Showing 1–27 of 27 results for author: Preston, H

Picture an Astronomer: Best Practices for Retaining Talent in Astrophysics

Authors: Ava Polzin, Katherine E. Whitaker, C. Megan Urry, Henna Abunemeh, Sanyukta Agarwal, Aadya Agrawal, Nathaniel Alden, Ann-Marsha Alexis, Sydney Andersen, Melanie Archipley, Yasmeen Asali, Katie Auchettl, Bradford Benson, Binod Bhattarai, Sarah Biddle, Madison Brady, Katelyn Breivik, Disha Chakraborty, Mikel Charles, Hsiao-Wen Chen, Josephine Chishala, Anirudh Chiti, Panagiota Eleftheria Christopoulou, Mi Dai, Flaminia Fortuni , et al. (63 additional authors not shown)

Abstract: Women are consistently underrepresented in astrophysics yet are simultaneously subject to disproportionate attrition at every career stage. This disparity between demonstrated efficacy in job performance and ultimate career outcome was the primary motivation for the Picture an Astronomer series, which included both targeted public outreach to increase representation of women in astrophysics and hi… ▽ More Women are consistently underrepresented in astrophysics yet are simultaneously subject to disproportionate attrition at every career stage. This disparity between demonstrated efficacy in job performance and ultimate career outcome was the primary motivation for the Picture an Astronomer series, which included both targeted public outreach to increase representation of women in astrophysics and high-level, solution-oriented discussions among professional astronomers. In March 2025, more than 200 astronomers came together in a hybrid-format symposium focused on the state of the field for female scientists, combining scientific exchange with discussions of policies and practices to strengthen retention of talent in the field. This white paper is the result of those discussions, offering a wide range of recommendations developed in the context of gendered attrition in astrophysics but which ultimately support a healthier climate for all scientists alike. △ Less

Submitted 30 December, 2025; originally announced December 2025.

Comments: White paper from Picture an Astronomer symposium

Personalizing Prostate Cancer Education for Patients Using an EHR-Integrated LLM Agent

Authors: Yuexing Hao, Jason Holmes, Mark R. Waddle, Brian J. Davis, Nathan Y. Yu, Kristin Vickers, Heather Preston, Drew Margolin, Corinna E. Lockenhoff, Aditya Vashistha, Saleh Kalantari, Marzyeh Ghassemi, Wei Liu

Abstract: Cancer patients often lack timely education and personalized support due to clinician workload. This quality improvement study develops and evaluates a Large Language Model (LLM) agent, MedEduChat, which is integrated with the clinic's electronic health records (EHR) and designed to enhance prostate cancer patient education. Fifteen non-metastatic prostate cancer patients and three clinicians recr… ▽ More Cancer patients often lack timely education and personalized support due to clinician workload. This quality improvement study develops and evaluates a Large Language Model (LLM) agent, MedEduChat, which is integrated with the clinic's electronic health records (EHR) and designed to enhance prostate cancer patient education. Fifteen non-metastatic prostate cancer patients and three clinicians recruited from the Mayo Clinic interacted with the agent between May 2024 and April 2025. Findings showed that MedEduChat has a high usability score (UMUX 83.7 out of 100) and improves patients' health confidence (Health Confidence Score rose from 9.9 to 13.9). Clinicians evaluated the patient-chat interaction history and rated MedEduChat as highly correct (2.9 out of 3), complete (2.7 out of 3), and safe (2.7 out of 3), with moderate personalization (2.3 out of 3). This study highlights the potential of LLM agents to improve patient engagement and health education. △ Less

Submitted 17 November, 2025; v1 submitted 27 September, 2024; originally announced September 2024.

Journal ref: npj Digital Medicine 2025

IDEAS: Immersive Dome Experiences for Accelerating Science

Authors: Jacqueline K. Faherty, Mark SubbaRao, Ryan Wyatt, Anders Ynnerman, Neil deGrasse Tyson, Aaron Geller, Maria Weber, Philip Rosenfield, Wolfgang Steffen, Gabriel Stoeckle, Daniel Weiskopf, Marcus Magnor, Peter K. G. Williams, Brian Abbott, Lucia Marchetti, Thomas Jarrrett, Jonathan Fay, Joshua Peek, Or Graur, Patrick Durrell, Derek Homeier, Heather Preston, Thomas Müller, Johanna M Vos, David Brown , et al. (7 additional authors not shown)

Abstract: Astrophysics lies at the crossroads of big datasets (such as the Large Synoptic Survey Telescope and Gaia), open source software to visualize and interpret high dimensional datasets (such as Glue, WorldWide Telescope, and OpenSpace), and uniquely skilled software engineers who bridge data science and research fields. At the same time, more than 4,000 planetariums across the globe immerse millions… ▽ More Astrophysics lies at the crossroads of big datasets (such as the Large Synoptic Survey Telescope and Gaia), open source software to visualize and interpret high dimensional datasets (such as Glue, WorldWide Telescope, and OpenSpace), and uniquely skilled software engineers who bridge data science and research fields. At the same time, more than 4,000 planetariums across the globe immerse millions of visitors in scientific data. We have identified the potential for critical synergy across data, software, hardware, locations, and content that -- if prioritized over the next decade -- will drive discovery in astronomical research. Planetariums can and should be used for the advancement of scientific research. Current facilities such as the Hayden Planetarium in New York City, Adler Planetarium in Chicago, Morrison Planetarium in San Francisco, the Iziko Planetarium and Digital Dome Research Consortium in Cape Town, and Visualization Center C in Norrkoping are already developing software which ingests catalogs of astronomical and multi-disciplinary data critical for exploration research primarily for the purpose of creating scientific storylines for the general public. We propose a transformative model whereby scientists become the audience and explorers in planetariums, utilizing software for their own investigative purposes. In this manner, research benefits from the authentic and unique experience of data immersion contained in an environment bathed in context and equipped for collaboration. Consequently, in this white paper we argue that over the next decade the research astronomy community should partner with planetariums to create visualization-based research opportunities for the field. Realizing this vision will require new investments in software and human capital. △ Less

Submitted 21 September, 2020; v1 submitted 11 July, 2019; originally announced July 2019.

Comments: Astro2020 White Paper submission, 10 pages, 2 figures

Diffeomorphism-invariant averaging in quantum gravity and cosmology

Authors: Anthony W. H. Preston

Abstract: This thesis concerns research undertaken in two related topics concerning high-energy gravitational physics. The first is the construction of a manifestly diffeomorphism-invariant Exact Renormalization Group (ERG). This is a procedure that constructs effective theories of gravity by integrating out high-energy modes down to an ultraviolet cutoff scale without gauge-fixing. The manifest diffeomorph… ▽ More This thesis concerns research undertaken in two related topics concerning high-energy gravitational physics. The first is the construction of a manifestly diffeomorphism-invariant Exact Renormalization Group (ERG). This is a procedure that constructs effective theories of gravity by integrating out high-energy modes down to an ultraviolet cutoff scale without gauge-fixing. The manifest diffeomorphism invariance enables us to construct a fully background-independent formulation. This thesis will explore both the fixed-background and background-independent forms of the manifestly diffeomorphism-invariant ERG. The second topic is cosmological backreaction, which concerns the effect of averaging over high-frequency metric perturbations to the gravitational field equations describing the universe at large scales. This has been much studied the context of the unmodified form of General Relativity, but has been much less studied in the context of higher-derivative effective theories obtained by integrating out the high-energy modes of some more fundamental (quantum) theory of gravity. The effective stress-energy tensor for backreaction can be used directly as a diffeomorphism-invariant effective stress-energy tensor for gravitational waves without specifying the background metric. This thesis will construct the manifestly diffeomorphism-invariant ERG and compute the effective action at the classical level in two different schemes. We will then turn to cosmological backreaction in higher-derivative gravity, deriving the general form of the effective stress-energy tensor due to inhomogeneity for local diffeomorphism-invariant effective theories of gravity. This an exciting research direction, as it begins the construction of a quantum theory of gravity as well as investigating possible implications for cosmology. △ Less

Submitted 20 December, 2016; originally announced December 2016.

Comments: PhD thesis, 190 pages, 3 figures

Cosmological backreaction in higher-derivative gravity expansions

Authors: Anthony W. H. Preston

Abstract: We calculate a general effective stress-energy tensor induced by cosmological inhomogeneity in effective theories of gravity where the action is Taylor-expandable in the Riemann tensor and covariant derivatives of the Riemann tensor. This is of interest as an effective fluid that might provide an alternative to the cosmological constant, but it also applies to gravitational waves. We use an adapta… ▽ More We calculate a general effective stress-energy tensor induced by cosmological inhomogeneity in effective theories of gravity where the action is Taylor-expandable in the Riemann tensor and covariant derivatives of the Riemann tensor. This is of interest as an effective fluid that might provide an alternative to the cosmological constant, but it also applies to gravitational waves. We use an adaptation of Green and Wald's weak-averaging framework, which averages over perturbations in the field equation where the perturbation length scales are small compared to the averaging scale. In this adaptation, the length scale of the effective theory, $1/M$, is also taken to be small compared with the averaging scale. This ensures that the perturbation length scales remain in fixed proportion to the length scale of the effective theory as the cosmological averaging scale is taken to be large. We find that backreaction from higher-derivative terms in the effective action can continue to be important in the late universe, given a source of sufficiently high-frequency metric perturbations. This backreaction might also provide a window on exotic particle physics in the far ultraviolet. △ Less

Submitted 1 August, 2016; v1 submitted 19 May, 2016; originally announced May 2016.

Comments: 27 pages, 2 references added, minor clarifications made, comments added to introduction and discussion, some details moved to appendices

Journal ref: JCAP08(2016)038

Manifestly diffeomorphism invariant classical Exact Renormalization Group

Authors: Tim R. Morris, Anthony W. H. Preston

Abstract: We construct a manifestly diffeomorphism invariant Wilsonian (Exact) Renormalization Group for classical gravity, and begin the construction for quantum gravity. We demonstrate that the effective action can be computed without gauge fixing the diffeomorphism invariance, and also without introducing a background space-time. We compute classical contributions both within a background-independent fra… ▽ More We construct a manifestly diffeomorphism invariant Wilsonian (Exact) Renormalization Group for classical gravity, and begin the construction for quantum gravity. We demonstrate that the effective action can be computed without gauge fixing the diffeomorphism invariance, and also without introducing a background space-time. We compute classical contributions both within a background-independent framework and by perturbing around a fixed background, and verify that the results are equivalent. We derive the exact Ward identities for actions and kernels and verify consistency. We formulate two forms of the flow equation corresponding to the two choices of classical fixed-point: the Gaussian fixed point, and the scale invariant interacting fixed point using curvature-squared terms. We suggest how this programme may completed to a fully quantum construction. △ Less

Submitted 21 April, 2016; v1 submitted 29 February, 2016; originally announced February 2016.

Comments: 50 pages, 2 figures, minor typos fixed, minor additions to the Discussion, one reference added

Journal ref: JHEP 06 (2016) 012

Serendipitous Science from the K2 Mission

Authors: Derek L. Buzasi, Lindsey Carboneau, Carly Hessler, Andy Lezcano, Heather Preston

Abstract: The K2 mission is a repurposed use of the Kepler spacecraft to perform high-precision photometry of selected fields in the ecliptic. We have developed an aperture photometry pipeline for K2 data which performs dynamic automated aperture mask selection, background estimation and subtraction, and positional decorrelation to minimize the effects of spacecraft pointing jitter. We also identify seconda… ▽ More The K2 mission is a repurposed use of the Kepler spacecraft to perform high-precision photometry of selected fields in the ecliptic. We have developed an aperture photometry pipeline for K2 data which performs dynamic automated aperture mask selection, background estimation and subtraction, and positional decorrelation to minimize the effects of spacecraft pointing jitter. We also identify secondary targets in the K2 "postage stamps" and produce light curves for those targets as well. Pipeline results will be made available to the community. Here we describe our pipeline and the photometric precision we are capable of achieving with K2, and illustrate its utility with asteroseismic results from the serendipitous secondary targets. △ Less

Submitted 29 November, 2015; originally announced November 2015.

Comments: 6 pages, 9 figures To appear in IAU Focus Meeting 17, "Advances in Stellar Physics from Asteroseismology", ed. Piero Benvenuti

Low-energy V t2g orbital excitations in NdVO3

Authors: J. Laverock, B. Chen, A. R. H. Preston, D. Newby Jr., L. F. J. Piper, L. D. Tung, G. Balakrishnan, P. -A. Glans, J. -H. Guo, K. E. Smith

Abstract: The electronic structure of NdVO3, YVO3 has been investigated as a function of sample temperature using resonant inelastic soft x-ray scattering at the V L3-edge. Most of the observed spectral features are in good agreement with an atomic crystal-field multiplet model. However, a low energy feature is observed at ~0.4 eV that cannot be explained by crystal-field arguments. The resonant behaviour o… ▽ More The electronic structure of NdVO3, YVO3 has been investigated as a function of sample temperature using resonant inelastic soft x-ray scattering at the V L3-edge. Most of the observed spectral features are in good agreement with an atomic crystal-field multiplet model. However, a low energy feature is observed at ~0.4 eV that cannot be explained by crystal-field arguments. The resonant behaviour of this feature establishes it as due to excitations of the V t2g states. Moreover, this feature exhibits a strong sample temperature dependence, reaching maximum intensity in the orbitally-ordered phase of NdVO3, before becoming suppressed at low temperatures. This behaviour indicates that the origin of this feature is a collective orbital excitation, i.e. the bi-orbiton. △ Less

Submitted 17 November, 2014; originally announced November 2014.

Journal ref: J. Phys.: Condens. Matter 26, 455603 (2014)

Cosmological back-reaction in modified gravity and its implications for dark energy

Authors: Anthony W. H. Preston, Tim R. Morris

Abstract: We study the effective stress-energy tensor induced by cosmological inhomogeneity in $f(R)=R+cR^2$ and equivalent scalar-tensor theories, motivated both by models of early universe inflation and by phenomenological alternative cosmologies to the standard $Λ$-CDM. We use Green and Wald's framework for averaging over classical fluctuations of short-wavelength $λ$. By ensuring that the leading non-li… ▽ More We study the effective stress-energy tensor induced by cosmological inhomogeneity in $f(R)=R+cR^2$ and equivalent scalar-tensor theories, motivated both by models of early universe inflation and by phenomenological alternative cosmologies to the standard $Λ$-CDM. We use Green and Wald's framework for averaging over classical fluctuations of short-wavelength $λ$. By ensuring that the leading non-linear terms from the fluctuations of the Einstein terms and the corrections both contribute in the formal limit as $λ\to0$, we derive a diffeomorphism invariant effective stress-energy tensor whose trace is non-vanishing and of the right sign to potentially account for the current acceleration of the universe. However a more phenomenologically acceptable dark energy model would be required if this effect were to fully account for the current acceleration. △ Less

Submitted 11 August, 2014; v1 submitted 20 June, 2014; originally announced June 2014.

Comments: 23 pages, references updated, some arguments clarified

Journal ref: JCAP09(2014)017

Electronic structure of the kagome staircase compounds Ni3V2O8 and Co3V2O8

Authors: J. Laverock, B. Chen, A. R. H. Preston, K. E. Smith, N. R. Wilson, G. Balakrishnan, P. -A. Glans, J. -H. Guo

Abstract: The electronic structure of the kagome staircase compounds, Ni3V2O8 and Co3V2O8, has been investigated using soft x-ray absorption, soft x-ray emission, and resonant inelastic x-ray scattering (RIXS). Comparison between the two compounds, and with first principles band structure calculations and crystal-field multiplet models, provide unique insight into the electronic structure of the two materia… ▽ More The electronic structure of the kagome staircase compounds, Ni3V2O8 and Co3V2O8, has been investigated using soft x-ray absorption, soft x-ray emission, and resonant inelastic x-ray scattering (RIXS). Comparison between the two compounds, and with first principles band structure calculations and crystal-field multiplet models, provide unique insight into the electronic structure of the two materials. Whereas the location of the narrow (Ni,Co) d bands is predicted to be close to EF, we experimentally find they lie deeper in the occupied O 2p and unoccupied V 3d manifolds, and determine their energy via measured charge-transfer excitations. Additionally, we find evidence for a dd excitation at 1.5 eV in Ni3V2O8, suggesting the V d states may be weakly occupied in this compound, contrary to Co3V2O8. Good agreement is found between the crystal-field dd excitations observed in the experiment and predicted by atomic multiplet theory. △ Less

Submitted 1 May, 2013; originally announced May 2013.

Journal ref: Phys. Rev. B 87, 125133 (2013)

Photoemission evidence for crossover from Peierls-like to Mott-like transition in highly strained VO$_2$

Authors: J. Laverock, A. R. H. Preston, D. Newby Jr, K. E. Smith, S. Sallis, L. F. J. Piper, S. Kittiwatanakul, J. W. Lu, S. A. Wolf, M. Leandersson, T. Balasubramanian

Abstract: We present a spectroscopic study that reveals that the metal-insulator transition of strained VO$_2$ thin films may be driven towards a purely electronic transition, which does not rely on the Peierls dimerization, by the application of mechanical strain. Comparison with a moderately strained system, which does involve the lattice, demonstrates the crossover from Peierls- to Mott-like transitions. We present a spectroscopic study that reveals that the metal-insulator transition of strained VO$_2$ thin films may be driven towards a purely electronic transition, which does not rely on the Peierls dimerization, by the application of mechanical strain. Comparison with a moderately strained system, which does involve the lattice, demonstrates the crossover from Peierls- to Mott-like transitions. △ Less

Submitted 16 November, 2012; originally announced November 2012.

Journal ref: Phys. Rev. B 86, 195124 (2012)

Strain dependence of bonding and hybridization across the metal-insulator transition of VO2

Authors: J. Laverock, L. F. J. Piper, A. R. H. Preston, B. Chen, J. McNulty, K. E. Smith, S. Kittiwatanakul, J. W. Lu, S. A. Wolf, P. -A. Glans, J. -H. Guo

Abstract: Soft x-ray spectroscopy is used to investigate the strain dependence of the metal-insulator transition of VO2. Changes in the strength of the V 3d - O 2p hybridization are observed across the transition, and are linked to the structural distortion. Furthermore, although the V-V dimerization is well-described by dynamical mean-field theory, the V-O hybridization is found to have an unexpectedly str… ▽ More Soft x-ray spectroscopy is used to investigate the strain dependence of the metal-insulator transition of VO2. Changes in the strength of the V 3d - O 2p hybridization are observed across the transition, and are linked to the structural distortion. Furthermore, although the V-V dimerization is well-described by dynamical mean-field theory, the V-O hybridization is found to have an unexpectedly strong dependence on strain that is not predicted by band theory, emphasizing the relevance of the O ion to the physics of VO2. △ Less

Submitted 10 February, 2012; originally announced February 2012.

Journal ref: Phys. Rev. B 85, 081104(R) (2012)

Maximum entropy deconvolution of resonant inelastic x-ray scattering spectra

Authors: J. Laverock, A. R. H. Preston, D. Newby Jr, K. E. Smith, S. B. Dugdale

Abstract: Resonant inelastic x-ray scattering (RIXS) has become a powerful tool in the study of the electronic structure of condensed matter. Although the linewidths of many RIXS features are narrow, the experimental broadening can often hamper the identification of spectral features. Here, we show that the Maximum Entropy technique can successfully be applied in the deconvolution of RIXS spectra, improving… ▽ More Resonant inelastic x-ray scattering (RIXS) has become a powerful tool in the study of the electronic structure of condensed matter. Although the linewidths of many RIXS features are narrow, the experimental broadening can often hamper the identification of spectral features. Here, we show that the Maximum Entropy technique can successfully be applied in the deconvolution of RIXS spectra, improving the interpretation of the loss features without a severe increase in the noise ratio. △ Less

Submitted 10 February, 2012; v1 submitted 20 July, 2011; originally announced July 2011.

Journal ref: Phys. Rev. B 84, 235111 (2011)

Asteroseismology of red giants from the first four months of Kepler data: Global oscillation parameters for 800 stars

Authors: D. Huber, T. R. Bedding, D. Stello, B. Mosser, S. Mathur, T. Kallinger, S. Hekker, Y. P. Elsworth, D. L. Buzasi, J. De Ridder, R. L. Gilliland, H. Kjeldsen, W. J. Chaplin, R. A. Garcia, S. J. Hale, H. L. Preston, T. R. White, W. J. Borucki, J. Christensen-Dalsgaard, B. D. Clarke, J. M. Jenkins, D. Koch

Abstract: We have studied solar-like oscillations in ~800 red-giant stars using Kepler long-cadence photometry. The sample includes stars ranging in evolution from the lower part of the red-giant branch to the Helium main sequence. We investigate the relation between the large frequency separation (Delta nu) and the frequency of maximum power (nu_max) and show that it is different for red giants than for ma… ▽ More We have studied solar-like oscillations in ~800 red-giant stars using Kepler long-cadence photometry. The sample includes stars ranging in evolution from the lower part of the red-giant branch to the Helium main sequence. We investigate the relation between the large frequency separation (Delta nu) and the frequency of maximum power (nu_max) and show that it is different for red giants than for main-sequence stars, which is consistent with evolutionary models and scaling relations. The distributions of nu_max and Delta nu are in qualitative agreement with a simple stellar population model of the Kepler field, including the first evidence for a secondary clump population characterized by M ~> 2 M_sun and nu_max ~ 40-110 muHz. We measured the small frequency separations delta nu_02 and delta nu_01 in over 400 stars and delta nu_03 in over 40. We present C-D diagrams for l=1, 2 and 3 and show that the frequency separation ratios delta nu_02/Delta nu and delta nu_01/Delta nu have opposite trends as a function of Delta nu. The data show a narrowing of the l=1 ridge towards lower nu_max, in agreement with models predicting more efficient mode trapping in stars with higher luminosity. We investigate the offset epsilon in the asymptotic relation and find a clear correlation with Delta nu, demonstrating that it is related to fundamental stellar parameters. Finally, we present the first amplitude-nu_max relation for Kepler red giants. We observe a lack of low-amplitude stars for nu_max ~> 110 muHz and find that, for a given nu_max between 40-110 muHz, stars with lower Delta nu (and consequently higher mass) tend to show lower amplitudes than stars with higher Delta nu. △ Less

Submitted 21 October, 2010; originally announced October 2010.

Comments: 12 pages, 12 figures, accepted for publication in ApJ

Solar-like oscillations in red giants observed with Kepler: comparison of global oscillation parameters from different methods

Authors: S. Hekker, Y. Elsworth, J. De Ridder, B. Mosser, R. A. Garcia, T. Kallinger, S. Mathur, D. Huber, D. L. Buzasi, H. L. Preston, S. J. Hale, J. Ballot, W. J. Chaplin, C. Regulo, T. R. Bedding, D. Stello, W. J. Borucki, D. G. Koch, J. Jenkins, C. Allen, R. L. Gilliland, H. Kjeldsen, J. Christensen-Dalsgaard

Abstract: The large number of stars for which uninterrupted high-precision photometric timeseries data are being collected with \textit{Kepler} and CoRoT initiated the development of automated methods to analyse the stochastically excited oscillations in main-sequence, subgiant and red-giant stars. Aims: We investigate the differences in results for global oscillation parameters of G and K red-giant stars d… ▽ More The large number of stars for which uninterrupted high-precision photometric timeseries data are being collected with \textit{Kepler} and CoRoT initiated the development of automated methods to analyse the stochastically excited oscillations in main-sequence, subgiant and red-giant stars. Aims: We investigate the differences in results for global oscillation parameters of G and K red-giant stars due to different methods and definitions. We also investigate uncertainties originating from the stochastic nature of the oscillations. Methods: For this investigation we use Kepler data obtained during the first four months of operation. These data have been analysed by different groups using already published methods and the results are compared. We also performed simulations to investigate the uncertainty on the resulting parameters due to different realizations of the stochastic signal. Results: We obtain results for the frequency of maximum oscillation power (nu_max) and the mean large separation (<delta nu>) from different methods for over one thousand red-giant stars. The results for these parameters agree within a few percent and seem therefore robust to the different analysis methods and definitions used here. The uncertainties for nu_max and <delta nu> due to differences in realization noise are not negligible and should be taken into account when using these results for stellar modelling. △ Less

Submitted 17 August, 2010; originally announced August 2010.

Comments: 11 pages, 9 Figures and 7 tables, accepted for publication in Astronomy and Astrophysics

The asteroseismic potential of Kepler: first results for solar-type stars

Authors: W. J. Chaplin, T. Appourchaux, Y. Elsworth, R. A. Garcia, G. Houdek, C. Karoff, T. S. Metcalfe, J. Molenda-Zakowicz, M. J. P. F. G. Monteiro, M. J. Thompson, T. M. Brown, J. Christensen-Dalsgaard, R. L. Gilliland, H. Kjeldsen, W. J. Borucki, D. Koch, J. M. Jenkins, J. Ballot, S. Basu, M. Bazot, T. R. Bedding, O. Benomar, A. Bonanno, I. M. Brandao, H. Bruntt , et al. (83 additional authors not shown)

Abstract: We present preliminary asteroseismic results from Kepler on three G-type stars. The observations, made at one-minute cadence during the first 33.5d of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: About 20 modes of oscillation may be clearly distinguished in each star. We discuss the appearance of the oscillation spectra, use the frequencies a… ▽ More We present preliminary asteroseismic results from Kepler on three G-type stars. The observations, made at one-minute cadence during the first 33.5d of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: About 20 modes of oscillation may be clearly distinguished in each star. We discuss the appearance of the oscillation spectra, use the frequencies and frequency separations to provide first results on the radii, masses and ages of the stars, and comment in the light of these results on prospects for inference on other solar-type stars that Kepler will observe. △ Less

Submitted 18 January, 2010; v1 submitted 4 January, 2010; originally announced January 2010.

Comments: 16 pages, 3 figures, 1 table; now accepted for publication in ApJ Letters

Solar-like oscillations in low-luminosity red giants: first results from Kepler

Authors: T. R. Bedding, D. Huber, D. Stello, Y. P. Elsworth, S. Hekker, T. Kallinger, S. Mathur, B. Mosser, H. L. Preston, J. Ballot, C. Barban, A. M. Broomhall, D. L. Buzasi, W. J. Chaplin, R. A. Garcia, M. Gruberbauer, S. J. Hale, J. De Ridder, S. Frandsen, W. J. Borucki, T. Brown, J. Christensen-Dalsgaard, R. L. Gilliland, J. M. Jenkins, H. Kjeldsen , et al. (25 additional authors not shown)

Abstract: We have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30-minute sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the larg… ▽ More We have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30-minute sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the large separation of the oscillations (Delta nu) and the frequency of maximum power (nu_max). We focus on a sample of 50 low-luminosity stars (nu_max > 100 muHz, L <~ 30 L_sun) having high signal-to-noise ratios and showing the unambiguous signature of solar-like oscillations. These are H-shell-burning stars, whose oscillations should be valuable for testing models of stellar evolution and for constraining the star-formation rate in the local disk. We use a new technique to compare stars on a single echelle diagram by scaling their frequencies and find well-defined ridges corresponding to radial and non-radial oscillations, including clear evidence for modes with angular degree l=3. Measuring the small separation between l=0 and l=2 allows us to plot the so-called C-D diagram of delta nu_02 versus Delta nu. The small separation delta nu_01 of l=1 from the midpoint of adjacent l=0 modes is negative, contrary to the Sun and solar-type stars. The ridge for l=1 is notably broadened, which we attribute to mixed modes, confirming theoretical predictions for low-luminosity giants. Overall, the results demonstrate the tremendous potential of Kepler data for asteroseismology of red giants. △ Less

Submitted 22 January, 2010; v1 submitted 1 January, 2010; originally announced January 2010.

Comments: accepted by ApJ Letters, to appear in special Kepler issue. Updated references

Band structure of ZnO from resonant x-ray emission spectroscopy

Authors: A. R. H. Preston, B. J. Ruck, L. F. J. Piper, A. DeMasi, K. E. Smith, A. Schleife, F. Fuchs, F. Bechstedt, J. Chai, S. M. Durbin

Abstract: Soft x-ray emission and absorption spectroscopy of the O K-edge are employed to investigate the electronic structure of wurtzite ZnO(0001). A quasiparticle band structure calculated within the GW approximation agrees well with the data, most notably with the energetic location of the Zn3d - O2p hybridized state and the anisotropy of the absorption spectra. Dispersion in the band structure is map… ▽ More Soft x-ray emission and absorption spectroscopy of the O K-edge are employed to investigate the electronic structure of wurtzite ZnO(0001). A quasiparticle band structure calculated within the GW approximation agrees well with the data, most notably with the energetic location of the Zn3d - O2p hybridized state and the anisotropy of the absorption spectra. Dispersion in the band structure is mapped using the coherent k-selective part of the resonant x-ray emission spectra. We show that a more extensive mapping of the bands is possible in the case of crystalline anisotropy such as that found in ZnO. △ Less

Submitted 5 September, 2008; v1 submitted 29 May, 2008; originally announced May 2008.

Comments: 5 pages, 5 figures

Polaris the Cepheid returns: 4.5 years of monitoring from ground and space

Authors: H. Bruntt, N. R. Evans, D. Stello, A. J. Penny, J. A. Eaton, D. L. Buzasi, D. D. Sasselov, H. L. Preston, E. Miller-Ricci

Abstract: We present the analysis of 4.5 years of nearly continuous observations of the classical Cepheid Polaris, which comprise the most precise data available for this star. We have made spectroscopic measurements from ground and photometric measurements from the WIRE star tracker and the SMEI instrument on the Coriolis satellite. Measurements of the amplitude of the dominant oscillation (P = 4 days),… ▽ More We present the analysis of 4.5 years of nearly continuous observations of the classical Cepheid Polaris, which comprise the most precise data available for this star. We have made spectroscopic measurements from ground and photometric measurements from the WIRE star tracker and the SMEI instrument on the Coriolis satellite. Measurements of the amplitude of the dominant oscillation (P = 4 days), that go back more than a century, show a decrease from 120 mmag to 30 mmag (V magnitude) around the turn of the millennium. It has been speculated that the reason for the decrease in amplitude is the evolution of Polaris towards the edge of the instability strip. However, our new data reveal an increase in the amplitude by about 30% from 2003-2006. It now appears that the amplitude change is cyclic rather than monotonic, and most likely the result of a pulsation phenomenon. In addition, previous radial velocity campaigns have claimed the detection of long-period variation in Polaris (P > 40 days). Our radial velocity data are more precise than previous datasets, and we find no evidence for additional variation for periods in the range 3-50 days with an upper limit of 100 m/s. However, in the WIRE data we find evidence of variation on time-scales of 2-6 days, which we interpret as being due to granulation. △ Less

Submitted 22 April, 2008; originally announced April 2008.

Comments: Accepted by ApJ. 8 pages. Quality of figures 1+3+7 degraded

Near-Zero Moment Ferromagnetism in the Semiconductor SmN

Authors: C. Meyer, B. J. Ruck, J. Zhong, S. Granville, A. R. H. Preston, G. V. M. Williams, H. J. Trodahl

Abstract: The magnetic behaviour of SmN has been investigated in stoichiometric polycrystalline films. All samples show ferromagnetic order with Curie temperature (T_c) of 27 +/- 3 K, evidenced by the occurrence of hysteresis below T_c. The ferromagnetic state is characterised by a very small moment and a large coercive field, exceeding even the maximum applied field of 6 T below about 15 K. The residual… ▽ More The magnetic behaviour of SmN has been investigated in stoichiometric polycrystalline films. All samples show ferromagnetic order with Curie temperature (T_c) of 27 +/- 3 K, evidenced by the occurrence of hysteresis below T_c. The ferromagnetic state is characterised by a very small moment and a large coercive field, exceeding even the maximum applied field of 6 T below about 15 K. The residual magnetisation at 2 K, measured after cooling in the maximum field, is 0.035 mu_B per Sm. Such a remarkably small moment results from a near cancellation of the spin and orbital contributions for Sm3+ in SmN. Coupling to an applied field is therefore weak, explaining the huge coercive field . The susceptibility in the paramagnetic phase shows temperature-independent Van Vleck and Curie-Weiss contributions. The Van Vleck contribution is in quantitative agreement with the field-induced admixture of the J=7/2 excited state and the 5/2 ground state. The Curie-Weiss contribution returns a Curie temperature that agrees with the onset of ferromagnetic hysteresis, and a conventional paramagnetic moment with an effective moment of 0.4 mu_B per Sm ion, in agreement with expectations for the crystal-field modified effective moment on the Sm3+ ions. △ Less

Submitted 16 October, 2008; v1 submitted 9 April, 2008; originally announced April 2008.

Comments: 5 pages, 3 figures

Oscillating K giants with the WIRE satellite: determination of their asteroseismic masses

Authors: D. Stello, H. Bruntt, H. Preston, D. Buzasi

Abstract: Mass estimates of K giants are generally very uncertain. Traditionally, stellar masses of single field stars are determined by comparing their location in the Hertzsprung-Russell diagram with stellar evolutionary models. Applying an additional method to determine the mass is therefore of significant interest for understanding stellar evolution. We present the time series analysis of 11 K giants… ▽ More Mass estimates of K giants are generally very uncertain. Traditionally, stellar masses of single field stars are determined by comparing their location in the Hertzsprung-Russell diagram with stellar evolutionary models. Applying an additional method to determine the mass is therefore of significant interest for understanding stellar evolution. We present the time series analysis of 11 K giants recently observed with the WIRE satellite. With this comprehensive sample, we report the first confirmation that the characteristic acoustic frequency, nu_max, can be predicted for K giants by scaling from the solar acoustic cut-off frequency. We are further able to utilize our measurements of nu_max to determine an asteroseismic mass for each star with a lower uncertainty compared to the traditional method, for most stars in our sample. This indicates good prospects for the application of our method on the vast amounts of data that will soon come from the COROT and Kepler space missions. △ Less

Submitted 14 January, 2008; originally announced January 2008.

Comments: 4 pages, 3 figures. Accepted for publication in ApJL

Ferromagnetic redshift of the optical gap in GdN

Authors: H. J. Trodahl, A. R. H. Preston, J. Zhong, B. J. Ruck, N. Strickland, C. Mitra, W. R. L. Lambrecht

Abstract: We report measurements of the optical gap in a GdN film at temperatures from 300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is 1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split bands below the Curie temperature. The paramagnetic gap is larger than was suggested by very early experiments, and has permitted us to refine a (LSDA+U)-computed band… ▽ More We report measurements of the optical gap in a GdN film at temperatures from 300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is 1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split bands below the Curie temperature. The paramagnetic gap is larger than was suggested by very early experiments, and has permitted us to refine a (LSDA+U)-computed band structure. The band structure was computed in the full translation symmetry of the ferromagnetic ground state, assigning the paramagnetic-state gap as the average of the majority- and minority-spin gaps in the ferromagnetic state. That procedure has been further tested by a band structure in a 32-atom supercell with randomly-oriented spins. After fitting only the paramagnetic gap the refined band structure then reproduces our measured gaps in both phases by direct transitions at the X point. △ Less

Submitted 26 July, 2007; v1 submitted 21 May, 2007; originally announced May 2007.

Comments: 5 pages, 4 figures

Comparison between experiment and calculated band structures for DyN and SmN

Authors: A. R. H. Preston, S. Granville, D. H. Housden, B. Ludbrook, B. J. Ruck, H. J. Trodahl, A. Bittar, G. V. M. Williams, J. E. Downes, A. DeMasi, Y. Zhang, K. E. Smith, W. R. L. Lambrecht

Abstract: We investigate the electronic band structure of two of the rare-earth nitrides, DyN and SmN. Resistivity measurements imply that both materials have a semiconducting ground state, and both show resistivity anomalies coinciding with the magnetic transition, despite the different magnetic states in DyN and SmN. X-ray absorption and emission measurements are in excellent agreement with LSDA+U calcu… ▽ More We investigate the electronic band structure of two of the rare-earth nitrides, DyN and SmN. Resistivity measurements imply that both materials have a semiconducting ground state, and both show resistivity anomalies coinciding with the magnetic transition, despite the different magnetic states in DyN and SmN. X-ray absorption and emission measurements are in excellent agreement with LSDA+U calculations, although for SmN the calculations predict a zero band gap. △ Less

Submitted 19 March, 2008; v1 submitted 28 March, 2007; originally announced March 2007.

Comments: 2 figures

Journal ref: Physical Review B 76, 245120 (2007)

Gravitational Lensing by Galaxy Quantization States

Authors: Franklin Potter, Howard G. Preston

Abstract: We show how our theory of large-scale gravitational quantization explains the large angle gravitational lensing by galaxies without requiring "dark matter". A galaxy is treated as a collective system of billions of stars in each quantization state with each star experiencing an average gravitational environment analogous to that for nucleons in the atomic nucleus. Consequently, each star is in a… ▽ More We show how our theory of large-scale gravitational quantization explains the large angle gravitational lensing by galaxies without requiring "dark matter". A galaxy is treated as a collective system of billions of stars in each quantization state with each star experiencing an average gravitational environment analogous to that for nucleons in the atomic nucleus. Consequently, each star is in an approximate finite depth square well type of gravitational potential. The "effective potential" is shown to be about ten times greater than the Newtonian gravitational potential, so the gravitational lensing effects of a galaxy are about ten times greater also, in agreement with the measured gravitational lensing. △ Less

Submitted 5 May, 2004; originally announced May 2004.

Comments: 6 pages

Altair: The Brightest Delta Scuti Star

Authors: D. L. Buzasi, H. Bruntt, T. R. Bedding, A. Retter, H. Kjeldsen, H. L. Preston, W. J. Mandeville, J. Catanzarite, T. Conrow, R. Laher

Abstract: We present an analysis of observations of the bright star Altair (alpha Aql) obtained using the star camera on the Wide-Field Infrared Explorer (WIRE) satellite. Although Altair lies within the delta Scuti instability strip, previous observations have not revealed the presence of oscillations. However, the WIRE observations show Altair to be a low-amplitude (Delta m < 1 ppt) delta Scuti star wit… ▽ More We present an analysis of observations of the bright star Altair (alpha Aql) obtained using the star camera on the Wide-Field Infrared Explorer (WIRE) satellite. Although Altair lies within the delta Scuti instability strip, previous observations have not revealed the presence of oscillations. However, the WIRE observations show Altair to be a low-amplitude (Delta m < 1 ppt) delta Scuti star with at least 7 modes present. △ Less

Submitted 6 May, 2004; originally announced May 2004.

Comments: 15 pages, 6 figure files (only 5 figures), 2 tables

Journal ref: Astrophys.J. 619 (2005) 1072-1076

Stellar Coronal Spectroscopy with the Chandra HETGS

Authors: David P. Huenemoerder, Bram Boroson, Norbert S. Schulz, Claude R. Canizares, Derek L. Buzasi, Heather L. Preston, Joel H. Kastner

Abstract: Spectroscopy with the Chandra High Energy Transmission Grating Spectrometer provides details on X-ray emission and activity from young and cool stars through resolution of emission lines from a variety of ions. We are beginning to see trends in activity regarding abundances, emission measures, and variability. Here we contrast spectra of TV Crt, a weak-lined T Tauri star, with TW Hya, a Classica… ▽ More Spectroscopy with the Chandra High Energy Transmission Grating Spectrometer provides details on X-ray emission and activity from young and cool stars through resolution of emission lines from a variety of ions. We are beginning to see trends in activity regarding abundances, emission measures, and variability. Here we contrast spectra of TV Crt, a weak-lined T Tauri star, with TW Hya, a Classical T Tauri star. TV Crt has a spectrum more like magnetic activity driven coronae, relative to the TW Hya spectrum, which we have interpreted as due to accretion-produced X-rays. We have also observed the long period system, IM Pegasi to search for rotational modulation, and to compare activity in a long period active binary to shorter period systems and to the pre-main sequence stars. We detected no rotational modulation, but did see long-duration flares. △ Less

Submitted 11 October, 2003; originally announced October 2003.

Comments: 5 pages, 2 figures; to be published in IAU Symposium 219: "Stars as Suns: Activity, Evolution, Planets" (Ed. A. Dupree and A. O. Benz)

Exploring Large-scale Gravitational Quantization without h-bar in Planetary Systems, Galaxies, and the Universe

Authors: Howard G. Preston, Franklin Potter

Abstract: We explore a theory of large-scale gravitational quantization, using the general relativistic Hamilton-Jacobi equation to create quantization conditions via a new scalar wave equation dependent upon the total mass and the total vector angular momentum only. Instead of h-bar, a local invariant quantity proportional to the total angular momentum dictates the quantization conditions. In the Schwarz… ▽ More We explore a theory of large-scale gravitational quantization, using the general relativistic Hamilton-Jacobi equation to create quantization conditions via a new scalar wave equation dependent upon the total mass and the total vector angular momentum only. Instead of h-bar, a local invariant quantity proportional to the total angular momentum dictates the quantization conditions. In the Schwarzschild metric the theory predicts eigenstates with quantized energy per mass and angular momentum per mass. We find excellent agreement to the orbital spacings of the satellites of the Jovian planets and to the planet spacings in the Solar System. For galaxies we derive the baryonic Tully-Fisher relation and the MOND acceleration, so galaxy velocity curves are explained without requiring 'dark matter'. For the universe, we derive a new Hubble relation that accounts for the accelerated expansion with a matter density at about 5% of the critical matter/energy density, with the remainder being large-scale quantization zero-point energy. A possible laboratory test is proposed. △ Less

Submitted 28 March, 2003; originally announced March 2003.

Comments: 16 pages