The New EXor Outburst of ESO-H$\alpha$ 99 observed by Gaia ATLAS and TESSJournal: AJFacilities: ATLAS, FTN, Gaia, OCA:IRIS, IRTF, TESS, UKIRT

The Vela Molecular Ridge (VMR) is a massive molecular cloud complex straddling the border of Vela and Puppis along the Galactic plane between Galactic longitudes 259^∘ and 272^∘. It was first noted by Dame et al. 1987 and May, Murphy & Thaddeus 1988. Murphy & May 1991 identified four separate clouds, which they called A, B, C, and D. A detailed CO survey of the entire VMR was presented by Yamaguchi et al. 1999. The distance to the VMR is poorly known, over the years numerous estimates have been made to individual regions in the VMR, ranging from 0.7 kpc to 2 kpc (Pettersson 2008). The Gaia-DR2 parallax of ESO-H$\alpha$ 99 is “1.2518 $\pm$ 0.3176 mas” ($\approx$ 800 $\pm$ 300 pc), not a very precise value due to the faintness of this object. Here we adopt a newly determined distance to VMR-D of 1007$\pm$30 pc determined by Pettersson and Reipurth (in preparation) based on Gaia-DR2 parallax data of all H$\alpha$ emitting stars in this region. This new distance measurement places the VMR between the foreground Gum Nebula and the background Carina-Sagittarius arm (Pettersson 2008), producing a complex line-of-sight. In particular it should be noted that the VMR is seen behind the OB association Trumpler 10. Star formation is abundant all along the VMR as was pointed out by Liseau et al. 1992, Giannini et al. 2007, and Massi et al. 2007.

The northwestern part of the ridge is VMR-D, which is dominated by three HII regions, RCW 27, 32, and 33 (Rodgers, Campbell, & Whiteoak 1960). All of these regions are actively forming stars, which have been studied at multiple wavelengths, e.g., by Strafella et al. 2010; Strafella et al. 2015. It appears that RCW 27 is a particularly fertile star-forming region. Pettersson & Reipurth 1994 carried out a large survey for H$\alpha$ emission stars towards RCW 27, 32, and 33, many of which have subsequently been identified as T Tauri stars, e.g., by Prizinzano et al. 2018. One of these young emission-line stars is ESO-H$\alpha$ 99, the object of this paper. ESO-H$\alpha$ 99 was detected by IRAS and is the dominant flux source of IRAS 08370-4030. This star is partly embedded in the dense high-extinction cloud Sandqvist 1, which is seen against the RCW 27 HII region (Sandqvist & Lindroos 1976). Prominent bright rims are seen in the infrared Spitzer image (Figure 1, bottom panel), which are likely excited by the OB star in RCW 27. In contrast, no bright rims are seen in the red optical image (Figure 1 top panel) suggesting that we are seeing the Sandquist 1 cloud from the un-illuminated back side.

Figure 2 shows the spectral energy distribution (SED) of ESO-H$\alpha$ 99 based on ground-based and space-based survey data obtained from VizieR. The infrared and far-infrared data are from the compilation of catalog data by Abrahamyan et al. 2015 and include data from WISE, MSX, AKARI, and IRAS. The 2MASS data are from the point source catalog (Skrutskie et al. 2006). The photographic data points (DSS) are from the guide-star catalog of Lasker et al. 2008. Gaia data are from data release DR 1 and DR 2 (Gaia Collaboration et al. 2016; Gaia Collaboration et al. 2018). Other optical data are from Prizinzano et al. 2018. Note that the SED in Figure 2 does not include the IRAS 100 $\mu$m data point because of the low spatial resolution of that data point and likely possible contamination from neighboring far-infrared sources, the closest bright one being IRAS08367-4028.

The most important unsubstantiated assumption in this SED is that all the measurements refer to the quiescent state of the object. We do not have a well measured light curve of this object prior to the start of regular monitoring by Gaia and ATLAS to place any of the SED data points into the context of a light curve. It is quite possible that the substantial scatter in the flux measurements at optical wavelengths, well above the errors indicated in Figure 2, is due to variability. The SED is a typical “flat spectrum” distribution (Greene et al. 1994). The flat part of the SED extends from the near-infrared $K$ band at 2.15 $\mu$m out to 160 $\mu$m, characterizing ESO-H$\alpha$ 99 as a YSO in transition from a deeply embedded infrared object (Class I) to a moderately embedded classical T Tauri star. At optical and near-infrared wavelengths, the SED rises steeply, indicating substantial dust obscuration in the line of sight. The integral pre-outburst luminosity of ESO-H$\alpha$ 99 from optical wavelengths to the AKARI 160 $\mu$m data point, a lower limit to the bolometric luminosity since we do not have sub-mm data to complete the SED, is 34 $L_{\odot}$, adopting again the distance of 1007 pc (Pettersson & Reipurth, in preparation). This pre-outburst luminosity is high compared to most other T Tauri stars, which are thought to be the precursors of EXor and FUor outbursts, and instead is up in the range of some Herbig Ae/Be stars, specifically the “group 2”, the flat-spectrum objects as defined by Hillenbrand et al. 1992.

Deep infrared images were obtained in the $J$ and $H$ bands with the Wide-Field Camera (WFCAM) described by Casali et al. 2007 on UKIRT in March and April of 2019. The source reached $K_{s}\approx$ 8 mag near maximum brightness, leading to saturation in the $K$-band in the shortest integration times possible with WFCAM on UKIRT. We used the saturated UKIRT $K$-band images primarily to show the reflection nebula in Figure 3. We also used UKIRT for deep imaging in the $1-0~S(1)$ filter to search for shock-excited outflow features and the one emission knot that we found is indicated in Figure 3.

Figure 3 shows a $H$, $S(1)$, and $K$ RGB color composite image obtained with UKIRT. The star ESO-H$\alpha$ 99 is associated with a reflection nebula that outlines what appears to be the walls of an outflow cavity. Farther to the north, 72″ from the star at P.A. 14°, a single small patch of $S(1)$ emission with vaguely indicated bow-shock shape, lying roughly on the axis of the reflection nebula, strongly suggests a shock-excited bow shock in a well-collimated outflow. This feature is clearly seen in the UKIRT $S(1)$ images and was confirmed by $S(1)$ images with lower resolution from the IRIS telescope. The emission feature is also present on Digital Sky Survey red plates, but not on B and I plates. An association of this emission feature with ESO-H$\alpha$ 99 cannot be conclusively proven without proper motion data, but is strongly suggested based on its morphology and location on the symmetry axis of the reflection nebula. This shock front has now been included in the catalog of Molecular Hydrogen Objects (Davis et al. 2010) as MHO 1520. ²² 2 The MHO catalog is now hosted by D. Froebrich at the University of Kent, U.K. About 50$\arcsec$ to the south of ESO-H$\alpha$ 99 a separate reflection nebula and some $S(1)$ emission associated with the 2MASS source 08385506$-$4042062 are visible in Figure 3. 2MASS 08385506$-$4042062 is not visible at optical wavelengths, and is fainter than ESO-H$\alpha$ 99 at all wavelengths shown here, indicating relatively low luminosity. It contributes to the flux of the far-infrared source IRAS08370$-$4030 but is only a minor contribution at any wavelength.

Most of the optical light curve of ESO-H$\alpha$ 99 is based on archival data from the ATLAS project described by Tonry et al. 2018. Photometry in the ATLAS “Orange” ($O$) filter is shown in Figures 4 and 5. ATLAS usually takes more than one image of any given region of the sky in each suitable night to follow fast-moving asteroids. For the light curves in Figures 4 and 5, we have median-combined the individual measurements for each night when the photometric zero points and sky brightness were stable.

During its Cycle 1 Sector 8 observations, the Transiting Exoplanet Survey Satellite (TESS) described by Ricker et al. 2015 observed the ESO-H$\alpha$ 99 outburst from 2019-Feb-02 to 2019-Feb-28 with Camera 3. We have reduced these observations using an image subtraction pipeline optimized for TESS Full-Frame Images. This pipeline has previously been applied successfully to TESS observations of supernovae (Vallely et al. 2019; Fausnaugh et al. 2019) and of the tidal disruption event ASASSN-19bt (Holoien et al. 2019). Because the reference image was constructed from images containing a considerable amount of flux from the EXor, fluxes in the raw difference light curve are systematically lower than their intrinsic values. We correct for this discrepancy by scaling and shifting the differential flux levels to match four ATLAS photometry points obtained concurrently with the TESS observations. The TESS photometry, as well as these ATLAS observations, are shown in the lower panel of Figure 4. It covered the second interim minimum in the rise of ESO-H$\alpha$ 99 to preliminary brightness maximum.

We obtained $J$, $H$, and $K_{s}$ band imaging photometry of ESO-H$\alpha$ 99 with the IRIS 0.8m telescope (Hodapp et al. 2010) and 1024$\times$1024 2.5 $\mu$m IRIS infrared camera of the Universitätssternwarte Bochum on Cerro Armazones, Chile. In addition to the monitoring with IRIS, we have also obtained deeper images with better spatial resolution were obtained in the $J$ and $H$ bands with the Wide-Field Camera (WFCAM) as described above. The WFCAM filters conform to the “MKO” standard described by Tokunaga, Simons, & Vacca 2002 and further characterized in Tokunaga & Vacca 2005. Both the UKIRT and IRIS data were calibrated against the 2MASS point source catalog (Skrutskie et al. 2006) by the Cambridge Astronomical Survey Unit (CASU) using the procedures described by Hodgkin et al. 2009. The source reached $K_{s}\approx$ 8 mag near maximum brightness, leading to saturation in the $K$-band in the shortest integration times possible with WFCAM on UKIRT. In $J$ and $H$, both the UKIRT and IRIS photometry were used, while the $K_{s}$-band photometry is only based on IRIS data. The infrared photometry is shown as light curves in Figure 5 and is the basis for the color-color diagram in Figure 6

Infrared photometry only started in March of 2019 after the optical outburst had been noted by ATLAS. The $J$, $H$, and $K_{s}$ light curve essentially started at the maximum brightness and shows one interim minimum. The total outburst amplitude in the infrared can only be determined in comparison with the DENIS (Epchtein et al. 1997) and 2MASS (Skrutskie et al. 2006) catalog values: DENIS gives the following magnitudes: $I$=15.938, $J$=13.063, $K_{s}$=9.681 obtained on JD 2450099.72 and $I$=15.810, $J$=13.028, $K_{s}$=9.581 obtained on JD 2451207.69 while 2MASS gives $J$=13.099, $H$=11.085, and $K_{s}$=9.414 obtained on JD 2451236.55 The USNOA2.0 red photographic magnitude is 16.40 (Monet et al. 1998), consistent with our pre-outburst brightness in the ATLAS $O$ filter. The DENIS and 2MASS near-infrared pre-outburst magnitudes are consistent with each other, and the two DENIS measurements are consistent over an interval of 1108 days, so we assume that all these measurements represent the quiescent state of ESO-H$\alpha$ 99. In Figure 5 the – presumably quiescent – 2MASS catalog magnitudes, whose filter bandpasses match the IRIS photometry, are indicated by left-pointing arrows.

We have also obtained $V$-band photometry with the 40cm Bochum Monitoring Telescope (BMT) on Cerro Armazones (Ramolla et al. 2013), where the object remained observable longer than from Hawaii. The photometry was calibrated against a set of in-field non-variable stars from the APASS all-sky catalog (Munari et al. 2014) and is included as green triangle symbols in Figure 4.

Finally, on 2019 March 16, UT, we obtained a near-infrared spectrum of ESO-H$\alpha$ 99 using the NASA Infrared Telescope Facility (IRTF) with the SPEX instrument (Rayner et al. 2003) in short cross-dispersed (SXD) mode with a 0$\farcs$3 slit, giving a spectral resolution of R=2000. Low-resolution optical spectra of ESO-H$\alpha$ 99 were obtained with the Faulkes Telescope North on Haleakala, Maui, on 2019 March 17 (UT) and 2019 April 5 (UT) using a slit width of 1$\farcs$6 and 30″ slit length of the FLOYDS spectrograph. This resulted in a FWHM of the [O I] night sky lines of 14 Å and a spectral resolution of R = 425 at the wavelength of the Na I doublet.

The optical and infrared spectra from March 17 and 16 (UTC), respectively, were merged into Figure 7. It turned out that these spectra were obtained close to the maximum brightness reached by ESO-H$\alpha$ 99 during this outburst.

The spectrum near maximum brightness in Figure 7, combining optical data from the Faulkes telescope and near-infrared data from the IRTF/SPEX, shows prominent H I, He I, Ca II, and CO bandhead emission, the only noticeable exception being Na I D line in absorption. Note, however, that Na I is in emission in the near-infrared. Besides the emission lines, a continuum is present, but at the spectral resolution of our data, we cannot detect absorption lines that could be used for spectral classification. The H₂ 1–0 S(1) line is weakly detected in emission at the position of the star itself, indicating shock-excitation of H₂ near the star.

As part of a spectroscopic survey of the brighter stars of the sample of H$\alpha$ emission line stars in the VMR-D region of Pettersson & Reipurth 1994 a spectrum of ESO-H$\alpha$ 99 was obtained on February 22, 1993 at the ESO 3.6m telescope with the OPTOPUS multi-object spectrograph and an exposure time of 2 $\times$ 30 min. The original data are no longer available, but a plot of the spectrum is. For the analysis in this paper, we have digitized this plot. Although we do not know the photometric brightness of the star at the time this spectrum was taken, absent any indication of a prior outburst at that time, we assume that the spectrum represents the quiescent state. The pre-outburst spectrum is the one shown in red in the comparison of spectra in Figure 8.

The observed outburst amplitude is strongly wavelength dependent, being much larger at optical wavelengths than in the near-infrared. The optical light curve in the ATLAS $O$ filter of ESO-H$\alpha$ 99 (Figure 4) shows a rise of about 4.4 magnitudes from the quiescent brightness ($\approx$ 19.1 mag) up to the maximum of brightness observed until now ($O$ = 14.69 mag). Just prior to the rise, around MJD 58185, both the ATLAS and Gaia data indicate a brief ($\approx$30 day) dip in brightness of about 1.3 mag in ATLAS $O$ below the quiescent brightness. The immediately following rise in brightness to the maximum is not steady, rather, the ATLAS data show three interim minima superposed on the overall rise of the outburst light curve. As we will discuss below in the context of the spectroscopy, at least one of these interim minima in ESO-H$\alpha$ 99 is associated with spectral changes. We can exclude that these are merely caused by extinction variations. The few sparse Gaia data obtained after ESO-H$\alpha$ 99 became observable again in July of 2019 show variations of about 0.5 mag again, similar to the interim minima observed earlier with both Gaia and ATLAS data. We conclude that the semi-periodic fluctuations with typical periods of about one month are continuing now superposed on a fairly stable plateau near maximum brightness.

Quasi-periodicity on the order of one month as observed in ESO-H$\alpha$ 99 superposed on the outburst is too long for the expected rotational period of a still rapidly rotating young star and the magnetically coupled accretion flows, but must be related to regions of the rotating disk farther away from the star.

Similar, though shorter period fluctuation superposed on the years-long outburst have been observed by Sicilia-Aguilar et al. 2017 in ASASSN 13db and interpreted as rotational modulation of starspots that are coming and fading on timescales of months.

About 2 years before the 2019 major outburst, the ATLAS data indicate a minor maximum around MJD 57800 (2017 Feb. 16) The coverage of this prior maximum by ATLAS is poor, and there are no confirming Gaia measurements. We can only state that this maximum has lasted for at least a few months, and that the amplitude above the quiescent brightness was about 2.5 mag in ATLAS $O$ filter, substantially less than the present maximum. Despite the limitations of the available data, we note that this smaller prior maximum is entirely consistent with the characteristics of other EXors, as was reviewed in the Section 1, and in particular appears similar to the sporadic outbursts of the prototypical EX Lupi (McLaughlin 1946; Herbig et al. 2001; Herbig 2007), where Aspin et al. 2010 distinguish “characteristic” and “extreme” outbursts. In this terminology, the present 2019 outburst of ESO-H$\alpha$ 99 qualifies as an “extreme” outburst, while the smaller 2017 outburst would be “characteristic”.

The outburst of ESO-H$\alpha$ 99 was fortuitously observed by TESS. The data come from the Full Frame Images and give photometry on a 30 min cadence. The resulting finely resolved light curve during the 28 days of observations is shown in the lower panel of Figure 4. The TESS observations covered the second of the three minor ($\approx$ month-long) interim minima superposed on the overall rise to maximum brightness. The TESS light curve shows multiple local maxima and minima with typical rise and fall time of order of one day. A period analysis using the Lomb-Scargle algorithm developed by Lomb 1976 and Scargle 1982, and implemented in the Period Analysis Software (PERANSO) written by T. Vanmunster did not find any strict periodicity of these short term variations, rather, there is significant power in periods ranging from 2 to 10 days, after “whitening” out the power associated with the overall minor minimum. These variations on timescales of several days on top of the longer term features of the light curve are similar to the variations during quiescence found by Cody et al. 2017 using K2 data for YSOs with less substantial disks and “bursts“ (rather than major “outbursts”) of order of tens of percent in flux relative to the quiescent, low accretion state.

Variability of FU Ori during its on-going outburst was studied by Siwak et al. 2013; Siwak et al. 2018 and attributed to a small number of unstable accretion ”tongues” (referred to as “funnels” here) and their associated hotspots, revolving around the star.

Stable accretion funnels and stationary hotspots would lead to rotationally modulated periodic variability on timescales of the rotation of the star, i.e., of order of a few days. This is not what we observe, however. The short term variations observed by TESS cannot be described as quasi-perodic. At best, they indicate that “every few days, something changes”. This is best understood if we assume that we observe several active accretion funnels at any given time and their varying accretion rates, coupled with some rotational modulation of this complex scenery.

Figure 6 shows the near-infrared $J-H/H-K$ color-color diagram of ESO-H$\alpha$ 99 in both quiescence and outburst. We included similar objects observed by Lorenzetti et al. 2012 and V2492 Cyg, studied in detail by Aspin 2011a for comparison, as it is the most reddened EXor known to date.

Figure 6 shows that ESO-H$\alpha$ 99 shares the colors of the other two deeply embedded EXor objects, V1647 Ori and V2492 Cyg, and shows similar color changes from the quiescent state to the outburst maximum. In general, all EXors get “bluer” during outburst and the color-color path from quiescent to outburst location is roughly parallel to the interstellar reddening vector, if anything slightly less steep. While the color changes are therefore consistent with a clearing of obscuring material during outburst, this is certainly not the only mechanism at work in ESO-H$\alpha$ 99 since we are also seeing major changes in the line ratios in the spectrum. Those spectral changes cannot be explained simply by a varying amount of line-of-sight extinction. Some contribution from changing extinction, possibly caused by irregularities in the inner disk orbiting in and out of the line of sight is quite possible, but cannot be confirmed with the available data.

We have compared the pre-outburst spectral energy distribution of ESO-H$\alpha$ 99 (Figure 2) with the SEDs of other EXors and FUors using the VizieR database and its Photometry Viewer. This database is readily available, so we do not show all SEDs here. The wavelength coverage of those SEDs is quite heterogeneous, so we will not discuss the similarities and differences quantitatively. The SEDs of most classical, i.e., low extinction EXors, some of which are included in the near-infrared color-color diagram in Figure 6 and occupy the low-color-value area of this diagram, have the peak of their SED at near-infrared wavelengths, and decline from there to the long-wavelength limit of the available data. Only the deeply embedded EXors show flat or nearly flat SEDs: NY Ori is flat from 1 to 20 $\mu$m, but has no data beyond that. Deeply embedded, very red EXors such as V2492 Cyg, PV Cep, and V1647 Ori have essentially flat SEDs out to 100 $\mu$m. The SED of ESO-H$\alpha$ 99 is very similar to those three deeply embedded flat SED EXors. This flat 10 - 100 $\mu$m SED also puts ESO-H$\alpha$ 99 in the same SED class as OO Ser, the ”deeply embedded outburst star” (Hodapp et al. 1996) that falls outside of the classical FUor vs. EXor classification scheme.

We also note that the classical FUors FU Ori and V1057 Cyg have SED peaks at near-infrared wavelengths. The FUor V2775 Ori, however, (Fischer et al. 2012) has a flat SED similar to the one discussed here. Deeply embedded FUor-like objects, i.e., objects spectroscopically similar to a FUor but no historically observed outburst, such as L1551 IRS5, have SEDs rising throughout the 10 - 100 $\mu$m range.

While EX Lupi, the prototypical EXor, is not associated with much nebulosity and is not deeply embedded, ESO-H$\alpha$ 99 is highly reddened and is associated with a reflection nebula with a bubble-like morphology, probably light scattered off the walls of an outflow cavity. Both its red colors and the presence of a reflection nebula support the fact that ESO-H$\alpha$ 99 is a very young object, still deeply embedded in its parent molecular cloud, and surrounded by a substantial gas and dust disk. We have found one small region of shock-excited $H_{2}~1-0~S(1)$ emission with indication of a bow-shock morphology near the symmetry axis of the reflection nebula, strongly suggesting that this feature is the result of a highly collimated outflow from ESO-H$\alpha$ 99. Such highly collimated outflows are frequently found in YSOs, but are not typical of EXors or FUors. We interpret this as additional evidence that ESO-H$\alpha$ 99 is, within the broad EXor class of outbursts, one of the youngest of this class of objects.

We compare low-resolution optical spectra of ESO-H$\alpha$ 99 at three epochs: the pre-outburst spectrum from 1993 Feb. 22, presumably during a quiescent phase, the spectrum taken very close to the maximum of the current outburst on 2019 March 17 shown in Figure 7, and the spectrum taken on 2019 April 5 when the ATLAS $O$ magnitude was 0.8 mag lower than at the maximum.

We do not have coincident photometry for the pre-outburst spectrum from 1993 Feb. 22. From Figure 4 it is clear that ESO-H$\alpha$ 99 had shown some variability prior to the present outburst. On the other hand, none of the pre-outburst data (POSS, 2MASS etc.) give any indication of a prior large outburst. Our measured quiescent pre-outburst brightness in the ATLAS “Orange” filter is about $O$ = 19.1 mag We therefore make the assumption that at the time of the 1993 spectrum, the brightness was $O$ $\approx$ 19 mag.

For the two epochs in 2019, we have closely coincident ATLAS “Orange” filter photometry. For 2019 03 17, ATLAS obtained photometry in the same night, about 1.25 hours after the spectrum. For the 2019 04 05 spectrum, the closests ATLAS photometry is from the following night, 25.7 hours after the spectrum. The epochs of the recent spectra are indicated by blue arrows in Figures 4 and 5 and place those spectra in context of the light curve.

Of the two 2019 optical spectra, the first was obtained near the maximum of the observed light curve during this ongoing outburst, and the second in the following minor interim minimum in the light curve, after which the object brightened again. In the middle panel of Figure 4 and in Figure 5, the brightness maximum at the time of the first spectrum and the interim minimum on 2019 April 5 are documented by ATLAS $O$, and infrared $J$, $H$, and $K$ photometry. Table 1 lists the dates, ATLAS $O$ magnitudes, and the equivalent width of four spectral lines, both permitted and forbidden.

For now, we can leave the question open whether March 17 marked the overall maximum on the current outburst, or whether it is merely a preliminary interim maximum. After it reemerged from behind the Sun in August 2019, we have obtained a few new Gaia data points indicating that ESO-H$\alpha$ 99 has experienced another high point near the magnitude of the March 2019 maximum and is otherwise staying within a magnitude of the maximum brightness with some variations. The important point is that changes in broadband photometry are associated with changes in the emission line spectrum.

The early 1993 optical spectrum of ESO-H$\alpha$ 99 offers some insight into what we assume is the quiescent state of the star: First, the 26-year old spectrum shows the forbidden lines of [O I] at 6300 and 6363 Å and blended [S II] 6716/6731 Å, which together with strong H$\alpha$ emission are characteristic of Herbig-Haro shocks. The molecular hydrogen shock front MHO 1520 approximately along the line defined by the outflow cavity associated with ESO-H$\alpha$ 99 (Figure 3) shows that outflow activity must have occurred in the more distant past, certainly prior to the present outburst.

Second, a forest of permitted Fe II and forbidden [Fe II] lines is seen in the 5000-5500 Å region. These lines are only seen in the most active T Tauri stars (Herbig 1962), and the spectrum shows a strong semblance to that of the HH 32 driving source AS 353A (Eisloeffel et al. 1990) and the HH 46/47 driving source (Reipurth & Heathcote 1991).

Third, the Na I doublet at $\lambda$5890/5896 Å is in absorption, fairly weak and unresolved in the 1993 spectrum as well as in the 2019 spectra. The Na I absorption blends with the He I line at 5876 Å. Helium emission lines have been observed in several other EXors, e.g., NY Ori, V1118 Ori, and V350 Ori by Herbig 2008.

Altogether, the 1993 quiescent spectrum is indicative of a highly active, strongly accreting YSO star that is driving a shocked outflow.

During the present outburst, the spectrum has changed substantially. Referring to Table I, the equivalent width of all the emission lines was much higher in quiescence (1993) than during the outburst. H$\alpha$ line was much higher ($\sim$75 Å) than in the 2019 March 17 spectrum ($\sim$37 Å). On 2019 April 5, at a time when the broadband integrated flux was only about one half of that at maximum, the spectrum has an H$\alpha$ equivalent width between those two extreme points: EW $\sim$62 Å.

The spectral changes observed between those two 2019 spectra clearly indicate that this interim minimum was not just caused by changing amounts of obscuring dust in the line of sight, which should not affect the equivalent width of H$\alpha$. Rather, the photometric and spectral variations show real changes in the components of the accretion flow: inner disk, accretion funnels, and possibly a surface hot spot on the star.

Comparing the change of H$\alpha$ equivalent width to the change in broadband flux in Table 1, it is clear that the flux of the H$\alpha$ line does increase with increasing brightness, but that the continuum rises disproportionately more strongly, so that the equivalent width is reduced by about a factor of two between quiescence and the brightness maximum. Similarly, for the two recent epochs that we have a measured brightness for, the H$\alpha$ equivalent width increases by a factor of 1.6 during a decline on a factor 0.48 in broadband brightness, showing again that the continuum changes disproportionally more than the H$\alpha$ line flux.

The disproportionate rise of the continuum is even more pronounced for the forbidden lines, the brighest of which is [O I] at 6300 Å. From quiescence to the maximum, a factor of $\approx$ 50 in broadband flux, the equivalent width is reduced by a factor of $\approx$ 13. So while the line flux in [O I] 6300 Å does increase somewhat during maximum, the broadband flux increases 13 times more, showing that during maximum, the conditions are less favorable for the emission of forbidden lines, probably because of higher densities in the emitting regions. Given the uncertainty of our assumption for the pre-outburst brightness when the 1993 Feb. 22 spectrum was taken, the ESO-H$\alpha$ 99 spectral evolution is consistent with the flux in those forbidden lines being unchanged during the outburst, and simply being diluted by increased continuum or that line flux increasing by much less than the continuum. Comparing just the 2019 March 17 (maximum) and 2019 April 05 (post maximum) equivalent widths of [O I]6300 Å and the blended [S II] lines, we clearly note that the equivalent width changes inversely proportional to the broadband flux, i.e., the absolute line flux stays constant. The accretion sensitive (Muzerolle et al. 1998) permitted CaII triplet lines show an increase of a factor 1.3 during the decline by a factor of 0.48, meaning that the absolute line flux has diminished, but not proportional to the continuum flux.

We are now comparing the spectral evolution of ESO-H$\alpha$ 99 with several other EXor objects and one higher luminosity eruptive variable. The optical spectrum of ESO-H$\alpha$ 99 and its changes during the outburst are quite different from those observed in the 2008 outburst of the prototypical EX Lupi. Kospal et al. 2008; Sicilia-Aguilar et al. 2012; Sicilia-Aguilar et al. 2015 observed a very rich emission line spectrum, compared to other T Tauri stars, and a substantial increase in the number of metal emission lines during outburst, more than most other EXor-type outbursts, but note the absence of forbidden lines. As a caveat to the latter statement, in high resolution spectra, emission of [O I] was detected in EX Lupi by Banzatti et al. 2019 both in quiescence and, more strongly, during the 2008 outburst.

Similary, Holoien et al. 2014 observed in the case of the 2013 EXor eruption of ASASSN13db that “during the outburst, the spectra are dominated by a forest of emission lines, mostly neutral metallic lines from Fe I”. The longer duration 2014-2017 outburst of this object was studied by Sicilia-Aguilar et al. 2017 and they note again the similarity of the outburst spectrum to that of EX Lupi.

The spectral changes in these two objects are clearly different from the eruption of ESO-H$\alpha$ 99 where we observe a reduction in the prominence of the Fe I line forest, and a substantial reduction in the equivalent width, i.e. the importance relative to the continuum, for the forbidden lines of [O I] and [S II].

In contrast, the evolution of the spectral lines during the outburst of ESO-H$\alpha$ 99 has more similarities to the higher luminosity case PV Cep. Kun et al. 2011 found nearly constant flux, i.e. strongly increasing equivalent widths, in forbidden lines during the fading. However, they also noted little change in the equivalent width of H$\alpha$ in the fading phase different from what we observe in ESO-H$\alpha$ 99.

We conclude that during the ESO-H$\alpha$ 99 outburst, its spectrum has changed substantially. The H$\alpha$ line changes less than the continuum during the outburst, and the forbidden [O I] and [S II] lines change substantially less that the continuum, and are, in fact, consistent with these forbidden line fluxes being constant.

We conclude that in ESO-H$\alpha$ 99, the region emitting the forbidden lines may not be affected at all by the processes leading to the continuum outburst. The increase in the optical and near-infrared continuum is qualitatively similar to the appearance of a strong continuum during FUor outbursts. We speculate that in in ESO-H$\alpha$ 99, the rise in the continuum is less than in typical FUor outbursts, leaving the most prominent emission lines observable.