21 tables. Accepted for publication in Astronomy and Astrophysics

Journal ref: A&A 645, A117 (2021)

The pulsating white dwarf G117-B15A: still the most stable optical clock known

Authors: S. O. Kepler, D. E. Winget, Zachary P. Vanderbosch, Barbara Garcia Castanheira, J. J. Hermes, Keaton J. Bell, Fergal Mullally, Alejandra D. Romero, M. H. Montgomery, Steven DeGennaro, Karen I. Winget, Dean Chandler, Elizabeth J. Jeffery, Jamile K. Fritzen, Kurtis A. Williams, Paul Chote, Staszek Zola

Abstract: The pulsating hydrogen atmosphere white dwarf star G 117-B15A has been observed since 1974. Its main pulsation period at 215.19738823(63) s, observed in optical light curves, varies by only (5.12+/-0.82)x10^{-15} s/s and shows no glitches, as pulsars do. The observed rate of period change corresponds to a change of the pulsation period by 1 s in 6.2 million years. We demonstrate that this exceptio… ▽ More The pulsating hydrogen atmosphere white dwarf star G 117-B15A has been observed since 1974. Its main pulsation period at 215.19738823(63) s, observed in optical light curves, varies by only (5.12+/-0.82)x10^{-15} s/s and shows no glitches, as pulsars do. The observed rate of period change corresponds to a change of the pulsation period by 1 s in 6.2 million years. We demonstrate that this exceptional optical clock can continue to put stringent limits on fundamental physics, such as constraints on interaction from hypothetical dark matter particles, as well as to search for the presence of external substellar companions. △ Less

Submitted 3 November, 2020; v1 submitted 28 October, 2020; originally announced October 2020.

Comments: 14 pages, 2 figures, 2 tables. arXiv admin note: substantial text overlap with arXiv:astro-ph/0507487; replaced because Fig. 2 was overlaying text

SDSS J124043.01+671034.68: The partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?

Authors: Boris T. Gaensicke, Detlev Koester, Roberto Raddi, Odette Toloza, S. O. Kepler

Abstract: The white dwarf SDSS J124043.01+671034.68 (SDSS J1240+6710) was previously found to have an oxygen-dominated atmosphere with significant traces of neon, magnesium, and silicon. A possible origin via a violent late thermal pulse or binary interactions have been suggested to explain this very unusual photospheric composition. We report the additional detection of carbon, sodium, and aluminium in far… ▽ More The white dwarf SDSS J124043.01+671034.68 (SDSS J1240+6710) was previously found to have an oxygen-dominated atmosphere with significant traces of neon, magnesium, and silicon. A possible origin via a violent late thermal pulse or binary interactions have been suggested to explain this very unusual photospheric composition. We report the additional detection of carbon, sodium, and aluminium in far-ultraviolet and optical follow-up spectroscopy. No iron-group elements are detected, with tight upper limits on iron, cobalt and nickel, suggesting that the star underwent partial oxygen burning, but failed to ignite silicon burning. Modelling the spectral energy distribution and adopting the distance based on the Gaia parallax, we infer a low white dwarf mass, M(wd)=0.41+/-0.05Msun. The large space velocity of SDSS J1240+6710, computed from the Gaia proper motion and its radial velocity, is compatible with a Galactic rest-frame velocity of ~250km/s in the opposite direction with respect to the Galactic rotation, strongly supporting a binary origin of this star. We discuss the properties of SDSS J1240+6710 in the context of the recently identified survivors of thermonuclear supernovae, the D6 and LP 40-365 stars, and conclude that it is unlikely related to either of those two groups. We tentatively suggest that SDSS J1240+6710 is the partially burned remnant of a low-mass white dwarf that underwent a thermonuclear event. △ Less

Submitted 12 June, 2020; originally announced June 2020.

Comments: Accepted for publication in MNRAS

Pulsation in the white dwarf HE 1017-1352: confirmation of the class of hot DAV stars

Authors: Alejandra D. Romero, L. Antunes Amaral, S. O. Kepler, L. Fraga, D. Kurtz, H. Shibahashi

Abstract: We report the detection of periodic variations on the T_eff ~32 000 K DA white dwarf star HE 1017-1352. We obtained time series photometry using the 4.1 m SOAR telescope on three separate nights for a total of 16.8 h. From the frequency analysis we found four periods of 605 s, 556 s, 508 s and 869 s with significant amplitudes above the 1/1000 false alarm probability detection limit. The detected… ▽ More We report the detection of periodic variations on the T_eff ~32 000 K DA white dwarf star HE 1017-1352. We obtained time series photometry using the 4.1 m SOAR telescope on three separate nights for a total of 16.8 h. From the frequency analysis we found four periods of 605 s, 556 s, 508 s and 869 s with significant amplitudes above the 1/1000 false alarm probability detection limit. The detected modes are compatible with low harmonic degree g-mode non-radial pulsations with radial order higher than ~ 9. This detection confirms the pulsation nature of HE 1017-1352 and thus the existence of the new pulsating class of hot DA white dwarf stars. In addition, we detect a long period of 1.52 h, compatible with a rotation period of DA white dwarf stars. △ Less

Submitted 1 June, 2020; originally announced June 2020.

Comments: 6 pages, 3 figures. Accepted for publication in the Monthly Notices of the Royal Astronomical Society Letters

$\textit{Gaia}$ white dwarfs within 40 pc I: spectroscopic observations of new candidates

Authors: P. -E. Tremblay, M. A. Hollands, N. P. Gentile Fusillo, J. McCleery, P. Izquierdo, B. T. Gänsicke, E. Cukanovaite, D. Koester, W. R. Brown, S. Charpinet, T. Cunningham, J. Farihi, N. Giammichele, V. van Grootel, J. J. Hermes, M. J. Hoskin, S. Jordan, S. O. Kepler, S. J. Kleinman, C. J. Manser, T. R. Marsh, D. de Martino, A. Nitta, S. G. Parsons, I. Pelisoli , et al. (8 additional authors not shown)

Abstract: We present a spectroscopic survey of 230 white dwarf candidates within 40 pc of the Sun from the William Herschel Telescope and Gran Telescopio Canarias. All candidates were selected from $\textit{Gaia}$ Data Release 2 (DR2) and in almost all cases had no prior spectroscopic classifications. We find a total of 191 confirmed white dwarfs and 39 main-sequence star contaminants. The majority of stell… ▽ More We present a spectroscopic survey of 230 white dwarf candidates within 40 pc of the Sun from the William Herschel Telescope and Gran Telescopio Canarias. All candidates were selected from $\textit{Gaia}$ Data Release 2 (DR2) and in almost all cases had no prior spectroscopic classifications. We find a total of 191 confirmed white dwarfs and 39 main-sequence star contaminants. The majority of stellar remnants in the sample are relatively cool ($\langle T_{\rm eff} \rangle$ = 6200 K), showing either hydrogen Balmer lines or a featureless spectrum, corresponding to 89 DA and 76 DC white dwarfs, respectively. We also recover two DBA white dwarfs and 9--10 magnetic remnants. We find two carbon-bearing DQ stars and 14 new metal-rich white dwarfs. This includes the possible detection of the first ultra-cool white dwarf with metal lines. We describe three DZ stars for which we find at least four different metal species, including one which is strongly Fe- and Ni-rich, indicative of the accretion of a planetesimal with core-Earth composition. We find one extremely massive (1.31 $\pm$ 0.01 M$_{\odot}$) DA white dwarf showing weak Balmer lines, possibly indicating stellar magnetism. Another white dwarf shows strong Balmer line emission but no infrared excess, suggesting a low-mass sub-stellar companion. High spectroscopic completeness ($>$99%) has now been reached for $\textit{Gaia}$ DR2 sources within 40 pc sample, in the northern hemisphere ($δ>$ 0 deg) and located on the white dwarf cooling track in the Hertzsprung-Russell diagram. A statistical study of the full northern sample is presented in a companion paper. △ Less

Submitted 26 June, 2020; v1 submitted 1 June, 2020; originally announced June 2020.

Comments: 37 pages, 25 figures. Accepted for publication in MNRAS

New white dwarf envelope models and diffusion. Application to DQ white dwarfs

Authors: D. Koester, S. O. Kepler, A. W. Irwin

Abstract: Recent studies of the atmospheres of carbon-rich (DQ) white dwarfs have demonstrated the existence of two different populations that are distinguished by the temperature range, but more importantly, by the extremely high masses of the hotter group. The classical DQ below 10000 K are well understood as the result of dredge-up of carbon by the expanding helium convection zone. The high-mass group po… ▽ More Recent studies of the atmospheres of carbon-rich (DQ) white dwarfs have demonstrated the existence of two different populations that are distinguished by the temperature range, but more importantly, by the extremely high masses of the hotter group. The classical DQ below 10000 K are well understood as the result of dredge-up of carbon by the expanding helium convection zone. The high-mass group poses several problems regarding their origin and also an unexpected correlation of effective temperature with mass. We propose to study the envelopes of these objects to determine the total hydrogen and helium masses as possible clues to their evolution. We developed new codes for envelope integration and diffusive equilibrium that are adapted to the unusual chemical composition, which is not necessarily dominated by hydrogen and helium. Using the new results for the atmospheric parameters, in particular, the masses obtained using Gaia parallaxes, we confirm that the narrow sequence of carbon abundances with Teff in the cool classical DQ is indeed caused by an almost constant helium to total mass fraction, as found in earlier studies. This mass fraction is smaller than predicted by stellar evolution calculations. For the warm DQ above 10000 K, which are thought to originate from double white dwarf mergers, we obtain extremely low hydrogen and helium masses. The correlation of mass with Teff remains unexplained, but another possible correlation of helium layer masses with Teff as well as the gravitational redshifts casts doubt on the reality of both and suggests possible shortcomings of current models. △ Less

Submitted 24 February, 2020; originally announced February 2020.

Comments: accepted by Astronomy & Astrophysics

Evidence of Spectral Evolution on the white dwarf sample from the Gaia Mission

Authors: G. Ourique, S. O. Kepler, A. D. Romero, T. S. Klippel, D. Koester

Abstract: Since the Gaia data release 2, several works were published describing a bifurcation in the observed white dwarf colour$-$magnitude diagram for $\mbox{$G_{\mathrm{BP}}$}-\mbox{$G_{\mathrm{RP}}$} > 0$. Some possible explanations in the literature include the existence of a double population with different initial mass function or two distinct populations, one formed by hydrogen$-$ and one formed by… ▽ More Since the Gaia data release 2, several works were published describing a bifurcation in the observed white dwarf colour$-$magnitude diagram for $\mbox{$G_{\mathrm{BP}}$}-\mbox{$G_{\mathrm{RP}}$} > 0$. Some possible explanations in the literature include the existence of a double population with different initial mass function or two distinct populations, one formed by hydrogen$-$ and one formed by helium$-$envelope white dwarfs. We propose instead spectral evolution to explain the bifurcation. From a population synthesis approach, we find that the spectral evolution occurs for effective temperature below ${\simeq}11\,000\,\mathrm{K}$ and masses mainly between $0.64\,\mathrm{M}_\odot$ and $0.74\,\mathrm{M}_\odot$, which correspond to around $16$ per cent of all DA white dwarfs. We also find the Gaia white dwarf colour-magnitude diagram indicates a star formation history that decreases abruptly for objects younger than $1.4\,\mathrm{Gyr}$ and a top-heavy initial mass function for the white dwarf progenitors. △ Less

Submitted 13 January, 2020; originally announced January 2020.

Comments: 9 pages, 9 figures, accepted by MNRAS

On the existence of warm H-rich pulsating white dwarfs

Authors: Leandro G. Althaus, Alejandro H. Córsico, Murat Uzundag, Maja Vučković, Andrzej S. Baran, Keaton J. Bell, María E. Camisassa, Leila M. Calcaferro, Francisco C. De Gerónimo, S. O. Kepler, Roberto Silvotti

Abstract: The possible existence of warm ($T_{\rm eff}\sim19\,000$ K) pulsating DA white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical studies more than 30 yr ago. However, to date, no pulsating warm DA WD has been discovered. We re-examine the pulsational predictions for such WDs on the basis of new full evolutionary sequences. We analyze all the warm DAs observed by TESS satell… ▽ More The possible existence of warm ($T_{\rm eff}\sim19\,000$ K) pulsating DA white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical studies more than 30 yr ago. However, to date, no pulsating warm DA WD has been discovered. We re-examine the pulsational predictions for such WDs on the basis of new full evolutionary sequences. We analyze all the warm DAs observed by TESS satellite up to Sector 9 in order to search for the possible pulsational signal. We compute WD evolutionary sequences with H content in the range $-14.5 \lesssim \log(M_{\rm H}/M_{\star}) \lesssim -10$, appropriate for the study of warm DA WDs. We use a new full-implicit treatment of time-dependent element diffusion. Non-adiabatic pulsations were computed in the effective temperature range of $30\,000-10\,000$ K, focusing on $\ell= 1$ $g$ modes with periods in the range $50-1500$ s. We find that extended He/H transition zones inhibit the excitation of $g$ modes due to partial ionization of He below the H envelope, and only in the case that the H/He transition is assumed much more abrupt, models do exhibit pulsational instability. In this case, instabilities are found only in WD models with H envelopes in the range of $-14.5 \lesssim \log(M_{\rm H}/M_{\star}) \lesssim -10$ and at effective temperatures higher than those typical of ZZ Ceti stars, in agreement with previous studies. None of the 36 warm DAs observed so far by TESS satellite are found to pulsate. Our study suggests that the non-detection of pulsating warm DAs, if WDs with very thin H envelopes do exist, could be attributed to the presence of a smooth and extended H/He transition zone. This could be considered as an indirect proof that element diffusion indeed operates in the interior of WDs. △ Less

Submitted 6 November, 2019; originally announced November 2019.

Comments: 13 pages, 15 figures, 2 tables. Accepted for publication in Astronomy and Astrophysics

Journal ref: A&A 633, A20 (2020)

Ground based observation of ZZ Ceti stars and the discovery of four new variables

Authors: Alejandra D. Romero, L. Antunes Amaral, T. Klippel, D. Sanmartim, L Fraga, G. Ourique, I. Pelisoli, G. R. Lauffer, S. O. Kepler, D. Koester

Abstract: We perform ground based photometric observations of 22 DA white dwarf stars, 10 already known ZZ Cetis and 12 candidates with atmospheric parameters inside the classical instability strip. We report on the discovery of four new variable DA white dwarf stars. Two objects are near the middle of the instability strip, SDSS J082804.63+094956.6 and SDSS J094929.09+101918.8, and two red edge pulsators,… ▽ More We perform ground based photometric observations of 22 DA white dwarf stars, 10 already known ZZ Cetis and 12 candidates with atmospheric parameters inside the classical instability strip. We report on the discovery of four new variable DA white dwarf stars. Two objects are near the middle of the instability strip, SDSS J082804.63+094956.6 and SDSS J094929.09+101918.8, and two red edge pulsators, GD 195 and L495-82. In addition, we classified four objects as possible variables, since evidence of variability was detected in the light curve, but the S/N was not sufficient to establish a definite detection. Follow-up observations were performed for 10 know ZZ Ceti stars to verify period stability and search for new periodicities. For each confirmed variable, we perform a detailed asteroseismological fit and compare the structural parameters obtained from the best fit models with those obtained from spectroscopy and photometry from Gaia. Finally we present a study of the asteroseismological properties of a sample of 91 ZZ Ceti stars. △ Less

Submitted 12 September, 2019; originally announced September 2019.

Comments: 18 pages, 12 figures. Accepted for publication in MNRAS

Pulsating white dwarfs: new insights

Authors: Alejandro H. Córsico, Leandro G. Althaus, Marcelo M. Miller Bertolami, S. O. Kepler

Abstract: White dwarf stars constitute the final evolutionary stage for more than 95 per cent of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components ---including the star formation history of the Milky Way. In addition, white d… ▽ More White dwarf stars constitute the final evolutionary stage for more than 95 per cent of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components ---including the star formation history of the Milky Way. In addition, white dwarfs are tracers of the evolution of planetary systems along several phases of stellar evolution. Also, white dwarfs are used as laboratories for astro-particle physics, being their interest focused on physics beyond the standard model. The last decade has witnessed a great progress in the study of white dwarfs. In particular, a wealth of information of these stars from different surveys has allowed us to make meaningful comparison of evolutionary models with observations. While some information like surface chemical composition, temperature and gravity of isolated white dwarfs can be inferred from spectroscopy, and the total mass and radius can be derived as well when they are in binaries, the internal structure of these compact stars can be unveiled only by means of asteroseismology, an approach based on the comparison between the observed pulsation periods of variable stars and the periods predicted by appropriate theoretical models. The asteroseismological techniques allow us to infer details of the internal chemical stratification, the total mass, and even the stellar rotation profile. In this review, we first revise the evolutionary channels currently accepted that lead to the formation of white-dwarf stars, and then, we give a detailed account of the different sub-types of pulsating white dwarfs known so far, emphasizing the recent observational and theoretical advancements in the study of these fascinating variable stars. △ Less

Submitted 28 June, 2019; originally announced July 2019.

Comments: 104 pages, 25 figures, 14 tables. To be published in The Astronomy and Astrophysics Review

Carbon-rich (DQ) white dwarfs in the Sloan Digital Sky Survey

Authors: Detlev Koester, S. O. Kepler

Abstract: Among the spectroscopically identified white dwarfs, a fraction smaller than 2% have spectra dominated by carbon lines, mainly molecular C2, but also in a smaller group by CI and CII lines. These are together called DQ white dwarfs. We want to derive atmospheric parameters Teff,log g, and carbon abundances for a large sample of these stars and discuss implications for their spectral evolution. Slo… ▽ More Among the spectroscopically identified white dwarfs, a fraction smaller than 2% have spectra dominated by carbon lines, mainly molecular C2, but also in a smaller group by CI and CII lines. These are together called DQ white dwarfs. We want to derive atmospheric parameters Teff,log g, and carbon abundances for a large sample of these stars and discuss implications for their spectral evolution. Sloan Digital Sky Survey spectra and ugriz photometry were used, together with GAIA Data Release 2 parallaxes and G band photometry. These were fitted to synthetic spectra and theoretical photometry derived from model atmospheres. We found that the DQs hotter than Teff ~10000 K have masses ~0.4 Msun larger than the cooler ones, which have masses typical for the majority of white dwarfs, ~0.6 Msun. A significant fraction of the hotter objects with Teff > 14500 K have atmospheres dominated by carbon. △ Less

Submitted 16 July, 2019; v1 submitted 27 May, 2019; originally announced May 2019.

Comments: Accepted by Astronomy & Astrophysics

White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14

Authors: S. O. Kepler, Ingrid Pelisoli, Detlev Koester, Nicole Reindl, Stephan Geier, Alejandra D. Romero, Gustavo Ourique, Cristiane de Paula Oliveira, Larissa A. Amaral

Abstract: White dwarfs carry information on the structure and evolution of the Galaxy, especially through their luminosity function and initial-to-final mass relation. Very cool white dwarfs provide insight into the early ages of each population. Examining the spectra of all stars with $3σ$ proper motion in the Sloan Digital Sky Survey Data Release 14, we report the classification for 20 088 spectroscopical… ▽ More White dwarfs carry information on the structure and evolution of the Galaxy, especially through their luminosity function and initial-to-final mass relation. Very cool white dwarfs provide insight into the early ages of each population. Examining the spectra of all stars with $3σ$ proper motion in the Sloan Digital Sky Survey Data Release 14, we report the classification for 20 088 spectroscopically confirmed white dwarfs, plus 415 hot subdwarfs, and 311 cataclysmic variables. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs), warm helium atmosphere white dwarfs (DBs), hot subdwarfs (sdBs and sdOs), and estimate photometric Teff for white dwarf stars with continuum spectra (DCs). We find 15793 sdAs and 447 dCs between the white dwarf cooling sequence and the main sequence, especially below Teff= 10000 K; most are likely low-mass metal-poor main sequence stars, but some could be the result of interacting binary evolution. △ Less

Submitted 13 April, 2019; v1 submitted 2 April, 2019; originally announced April 2019.

Comments: 18 pages, 13 figures

The white dwarf mass-radius relation and its dependence on the hydrogen envelope

Authors: Alejandra D. Romero, S. O. Kepler, S. R. G. Joyce, G. R. Lauffer, A. H. Córsico

Abstract: We present a study of the dependence of the mass-radius relation for DA white dwarf stars on the hydrogen envelope mass and the impact on the value of log g, and thus the determination of the stellar mass. We employ a set of full evolutionary carbon-oxygen core white dwarf sequences with white dwarf mass between 0.493 and 1.05 Msun. Computations of the pre-white dwarf evolution uncovers an intrins… ▽ More We present a study of the dependence of the mass-radius relation for DA white dwarf stars on the hydrogen envelope mass and the impact on the value of log g, and thus the determination of the stellar mass. We employ a set of full evolutionary carbon-oxygen core white dwarf sequences with white dwarf mass between 0.493 and 1.05 Msun. Computations of the pre-white dwarf evolution uncovers an intrinsic dependence of the maximum mass of the hydrogen envelope with stellar mass, i.e., it decreases when the total mass increases. We find that a reduction of the hydrogen envelope mass can lead to a reduction in the radius of the model of up to ~12%. This translates directly into an increase in log g for a fixed stellar mass, that can reach up to 0.11 dex, mainly overestimating the determinations of stellar mass from atmospheric parameters. Finally, we find a good agreement between the results from the theoretical mass-radius relation and observations from white dwarfs in binary systems. In particular, we find a thin hydrogen mass of MH ~ 2 10^-8 Msun, for 40 Eridani B, in agreement with previous determinations. For Sirius B, the spectroscopic mass is 4.3% lower than the dynamical mass. However, the values of mass and radius from gravitational redshift observations are compatible with the theoretical mass-radius relation for a thick hydrogen envelope of MH = 2 10^-6 Msun. △ Less

Submitted 2 April, 2019; v1 submitted 14 January, 2019; originally announced January 2019.

Comments: Accepted for publication in MNRAS

Journal ref: 2019MNRAS.484.2711R

Phase Transitions in Neutron Stars

Authors: V. Dexheimer, L. T. T. Soethe, J. Roark, R. O. Gomes, S. O. Kepler, S. Schramm

Abstract: In this paper we review the most common descriptions for the first order phase transition to deconfined quark matter in the core of neutron stars. We also present a new description of these phase transitions in the core of proto-neutron stars, in which more constraints are enforced so as to include trapped neutrinos. Finally, we calculate the emission of gravitational waves associated with deconfi… ▽ More In this paper we review the most common descriptions for the first order phase transition to deconfined quark matter in the core of neutron stars. We also present a new description of these phase transitions in the core of proto-neutron stars, in which more constraints are enforced so as to include trapped neutrinos. Finally, we calculate the emission of gravitational waves associated with deconfinement phase transitions, discuss the possibility of their detection, and how this would provide information about the equation of state of dense matter. △ Less

Submitted 10 January, 2019; originally announced January 2019.

Comments: Invited review for the International Journal of Modern Physics E

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences VI. Thin H-envelope sequences and asteroseismology of ELMV stars revisited

Authors: Leila M. Calcaferro, Alejandro H. Córsico, Leandro G. Althaus, Alejandra D. Romero, S. O. Kepler

Abstract: Some low-mass white-dwarf (LMWD) stars with H atmospheres show long-period g-mode pulsations, comprising the class of pulsating WDs called extremely low-mass variable (ELMV) stars. It is generally believed that these stars have thick H envelopes. However, from stellar evolution considerations, the existence of LMWDs with thin H envelopes is also possible. We present a thorough asteroseismological… ▽ More Some low-mass white-dwarf (LMWD) stars with H atmospheres show long-period g-mode pulsations, comprising the class of pulsating WDs called extremely low-mass variable (ELMV) stars. It is generally believed that these stars have thick H envelopes. However, from stellar evolution considerations, the existence of LMWDs with thin H envelopes is also possible. We present a thorough asteroseismological analysis of ELMV stars based on a complete set of fully evolutionary models that represents low-mass He-core WD stars harboring a range of H envelope thicknesses. Although there are currently nine ELMVs, here we only focus on those that exhibit more than three periods and whose periods do not show significant uncertainties. We considered g-mode adiabatic pulsation periods for low-mass He-core WD models with $M_*$ in the range [0.1554-0.4352]$M_{\odot}$, $T_{\rm eff}$ in the range [6000-10000]K, and H envelope thicknesses in the range -5.8<log($M_{\rm H}/M_*$)<-1.7. We explore the effects of employing different H-envelope thicknesses on the adiabatic pulsation properties of low-mass He-core WD models, and perform period-to-period fits to ELMVs to search for a representative model. We found that the mode-trapping effects of g modes depend sensitively on $M_{\rm H}$, with the trapping cycle and trapping amplitude larger for thinner H envelopes. Also, the asymptotic period spacing is longer for thinner H envelopes. Finally, we found asteroseismological models (when possible) characterized by canonical (thick) and by thin H envelope, with $T_{\rm eff}$ and $M_*$ in agreement with the spectroscopic determinations. The fact that we have found asteroseismological solutions with H envelopes thinner than canonical gives a clue of the possible scenario of formation of these stars. Indeed, in the light of our results, some of these stars could have been formed by binary evolution through unstable mass loss. △ Less

Submitted 26 October, 2018; originally announced October 2018.

Comments: 12 pages, 11 figures, 5 tables. Accepted for publication in Astronomy & Astrophysics. The full version of this work, with more analyzed stars, is available upon request

Journal ref: A&A 620, A196 (2018)

A Study of Cool White Dwarfs in the Sloan Digital Sky Survey Data Release 12

Authors: G. Ourique, A. D. Romero, S. O. Kepler, D. Koester, L. A. Amaral

Abstract: In this work we study white dwarfs where $30\,000\,\text{K} {>} \mathrm{T}_{\rm{eff}} {>} 5\,000\,\text{K}$ to compare the differences in the cooling of DAs and non-DAs and their formation channels. Our final sample is composed by nearly $13\,000$ DAs and more than $3\,000$ non-DAs that are simultaneously in the SDSS DR12 spectroscopic database and in the \textit{Gaia} survey DR2. We present the m… ▽ More In this work we study white dwarfs where $30\,000\,\text{K} {>} \mathrm{T}_{\rm{eff}} {>} 5\,000\,\text{K}$ to compare the differences in the cooling of DAs and non-DAs and their formation channels. Our final sample is composed by nearly $13\,000$ DAs and more than $3\,000$ non-DAs that are simultaneously in the SDSS DR12 spectroscopic database and in the \textit{Gaia} survey DR2. We present the mass distribution for DAs, DBs and DCs, where it is found that the DCs are ${\sim}0.15\,\mathrm{M}_\odot$ more massive than DAs and DBs on average. Also we present the photometric effective temperature distribution for each spectral type and the distance distribution for DAs and non-DAs. In addition, we study the ratio of non-DAs to DAs as a function of effective temperature. We find that this ratio is around ${\sim}0.075$ for effective temperature above ${\sim}22\,000\,\text{K}$ and increases by a factor of five for effective temperature cooler than $15\,000\,\text{K}$. If we assume that the increase of non-DA stars between ${\sim}22\,000\,\text{K}$ to ${\sim}15\,000\,\text{K}$ is due to convective dilution, $14{\pm}3$ per cent of the DAs should turn into non-DAs to explain the observed ratio. Our determination of the mass distribution of DCs also agrees with the theory that convective dilution and mixing are more likely to occur in massive white dwarfs, which supports evolutionary models and observations suggesting that higher mass white dwarfs have thinner hydrogen layers. △ Less

Submitted 9 October, 2018; v1 submitted 8 October, 2018; originally announced October 2018.

Comments: 9 pages, 10 figures, accepted by MNRAS