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Serendipitous Discovery of Nine White Dwarfs With Gaseous Debris Disks

Optical spectroscopic observations of white dwarf stars selected from catalogs based on the Gaia DR2 database reveal nine new gaseous debris disks that orbit single white dwarf stars, about a factor of two increase over the previously known sample. For each source we present gas emission lines identified and basic stellar parameters, including abundances for lines seen with low-resolution spectroscopy. Principle discoveries include: (1) the coolest white dwarf (Teff~12,720 K) with a gas disk; this star, WD0145+234, has been reported to have undergone a recent infrared outburst; (2) co-location in velocity space of gaseous emission from multiple elements, suggesting that different elements are well-mixed; (3) highly asymmetric emission structures toward SDSSJ0006+2858, and possibly asymmetric structures for two other systems; (4) an overall sample composed of approximately 25% DB and 75% DA white dwarfs, consistent with the overall distribution of primary atmospheric types found in the field population; and (5) never-before-seen emission lines from Na in the spectra of GaiaJ0611-6931, semi-forbidden Mg, Ca, and Fe lines toward WD0842+572, and Si in both stars. The currently known sample of gaseous debris disk systems is significantly skewed towards northern hemisphere stars, suggesting a dozen or so emission line stars are waiting to be found in the southern hemisphere.


INTRODUCTION
The photopsheres of white dwarf stars are "polluted" by remnant solid material from planetary systems that otherwise stably orbited their host star while it was on the main sequence (e.g., Zuckerman et al. 2007;Jura 2008;Farihi et al. 2009;Dufour et al. 2010;Klein et al. 2010;Melis et al. 2010;Gänsicke et al. 2012;Xu et al. 2013, and references therein). Prior to being accreted, this solid material must find its way into the host star's Roche radius and be tidally shredded into disks of dust and gas (e.g., Debes & Sigurdsson 2002;Jura 2003;Frewen & Hansen 2014;Vanderburg et al. 2015;Veras et al. 2016;Manser et al. 2019).
While the most heavily polluted white dwarf stars known are typically host to infrared excess emission and hence orbiting dust disks (e.g., Farihi 2016 and references therein), a subset of these objects additionally are host to disks of gaseous metallic material (Gänsicke et al. 2006(Gänsicke et al. , 2007(Gänsicke et al. , 2008Melis et al. 2010Melis et al. , 2012Brinkworth et al. 2012;Farihi et al. 2012;Hartmann et al. 2016;Xu et al. 2016;Manser et al. 2016aManser et al. ,b, 2020. Models of disk formation and evolution suggest that dusty material is a necessary pre-condition to producing gaseous material in white dwarf debris disks. The gaseous material may form either from repeated impacts of solids within the dust disk or from sublimation of dusty material at the inner edge of the debris disk; the gas subsequently viscously spreads throughout the disk (e.g., Jura 2008;Melis et al. 2010;Rafikov 2011a,b;Bochkarev & Rafikov 2011;Hartmann et al. 2011Hartmann et al. , 2016Metzger et al. 2012;Bear & Soker 2013;Kenyon & Bromley 2017).
Gaseous material may act to aerodynamically drag dust particles, removing angular momentum and transporting them to the inner disk and eventually the star. Detailed study of dust and gas disk components around single white dwarf stars can provide insight into the fate of solid material in planetary systems and hence what may very well happen to our own solar system billions of years from now (e.g., Debes & Sigurdsson 2002;Farihi et al. 2009;Frewen & Hansen 2014;Veras et al. 2016;Manser et al. 2016b;Xu et al. 2016;Cauley et al. 2018;Mustill et al. 2018;Grishin & Veras 2019;Malamud & Perets 2020;Maldonado et al. 2020a,b). Additionally, intensive monitoring of gas-disk white dwarf stars can possibly provide a way to elucidate the size, density, and orbit of disintegrating rocky bodies around white dwarf stars; for example what appears to be part of a core from a differentiated rocky object orbiting the heavily polluted star SDSSJ1228+1040 (Manser et al. 2019) and an intact gas giant planet being accreted by WD J091405.30+191412.25 (Gänsicke et al. 2019).
In this paper we report on the discovery of a set of nine new white dwarf stars that host gas disk emission lines in their optical spectra. Included in this set is the recently reported infrared "outburst" object WD0145+234 (Wang et al. 2019). In addition to detection of gas disk emission lines in this system (confirming it to be host to gaseous and dusty disk components), we also identify atmospheric pollution indicating that this material is being accreted by the host star. Below we describe our target selection strategy, observations conducted for newly discovered gas disk systems, measurements made, target-specific discussion, and an overall sample discussion.

TARGET SELECTION AND OBSERVATIONS
Stars were observed as part of a large-scale survey to identify the most heavily polluted white dwarfs through low-resolution optical spectroscopy. We began with the sample of Gentile Fusillo et al. (2019) which reports the probability of an object being a white dwarf and stellar parameters (e.g., T eff and logg) based on fits to photometric data. These determinations are heavily informed by Gaia DR2 data, thus we refer to it as the "Gaia DR2 sample". Targets with evidence for infrared excess emission or having a helium-dominated atmosphere received higher priority for follow-up. Infrared excess emission data came from 2MASS, VISTA, and WISE (e.g., Xu et al. 2020) while GALEX colors were used to identify helium-dominated atmosphere white dwarfs (where available; helium-dominated atmosphere targets can have more readily-identifiable atmospheric pollution at low spectral resolution than can hydrogen-dominated atmosphere objects). In this paper we do not assess the reality of any putative infrared excess emission detected for our target stars nor do we report on any infrared emission characteristics; this will be the subject of future work. In sum, we have observed ≈500 white dwarf stars as a part of this survey. After serendipitously finding two gas-disk hosting white dwarf stars, we began routinely using a spectral setup that covered the Ca II infrared triplet (IRT) region where the strongest gas disk emission lines are typically seen (e.g., Melis et al. 2010;Manser et al. 2016b, and references therein). Table A1 lists all observation dates and resulting data properties. Grating resolution optical spectroscopy was performed at Lick Observatory with the Kast Double Spectrograph mounted on the Shane 3 m telescope and at Gemini-South with GMOS (Hook et al. 2004;hereafter GMOS-S). In general, all Kast observations employed the blue and red arms with light split by the d57 dichroic around 5700Å. After splitting, blue light was passed through the 600/4310 grism while red light was in most observations passed through the 830/8460 grating (and in a few, the 600/7500 grating which provided broader wavelength coverage). Slit widths of 1, 1.5, and 2 were used depending on seeing and cloud extinction; integration times ranged between 30-60 minutes depending on target brightness and conditions. GMOS-S observations used the B600 grating, a slit size of 0.5 , were centered at a wavelength of 525 nm, and exposed for 15-20 minutes. GMOS-S spectra are recorded onto three detectors with a gap in wavelength coverage between each detector; we did not use multiple setups to cover these gaps. Final GMOS-S wavelength coverage is given in Table A1. Grating resolution data are reduced using standard IRAF long-slit tasks including bias subtraction, flat-fielding, wavelength calibration with arc lamps, and instrumental response calibration via observations of flux calibration standard stars. Arc lamp frames are not obtained close in time to science frames and as such the zero-point of the wavelength scale is not accurate.
Higher resolution observations were obtained for new gas disk white dwarf systems with the Keck I telescope and HIRES at Mauna Kea Observatory (Vogt et al. 1994) and with the Baade telescope and MagE at Las Campanas Observatory (Marshall et al. 2008). HIRES data were taken with the C5 decker (1.148 slit width), had exposure times of 30-80 minutes, and are reduced using the MAKEE software package which outputs heliocentric velocity-corrected spectra shifted to vacuum wavelengths. While HIRES wavelength coverage is quoted as continuous in Table A1, there are gaps in coverage between each of the three CCDs and sometimes between red orders. MagE data were taken with the 0.5 slit, integrated for 60 minutes, and are reduced with the facility Carnegie Python pipeline (Kelson et al. 2000;Kelson 2003). After reduction and extraction, polynomials are fit to each order to bring overlapping order segments into agreement before combining all orders of every exposure to generate a final spectrum for analysis. More details about reducing echelle data in the presence of gas disk emission lines can be found in Melis et al. (2010).

Atmospheric Parameters and Abundances
For each target we use low-resolution data to obtain atmospheric parameters (e.g., T eff , logg) and then measure abundances for any detected metallic absorption lines. In this paper we only report on elements detected in the low-resolution spectra, results from higher-resolution spectra will appear in future publications dedicated to atmospheric pollution. Fitting proceeds as in Melis et al. (2011), Melis & Dufour (2017), and references therein. We briefly summarize this process here.
To obtain atmospheric parameters we perform model fits to hydrogen Balmer lines and helium lines in the low resolution data. The method follows the "spectroscopic technique" described by Bergeron et al. (1992) and described at length in Liebert et al. (2005) and references therein. Gaia DR2 distances are used to help constrain the stellar radius and hence logg. Uncertainties for resulting T eff and logg values are not calculated individually for each star, but are typically ±1,000 K for T eff and ±0.1 dex for logg (e.g., Bergeron et al. 2019;Genest-Beaulieu & Bergeron 2019a,b and references therein).
When metallic absorption lines are present in the low-resolution spectra, we measure abundances by fitting synthetic model spectra to the data. A grid of local thermodynamic equillibrium (LTE) model atmospheres are generated with code similar to that described in Dufour et al. (2005Dufour et al. ( , 2007 where absorption line data are taken from the Vienna Atomic Line Database. Synthetic spectra in the grid cover a range of abundances typically from log[n(Z)/n(H)]= −3.0 to −8.0 in steps of 0.5 dex. We then determine the abundance of each element by fitting the various observed lines using a similar method to that described in Dufour et al. (2005). This is done by minimizing the value of χ 2 taken as the sum over all frequencies of the difference between the normalized observed and model fluxes (the synthetic spectra are multiplied by a constant factor to account for the solid angle and the slope of the spectra locally are allowed to vary by a first order polynomial to account for residuals from the spectral normalization procedure), all frequency points being given an equal weight. Interpolation between grid points allows us to achieve individual line abundances accurate to <0.05 dex. Uncertainties are conservatively set at 0.2 dex. Table 1 reports modeled T eff , logg, and abundances for Ca and/or Mg (the most readily detectable elements in low resolution optical spectra); corresponding white dwarf masses and radii are obtained from the MWDD 1 evolutionary models (Bédard et al. 2020). For the two heliumdominated atmosphere stars we also report measured hydrogen abundances in their atmospheres. For Gaia J0611−6931 we report on the additional detection of Si in Section 4.5.

Gaseous Emission Lines
Broad emission lines from several atomic transitions are detected in our optical spectra. Figure  1 shows Ca II IRT emission from one epoch for each source while Figures B.1 through B.33 show most other emission lines detected for each source and compare multiple epochs when such data are available. In many cases identifications for emission lines seen could be taken from past studies of white dwarfs with gaseous disks (e.g., Gänsicke et al. 2006;Manser et al. 2016b, and references therein). In some cases lines seen did not have corresponding identifications in the literature and we sought to identify them. The procedure used for obtaining and assessing new line identifications is given below after discussion regarding basic measurements made for each line.
For each feature we make several measurements. First is the emission line equivalent width which is uncorrected for line absorption in the feature. From this value, one may calculate a line flux; we do not do that here. Equivalent width uncertainties are obtained by taking the standard deviation of several measurements which differ by anchoring the continuum points for the measurement at multiple locations consistent within the spectrum noise level.
Next, we measure maximum gas velocities in the blue and red wings of the detected emission line and/or the full velocity width of a feature at zero intensity (effectively red−blue extent). The highest velocity gas emission (reported as v max sini) corresponds to the innermost orbit of emitting gas-phase metals around each white dwarf (e.g., Horne & Marsh 1986). This value is quoted as a product with sini where i is the unknown inclination angle of the disk (i of 0 • would be obtained for a face-on disk). Maximum gas velocities are calculated relative to the white dwarf systemic motion. To derive each object's systemic motion, we measure radial velocities for metal and hydrogen lines in high-resolution spectra and then correct them for the computed gravitational redshifts at the white dwarf photosphere. Gravitational redshifts are based on measured atmospheric parameters reported in Table 1 and have uncertainties of up to ∼15 km s −1 due to uncertainties in atmospheric parameter determinations; we do not include this additional error term in any analysis. Systemic velocities are reported in Table A2 along with other astrometric and kinematic quantities for each star. Last, if possible, we measure the velocity separation between peaks for double-peaked profiles. Peak separation gives a rough characterization of the outermost orbit of emitting gas-phase metals and is obtained by taking the difference of each transition's peak centroid wavelengths. These are measured by making several Gaussian fits to each peak, changing between each fit the anchor points; the average is the adopted value and standard deviation the uncertainty.
Measurements are only made for lines with sufficiently good signal-to-noise such that both peaks are well-detected and/or where one can clearly identify the line edges (where the line emission reaches zero intensity and the continuum emission level is recovered). In lower-resolution data, where the absolute wavelength scale is not accurate, we do not report maximum gas velocities relative to the white dwarf systemic motion. Full widths can be measured in all spectra as those are derived from the difference between blue and red extents of a line and are not affected by absolute wavelength scale inaccuracies. When there is the possibility of contamination by nearby transitions (whether in emission or absorption) we do not make measurements. For example, we typically cannot make measurements for the Fe I λ5169 and Mg I triplet features around 5170Å (e.g., see Figure B.24).
For new and unidentified lines we follow a two-step, iterative process in establishing and assessing line identifications; this procedure relies upon high-resolution data and thus was not attempted with Kast or GMOS-S spectra. We begin with the selection of lines whose identifications we believe are unambiguous (e.g., Ca II IRT, O I, select Fe II, and Ca II H+K lines). From this selection of lines we establish a range of transition characteristics (lower and upper energy levels, Einstein coefficients, and oscillator strengths) and measured line extents in the red and blue wings (v max sini). We then search near the wavelength of an unidentified line for any transition with comparable transition characteristics to the known lines that also produces v max sini values reasonably in agreement with those seen for that target star. We used the NIST Atomic Spectra Database Lines Form 2 and the database of van Hoof (2018) 3 to obtain line parameters. With "candidate" line identifications obtained in this manner, we then perform a self-consistency check of seeking out other lines from the same element and ionization state that we might expect to see based on energy levels, transition probabilities, and oscillator strengths. Confirmation of such lines solidifies a line identification.
We have newly identified several Fe I (including some semi-forbidden) and Fe II lines, a semiforbidden Ca I line, a Si I line, a semi-forbidden Mg I line, and Na D doublet emission. Ambiguous and unidentified lines remain; many likely originate from iron but it is not clear which specific transition is responsible. There remain two lines for which identifications are not possible because they were covered only with low-resolution spectra (see Section 4.4).

INDIVIDUAL SYSTEMS
Here we briefly discuss results for each target, including any literature work that may exist for each system.

SDSS J0006+2858
SDSS J0006+2858 was one of two stars (the other being GaiaJ2100+2122) serendipitously found to host unambiguous emission lines early in our Kast polluted white dwarf survey and prompted a shift in our observing strategy to cover Ca II IRT lines.
SDSS J0006+2858 hosts a rich emission spectrum with lines from oxygen, calcium, and iron seen . Emission structures in the 6200-6500Å region similar to those seen in the spectrum of Gaia J0510+2315 are seen (Section 4.4), it is not clear what transitions they originate from. Possible contributions from magnesium may also be present near 5170Å, but are inconclusive due to the presence of strong Fe II λ5169 and λ5197 emission. SDSS J0006+2858 hosts Ca II IRT line strengths comparable to some of the strongest known emitters (e.g., SDSS J1228+1040 and Gaia J0611−6931). Curiously, it also appears to host a third peak in its IRT lines that falls on the blue shoulder of the blue-ward major peak. Such a structure has not been previously seen for any other gas disk white dwarf star.
Of special note for this system is a significant asymmetry in the maximum blue and red velocities of its emission lines. On average, the blue line extent is roughly double that seen in the red (−800 km s −1 vs +400 km s −1 ; see Table B3 and Figure 2). Uncertainties in stellar atmospheric parameters (and hence gravitational redshift) combined with line measurement uncertainties are not capable of accounting for this difference. While this asymmetry is seen in all lines, the strength of the "extended" portion in the blue wing is suppressed in the Ca II IRT relative to what is seen for other transitions (including Ca II H+K). This extension in the blue wing is stable over at least a week timescale (07 July 2019 and 16 July 2019 HIRES measurements) and possibly over six month timescales (Figures B.1 and B.4).

WD0145+234
WD0145+234 was first reported as a spectroscopically confirmed white dwarf star with spectral type DA by McCook & Sion (1987). It was further studied in Gianninas et al. (2011) and Limoges et al. (2015) where they measured from spectroscopic observations a DA spectral type, an effective temperature of ≈13,000±200 K, and logg of ≈8.12±0.05. Gianninas et al. (2006) performed highspeed optical photometric monitoring of WD0145+234 over a couple hours in one night, finding no obvious variability over period ranges of 20-2000 seconds with a detection limit of 0.06%; they also derived from spectra T eff =12,470 K and logg of 8.06. Similar results in optical monitoring were found in the study performed by Bognár et al. (2018). Rebassa-Mansergas et al. (2019) identify WD0145+234 as an infrared excess candidate and associate its excess emission to a putative debris disk.
More remarkably, WD0145+234 was recently reported in the literature to have undergone an outburst in the mid-infrared (Wang et al. 2019). Pre-outburst archival observations demonstrate that it was host to atmospheric pollution (but past studies of this star did not note atmospheric pollution) and infrared excess emission indicating that the white dwarf star is in the process of consuming a rocky body from its remnant planetary system (Wang et al. 2019;Melis et al., in prep). The infrared excess emission is now brighter by over a magnitude, likely due to a fresh disintegration event off of a rocky body.
The infrared outburst at WD0145+234 began in mid-2018 (Wang et al. 2019); the observations of gas emission were in late 2019 (Table A1). Since no strong emission lines other than the Ca II IRT are seen (Table B4 and Figure B.5), and since no archival observations covering the Ca II IRT exist that we are aware of, it is not possible to comment on whether or not gaseous emission lines were present in spectra taken before the infrared outburst began. No obvious variability is seen in the gas emission lines (strength or structure) between the epochs presented here (Table B4 and Figure B.5).
An exhaustive study on the time variability of absorption lines and the composition of the parent body source of the material being accreted by WD0145+234 will appear elsewhere (Melis et al., in prep.).

SDSS J0347+1624
SDSS J0347+1624 was brought to our attention by Dennihy et al. (2020, submitted) as a new gas disk candidate. We had obtained observations of it in 2016 and obtained a further spectrum in 2019 to aid in the variability study being conducted by Dennihy et al. We present the Kast spectra obtained in both epochs here but defer to Dennihy et al. (2020, submitted) for a full discussion of this source.
Kast spectra from both epochs show clear emission from oxygen, calcium, and many iron lines (Figures 1 and B.6-B.8); unidentified emission in the 6200-6500Å region similar to that seen for Gaia J0510+2315 may also be present (Section 4.4). The Kast data do not reveal any detectable variation between iron lines in the 2016 and 2019 epochs (2016 epoch spectra did not cover beyond 7700Å and hence only covered iron emission line regions in common with the 2019 epoch; Tables A1  and B5).

Gaia J0510+2315
Emission lines from oxygen, magnesium, calcium, and iron are seen toward Gaia J0510+2315 (Figures B.9-B.14). Gaia J0510+2315 is the only star in the sample to host clear emission from Mg II, in this case near 7900Å (Figure B.13). It is also impressive in that both O I emission features are comparable in strength to the Ca II IRT emission lines (Table B6 and Figures B.13 and B.14).
No obvious variability is seen between epochs. Two emission structures that we can't identify appear in Kast data at wavelengths that are not covered by the HIRES spectra (Table 2, Figures B.9 and B.13). Emission in the 6200-6500Å region, likely from iron, is seen in Kast and HIRES spectra (Figure B.12).
Apparent asymmetry is seen in the blue and red wings of emission lines when one considers all maximum velocity measurements together ( Figure 2 and Table B6). Curiously, and unique to Gaia J0510+2315, the O I triplet near 7772Å (and possibly the O I complex near 8446Å) hosts a red extent that greatly exceeds measurements for any other line (Table B6 and Figures 2 and B.13). It is not clear if some blended emission lines are responsible for this or if the distribution of oxygen gas near Gaia J0510+2315 is different from other elements (in this case extending much closer to the star).

Gaia J0611−6931
GMOS-S and MagE spectra indicate that Gaia J0611−6931 hosts strong atmospheric metal pollution and hitherto unseen emission from the Na D doublet (Figure B.17). Also anomalous was the identification of Si I λ3905 emission which is otherwise only seen in WD0842+572 (Table 2). Gaia J0611−6931 otherwise hosts familiar transitions from oxygen, magnesium, calcium, and iron .
No variability is seen between emission lines in the GMOS-S and MagE epochs (Table B7). Possible emission is seen in a variety of places in the spectra, but is hard to confirm due to signal-to-noise constraints and systematics present in the data including fringing. Weak emission may be present near the core of the Hδ photospheric absorption line, a feature we cannot verify nor identify with the data in hand (Figure B.15; it is possible that similar emission may also be present for SDSS J0006+2858 and Gaia J2100+2122). The appearance of the line in the core of Hδ is reminiscent of what is seen in the core of the He I λ5876 line for helium-dominated atmosphere white dwarfs (Klein et al. 2020), but without similar cores in other Balmer transitions the self-reversal is probably not due to hydrogen.
In general there appears to be multiple emission complexes between 6100 and 6300Å (Figure B.17), but it is not clear if the emission is real and similar to what is seen in this region for other stars (Table 2, Figure B.12).
Beyond the unusual volatile emission from sodium, Gaia J0611−6931 also stands out as having some of the strongest Ca II IRT emission lines seen in any gas disk star (Table B7) and the second-highest magnesium abundance in the sample presented herein (Table 1). Additionally, we see even in lowresolution spectra photospheric absorption lines from Si II near 6350 and 6370Å (Figure B.17). We fit these lines in addition to Ca II and Mg II absorption lines finding a silicon abundance by number of log 10 (Si/H)=−4.8 (these values also provide a reasonable fit to the higher resolution MagE detections of these lines). Limits for iron and oxygen are not restrictive (oxygen especially being confounded by emission) so it is not possible to comment further on the composition of the parent body polluting Gaia J0611−6931. While it may be tempting to credit the presence of volatile emission seen for Gaia J0611−6931 to a volatile-rich nature of the parent body it is accreting, it is prudent to wait for complete disk and/or atmospheric abundances before arriving at such a conclusion (e.g., see the cautionary tale for over-interpreting gas emission strength as abundance in Matlovič et al. 2020).

Gaia J0644−0352
Only weak Ca II IRT emission lines are seen in HIRES data for this star (Figure 1). There may also possibly be a very weak hint of emission from Fe II λ5316, but with no other iron lines detected this is considered inconclusive. While the measurements have fairly high uncertainty (due to the low significance of the line detections), there could be a slight hint of asymmetry in the maximum blue and red velocity gas seen (Table B8).
Gaia J0644−0352 is notable for being one of two helium-dominated atmosphere stars in the sample. The star also has hydrogen detected in low-resolution spectra indicating the possibility of a parent body having some amount of water (e.g., Gentile Fusillo et al. 2017). A complete atmospheric abundance and oxygen budget analysis (e.g., Klein et al. 2010;Farihi et al. 2013) will be necessary to determine how much water the parent body contained.

WD0842+572
Initial detection and spectroscopy for WD0842+572 was conducted as a part of the Second Byurakan Sky Survey where the star was found to have a spectral type of DA (Balayan 1997a). Until recently this star was not prominently featured in any published work. In Swan et al. (2020), Spitzer warm IRAC photometry is presented along with an unsubstantiated note that the star is host to gas disk emission lines.
WD0842+572 is host to a selection of narrow emission lines with typically well-defined double-peak morphology. We find contributions from silicon, magnesium, calcium, and iron .
Several characteristics make WD0842+572 stand out compared to the rest of the sample. First is the highest atmospheric abundance of magnesium for any star in our sample (Table 1). Second is the clear detection of a semi-forbidden transition of magnesium, Mg I] λ4571 (Figure B.22), and a bevy of semi-forbidden neutral iron lines between 4300-5200Å (Figures B.21 and B.23). Additionally, semi-forbidden neutral calcium is seen in the red wing of the Hα photospheric absorption line (Figure B.25). Last is a preponderance of emission lines from neutral atomic species like Mg I, Si I, Ca I, and Fe I . Notably absent is emission from the O I triplet near 7772Å which is seen at most other systems presented herein.
Strong Ca II IRT lines are seen for this star (Figure B.26), but do not quite rise to the level of the strongest emitters known (Table B9). Possible variation is hinted at in the structure of lines seen between the Kast and HIRES epochs (separated by roughly six months). Whether this variation is due to the lower resolution in the Kast data or true changes in the lines is not clear from the data available; in any case, Kast line measurements differ from what is measured with HIRES at the 3σ level (see Table B9).

WD1622+587
Spectroscopic characterization of WD1622+587 was performed through the Second Byurakan Sky Survey where it was found to have a spectral type of DB (Balayan 1997b). The star received little attention following that initial observation and until now was not singled out individually in any published study.
Oxygen, calcium, and iron emission lines are seen in WD1622+587 (Figures B.27 to B.29). Most lines are only seen in the HIRES data, although slight hints of Ca II IRT emission are seen in Kast spectra (Figures B.28 and B.29); we do not attempt measurements for these low-significance lines. Oxygen emission in the 7772Å complex is comparable in strength to the individual Ca II IRT lines (Table B10), although no oxygen emission from the 8446Å complex is seen.

Gaia J2100+2122
Gaia J2100+2122 has an emission spectrum dominated by iron lines, a feature that made it a clear emission line star despite discovery Kast spectra not covering Ca II IRT lines (Figures B.30 and B.31). Emission is also seen from oxygen and calcium (Figures B.32 and B.33). Gaia J2100+211 also hosts unidentified emission in the 6200-6500Å region like in the spectrum of Gaia J0510+2315 (Figure B.12).
While most iron and oxygen lines are relatively similar between epochs, variability is present at the 3σ level for Ca II IRT lines between the HIRES and Kast epochs (Table B11 and Figure B.33). Gaia J2100+2122 thus stands out as the only star for which we saw variability beyond measurement uncertainties. Dennihy et al. (2020, submitted) showcase the variability for this source across several epochs. As an ensemble, the known gas disk-hosting white dwarf stars have an average effective temperature of ≈18,500 K with a standard deviation of ±4500 K. There are four sources in the sample with effective temperatures ≤ one standard deviation from the mean: SDSS J0738+1835 (T eff ≈14,000 K; Dufour et al. 2012), SDSS J1617+1620 (T eff ≈13,500 K; Brinkworth et al. 2012), SDSS J0959−0200 (T eff ≈13,300 K; Farihi et al. 2012), and WD0145+234 (T eff ≈12,720 K, Table 1). SDSS J0959−0200 and SDSS J1617+1620 are known to host highly variable IRT emission line strengths. On intra-night to ∼few month temporal baselines WD0145+234 shows no evidence for variability (Figure B.5).
WD0145+234 narrowly finds itself as the new "coolest" gas disk-hosting white dwarf known; it is not clear what role (if any) the recent infrared "outburst" may have had in producing the detected gas disk. All of the cool gas disk stars named in the previous paragraph have so far been found to host only Ca II IRT emission lines. An examination of summed Ca II IRT emission line equivalent width as a function of host white dwarf T eff reveals considerable scatter at all effective temperatures. However there appears to be an upper envelope of maximum observed Ca II IRT equivalent widths of ≈50Å for hotter white dwarfs that is significantly lower (≈10-20Å) for white dwarfs with T eff <16,000 K. With only four sources known to date in this temperature range it is not clear if this is the result of observational bias or actual physical processes at play (e.g., disk heating from energetic photons as described in Melis et al. 2010).
Three-dimensional Galactic UVW space motions are computed and presented for all systems discovered through this work (Table A2). Additionally, space motions were adopted for Ton 345, SDSS J1043+0855, and SDSS J1228+1040 from Melis et al. (2010). Kinematic measurements for HE 1349−2305 from Melis et al. (2012) are used to calculate UVW: U=+3 km s −1 , V=−11 km s −1 , W=−4 km s −1 . As a whole for the class of gas disk single white dwarf stars, we see consistently negative Galactic V velocities, similar to V velocities for white dwarfs with and without atmospheric pollution (see Table 6 of Zuckerman et al. 2003 and Table 4 of Zuckerman et al. 2010). As such, gas disk-hosting white dwarfs do not appear to be kinematically "special" compared to other white dwarf stars and join them in having space motions characteristic of an old population trailing the local standard of rest as they orbit around the Galactic Center.
Within the sample of new emission line stars presented herein we see a distribution of host star primary atmospheric types (hydrogen or DA, helium or DB) of ≈78% DA and 22% DB. For the 17 total gas disk-hosting systems now known, this distribution is 71% DA and 29% DB. Both are a reasonable match to what is observed in the field population of white dwarf stars (e.g., Kilic et al. 2020 and references therein). As such, it is safe to conclude there is no bias for gas disks to be present around DB or DA white dwarfs.
Previously identified gas disk white dwarfs have, with the exception of SDSS J1228+1040 and WD J091405.30+191412.25, typically only been seen to host emission from calcium and sometimes iron. SDSS J1228+1040 saw the additional identification of oxygen and magnesium (Manser et al. 2016b) while WD J091405.30+191412.25 hosts more unusual emission dominated by hydrogen, oxygen, and sulfur (the likes of which are due to accretion from a giant planet-like companion; Gänsicke et al. 2019). Two-thirds of the sample presented herein show at least oxygen, calcium, and iron while a small subset additionally show magnesium and silicon (Table 2). Neutral silicon emission from the 3905Å transition has never been documented before; we detect it unambiguously in the spectra of Gaia J0611−6931 and WD0842+572. Magnesium may be more prevalent than indicated by Table 2 as its neutral triplet transitions near 5170Å are typically confused with iron emission in that region. The Mg I λ8806 line appears to be comparably strong from Gaia J0611−6931 (Figure B.19) and SDSS J1228+1040 (Manser et al. 2016b) and is not blended with other lines. Unfortunately, the HIRES red setup used for this study did not cover the 8806Å region (it fell in an order gap). Gaia J0611−6931 is unique in the identification of sodium in its emission spectrum. The lack of hydrogen and other volatiles, combined with the detection of elements typically found in rocky minerals (both in emission and absorption), points to the gas originating in the destruction of rocky bodies. Future atmospheric pollution analyses and emission line modeling will reveal the elemental composition of the material producing the gas and being accreted by each host white dwarf. As noted in Melis et al. (2010), emission line profiles observed at high spectral resolution exhibit a range of complex structures (e.g., Figure 1). It is tempting to attribute the range of observed line widths for gas disk systems to different disk inclination angles. For example, WD0842+572 could be viewed nearly face-on while SDSS J0006+2858 could be viewed closer to edge-on (Tables  B9 and B3). However, without performing detailed physical modeling for the gas and/or dust disks in these systems, it is not possible to conclusively assign viewing geometries and thus arrive at gas disk parameters like inner and outer radii (e.g., as in Melis et al. 2010). Emission line modeling that reproduces the observed line intensities and shapes will be necessary to arrive at robust gas disk physical parameters (e..g, Gänsicke et al. 2019). Despite this, a speculative interpretation for the unusual characteristics seen in WD0842+572 is worth mentioning. The combination of its narrow line profiles, abundance of neutral emission line species, and presence of several semi-forbidden transitions could be seen as evidence for a face-on viewing angle. The nearly zero sini reduces the maximum velocity emission seen to its small (relative to other gas disk systems) value, while the presence of neutrals and semi-forbidden emission lines come from primarily seeing the lower-density region of an optically thick disk's upper atmosphere.
Variability in gas disk emission lines has been noted for several systems in the literature. Manser  Melis et al. (2012) to a blue-dominated peak structure and back to red-dominated again; a period of 1.4 years is estimated. Wilson et al. (2015) and Manser et al. (2016a) show variability in line profiles for Ton 345.
The above, typically more exhaustive, studies showcase a wide range of variability behaviors for gas disk emission systems. Variability in Gaia J2100+2122 (Table B11 and Section 4.9) and possibly in WD0842+572 (Table B9 and Section 4.7) are presented herein while Dennihy et al. (2020, submitted) showcase more extensive observations of SDSS J0347+1624 and Gaia J2100+2122. An interesting tiein to gas disk line variability comes from the recent explosion in dust disk variability studies. Xu & Jura (2014) showed that SDSS J0959−0200 experienced a dramatic drop in its infrared dust emission levels while Farihi et al. (2018) found similar behavior combined with re-brightening episodes for GD 56 (a star that does not host a known gaseous debris disk component). Wang et al. (2019) documented the "outburst" captured by the WISE satellite for WD0145+234. Swan et al. (2019Swan et al. ( , 2020) provide a more systematic study of infrared variability in white dwarf disk systems (those with dust only and both dust and gas) finding that gas disk-hosting systems as a class show more variability in their dust continuum emission than non-gas disk-hosting stars. They interpret this finding within the context of circumstellar material having a range of collisional activities, gas disk systems being the more collisionally active cousins of the dust-only systems.  Manser et al. (2016b) show that this can evolve into a maximum blue extent of −390 km s −1 and maximum red extent of +780 km s −1 (it is not clear if these are ever achieved at the same time). SDSS J0006+2858 simultaneously hosts a maximum blue extent for its gas emission of −800 km s −1 and maximum red extent of +400 km s −1 over the time observations were collected for it in this work (Figure 2 and Table B3). Similar to that found here for SDSS J0006+2858, elements other than calcium in the spectrum of SDSS J1228+1040 also show the same asymmetry indicating well-mixed atomic species (Manser et al. 2016b).
The only other system to show pronounced red/blue asymmetry similar to SDSS J0006+2858 and SDSS J1228+1040 is HE1349−2305. This star had in 2011 red and blue extents similar to that measured for SDSS J0006+2858 here (Melis et al. 2012). Dennihy et al. (2018) do not measure maximum red and blue gas extents in their monitoring study of HE1349−2305. Beyond these stars, possible asymmetries in maximum gas velocities may also be seen in Gaia J0510+2315 (Table B6) (Table B8). Thus, 6 out of 17 total gas disk systems show evidence for asymmetric line profiles. It is not clear at this time if all such systems cycle through phases of exhibiting these asymmetries or if an era of asymmetric line extents is part of an evolutionary sequence that ends in fully symmetric and stable gas emission lines.
Starting with the discovery of single white dwarf stars with gaseous debris disks, it was recognized that these systems are likely host to eccentric gas rings (Gänsicke et al. 2006(Gänsicke et al. , 2007(Gänsicke et al. , 2008Melis et al. 2010). This interpretation continued with the addition of new data sets and systems (e.g., Wilson et al. 2015;Manser et al. 2016b;Manser et al. 2016a;Manser et al. 2019). Indeed, an eccentric gaseous ring component has also been inferred for the transiting planetesimal system WD1145+147 (Cauley et al. 2018;Fortin-Archambault et al. 2020). Asymmetric gas emission line structures bolster such interpretations. Disk evolutionary models suggest that eccentric debris systems may reflect an early evolutionary state and that the material will eventually settle into a more circularized orbit (e.g., Veras et al. 2014;Nixon et al. 2020).
In conclusion, we present a set of nine new gas disk-hosting single white dwarf stars. Some properties of these systems are similar to previously discovered gas disk systems, while some display new and exotic features. Of special note is that only four of the known gas disk systems are found south of the celestial equator, and of those only two are below a Decl. of −5 • . This likely implies a significant number of southern-hemisphere gas disk-hosting white dwarf stars remain to be discovered (at least 10 to balance the current population).
C.M. and B.Z. acknowledge support from NSF grants SPG-1826583 and SPG-1826550. B.K. acknowledges support from the APS M. Hildred Blewett Fellowship. We thank E. Dennihy for useful discussion. Research at Lick Observatory is partially supported by a generous gift from Google. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. We acknowledge that Keck Observatory rests on land that is important and significant to the Hawaiian Natives in many ways. Similarly, we acknowledge that Lick Observatory resides on land traditionally inhabited by the Muwekma Ohlone Tribe of Native Americans. This research has made use of NASA's Astrophysics Data System, the National Institute of Standards and Technology Atomic Spectra Database Lines Form, the SIMBAD database, and the VizieR service.  An individual epoch of Ca II IRT emission for each of the nine white dwarfs discovered in this work to host a gas disk. In all cases, spectra are continuum normalized. Wavelengths for HIRES data are in vacuum and shifted to the heliocentric reference frame. Kast spectra have wavelengths presented in air and are not shifted to the heliocentric reference frame. MagE spectra have wavelengths in air and are shifted to the heliocentric reference frame. In some cases the HIRES spectra are smoothed with a 5-or 11-pixel boxcar for display purposes. More complete spectra and multiple epochs for each star are presented in Appendix B.  Velocity-space plots with representative emission lines for each element detected for SDSS J0006+2858 (left) and Gaia J0510+2315 (right). Continuum levels have been fit and divided into each HIRES spectral segment shown and individual segments offset by an additive constant for purposes of clarity. A velocity of 0 corresponds to the calculated systemic velocity for each star (Section 3.2 and Table A2). Blue vertical dashed lines give the measured average maximum gas velocities seen in the blue and red wings of emission lines (Section 3.2 and Tables B3 and B6). Clear velocity asymmetry is seen in the red and blue wings of all elements for SDSS J0006+2858 while more subtle velocity asymmetry is seen for Gaia J0510+2315. Extended red-wing emission in the O I λ8446 complex for SDSS J0006+2858 is due to a blend of multiple lines, the range delineated by the vertical dashed lines is centered on the lowest wavelength transition in the multiplet. For Gaia J0510+2315, O I lines near 7772 and 8446Å are significantly more extended than other emission lines (see Section 4.4). Note-Abundances are quoted relative to the primary atmospheric constituent, hydrogen (H) or helium (He). The primary atmospheric constituent is indicated in the "Primary Atm. Const." column. A "−" in the abundance column means no metal absorption lines are seen in the Kast spectra.
a Hydrogen is also detected in the atmospheres of GaiaJ0644−0352 and WD1622+587; we obtain log 10 (H/He)=−5.1 and log 10 (H/He)=−4.8 respectively.  Note-For each star and transition, a "−" means no line was seen in any epoch, "w" means a weak transition is seen, "Y" means a clear emission line is seen, and "?" means this wavelength was not covered by our observations or lies within a strong transition that prevents conclusive analysis. SDSS J0347+1624 was only observed with the Kast, all other stars have higher resolution data products.

Neutral Iron
Fe I]   Note-Resolving powers are derived from FWHM measurements of arclines in comparison spectra. HIRES wavelength coverage is quoted as continuous, but there are gaps in coverage between each of the three CCDs and sometimes between red orders. Note-"RV meas" is the average velocity measured for individual metal and hydrogen lines in high resolution data (see Section 3.2). "Grav. Redshift" is derived from the atmospheric parameters produced from models reported in Table 1. "Syst. Vel." is the calculated systemic velocity for each white dwarf star. We do not report nor propagate uncertainties for the gravitational redshift which could be up to 15 km s −1 . UVW space motions are reported relative to the Sun such that positive U is towards the Galactic center, positive V is in the direction of Galactic rotation, and positive W is toward the north Galactic pole (Johnson & Soderblom 1987).
a For the Gaia DR2 parallax uncertainty we add in quadrature any contribution from excess astrometric noise.   Iron and possibly magnesium emission line region for SDSS J0006+2858. For this and all remaining figures for SDSS J0006+2858, the continuum levels have been fit and divided into each spectrum. Hβ at 4863Å is cut out of each spectrum for plotting purposes. The HIRESb spectrum contains a gap between CCDs centered around 5025Å. HIRES data are smoothed with an 11-pixel boxcar for display purposes. Weak emission from an Fe I line near 5052Å may be present. It is possible that there is emission from Mg I between 5150-5190Å, but a clear identification is prevented due to dominating emission from Fe II in the same region. Highly structured emission features are seen in the HIRES spectra, including a "third" peak on the blue shoulder of the emission feature; this "third" peak appears to also be present in the Kast epoch.  Ca II IRT spectra for WD0145+234. Continuum levels have been fit and divided into each spectrum for this figure. Intra-night epochs taken with the Kast are marked as "2019-11-18-v#" for three observations taken throughout the night of UT 2019 November 18; no obvious variability is seen between these or other epochs. Iron, oxygen, and magnesium emission region for Gaia J0510+2315. O I emission is significantly broader than that measured for all other emission lines for this star (Table B6 and Figure 2). Mg II emission is contaminated by strong telluric absorption features. An unidentified emission feature is seen near 7506Å; this region of the spectrum is not covered by HIRES as it falls in between detectors. An inter-order gap also appears in the HIRES spectrum near 7850Å. Strong telluric absorption near 7600Å has been masked.        Iron and magnesium emission line region for WD0842+572. Based on the HIRESr spectrum, there may be numerous weak unidentified emission lines between 5200-5300Å. Spectra continuum levels have been fit and divided into each spectrum. Absorption lines from Fe and the Mg I triplet are apparent.  Iron emission region for WD1622+587. For all WD1622+587 figures the spectrum continuum levels have been fit and divided into each displayed spectrum. The spectrum shown here is smoothed with an 11-pixel boxcar for display purposes.   Figure B.30. Iron emission region for Gaia J2100+2122. Hβ is cut out of each spectrum for plotting purposes, HIRESb data also feature a gap between CCDs centered around 5025Å. For this and all figures for Gaia J2100+2122 the spectra continuum levels have been fit and divided into each spectrum displayed. The HIRES spectra in this figure are smoothed with an 11-pixel boxcar for display purposes. Si II photospheric absorption lines appear near 5040 and 5055Å. Another iron emission region for Gaia J2100+2122. Lines near 6382 and 6515Å are unidentified; they may be due to Fe. The HIRES spectra are smoothed with a 7-pixel boxcar and feature an order gap near 6420Å. Strong photospheric Si II absorption appears near 6350 and 6370Å; other absorption lines are from Earth's atmosphere.  Note-See Section 3.2 for descriptions of Peak Separation, vmaxsini, and Full Width and when these measurements can be made. The two different values reported for vmaxsini correspond to the maximum velocity gas seen in the blue and red wings of the emission features, respectively. Full velocity width of the emission feature is the velocity extent from the blue to the red wings and is only measured when a line is not blended with another transition.    Table B3.