# Using 3.4-$\bm{\mu}$m Variability towards White Dwarfs as a Signpost of
Remnant Planetary Systems

Joseph A. Guidry NSF Graduate Research Fellow  Department of Astronomy, Boston
University, Boston, MA 02215, USA  Institute for Astrophysical Research,
Boston University, Boston, MA 02215, USA  J. J. Hermes Department of
Astronomy, Boston University, Boston, MA 02215, USA  Institute for
Astrophysical Research, Boston University, Boston, MA 02215, USA  Kishalay De
NASA Einstein Fellow  MIT-Kavli Institute for Astrophysics and Space Research,
Cambridge, MA 02139, USA  Lou Baya Ould Rouis Department of Astronomy, Boston
University, Boston, MA 02215, USA  Institute for Astrophysical Research,
Boston University, Boston, MA 02215, USA  Brison B. Ewing Department of
Astronomy, Boston University, Boston, MA 02215, USA  B. C. Kaiser Department
of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel
Hill, NC 27599, USA

(Received April 18, 2024; Revised June 21, 2024; Accepted June 24, 2024)

###### Abstract

Roughly 2% of white dwarfs harbor planetary debris disks detectable via
infrared excesses, but only a few percent of these disks show a gaseous
component, distinguished by their double-peaked emission at the near-infrared
calcium triplet. Previous studies found most debris disks around white dwarfs
are variable at 3.4 and 4.5 $\mu$m, but they analyzed only a few of the now 21
published disks showing calcium emission. To test if most published calcium
emission disks exhibit large-amplitude stochastic variability in the near-
infrared, we use light curves generated from the unWISE images at 3.4 $\mu$m
that are corrected for proper motion to characterize the near-infrared
variability of these disks against samples of disks without calcium emission,
highly variable cataclysmic variables, and 3215 isolated white dwarfs. We find
most calcium emission disks are extremely variable: 6/11 with sufficient
signal-to-noise show high-amplitude variability in their 3.4-$\mu$m light
curves. These results lend further credence to the notion that disks showing
gaseous debris in emission are the most collisionally active. Under the
assumption that 3.4-$\mu$m variability is characteristic of white dwarfs with
dusty debris disks, we generate a catalog of 104 high-confidence near-infrared
variable white dwarfs, 84 of which are published as variable for the first
time. We do near-infrared spectroscopic follow-up of seven new candidate
3.4-$\mu$m variables, confirming at least one new remnant planetary system,
and posit that empirical near-infrared variability can be a discovery engine
for debris disks showing gaseous emission.

White dwarf stars (1799) — Debris disks (363) — Variable stars (1761) —
Exoplanet systems (484) — Infrared excess (788) — Transient detection (1957) —
Collisional processes (2286)

††journal: ApJ††facilities: WISE, NEOWISE, Gaia, LDT, SOAR, TESS, ASAS-
SN††software: Astropy (Astropy Collaboration et al., 2013, 2018), lmfit
(Newville et al., 2014), Matplotlib (Hunter, 2007), NumPy (Harris et al.,
2020) pandas (pandas development team, 2020), PypeIt (Prochaska et al., 2020),
SciPy (Virtanen et al., 2020), seaborn (Waskom, 2021), TOPCAT (Taylor, 2005),
SIMBAD and VizieR (curated by CDS, Strasbourg, France), and the NASA
Astrophysics Data System (ADS).

##  1 Introduction

More than 30% of all white dwarf stars are likely accreting close-in
circumstellar debris disks (Zuckerman et al., 2003, 2010; Koester et al.,
2014; Ould Rouis et al., 2024) composed of planetary bodies disrupted by tidal
forces (Jura, 2003). These debris disks are most commonly inferred from the
observation of metals polluting the otherwise pure hydrogen or pure helium
white dwarf photosphere. Mass fractions of these pollutants are largely
consistent with a rocky composition similar to bulk Earth (Xu et al., 2014; Xu
& Bonsor, 2021). Metal pollution is interpreted as a signpost for the presence
of a remnant planetary system bound to the white dwarf (see Veras, 2021, and
references therein).

Dust liberated from the disruption of these rocky bodies via ensuing grind-
down from planetesimal collisions (e.g., Malamud et al. 2021; Brouwers et al.
2022) can be detected as infrared emission in excess to the white dwarf
photosphere. Roughly 2% of white dwarfs exhibit a detectable infrared excesses
indicative of warm, dusty debris disks (Rocchetto et al., 2015; Wilson et al.,
2019; Rebassa-Mansergas et al., 2019). This smaller detection rate could be a
consequence of needing near-face-on viewing geometries if these disks tend to
be flat. Regardless, all white dwarfs with dusty disks show photospheric metal
pollution (see Farihi 2016 and references therein). These infrared excesses
peak at mid-infrared wavelengths ($\gtrsim 5\,\mu$m) and are challenging to
observe from the ground; white dwarfs with infrared excesses peaking in the
near-infrared ($\lesssim 5\,\mu$m) are more likely to host to an unresolved,
cool stellar or brown dwarf companion (e.g, Farihi et al. 2005; Girven et al.
2011; Dennihy et al. 2020b).

At the innermost radii of the debris disk, the temperatures should be
sufficient to sublimate the debris into gaseous states. This gas can be viewed
in absorption (e.g., Debes et al., 2012; Xu et al., 2016; Steele et al., 2021)
and as broad, double-peaked emission, mainly at the calcium triplet (Gänsicke
et al., 2006, 2007, 2008; Gänsicke, 2011; Farihi et al., 2012; Melis et al.,
2012; Wilson et al., 2014; Melis et al., 2020; Dennihy et al., 2020b; Gentile
Fusillo et al., 2021a). The latter class of “gaseous” debris disk systems are
observationally rare: Manser et al. (2020) calculate the occurrence rate of
white dwarfs showing emission at the Ca II triplet to be $\approx$0.1%.
Notably, all 21 published gas disk systems are metal polluted (Gentile Fusillo
et al., 2021a) and most show infrared excesses (see Farihi 2016 and references
therein; Gentile Fusillo et al. 2021a). Many of the white dwarfs hosting
gaseous disks show heavily polluted photospheres, though this circumstellar
gas so far does not appear to enhance accretion rates via drag effects (Rogers
et al., 2024a). This class of disks showing gaseous calcium is distinct from
cases like that of WD J0914+1914, which harbors a gas-only, dustless disk
fueled by the evaporation of a volatile rich atmosphere of a giant planet, and
not a disrupted rocky body (Gänsicke et al., 2019).

Throughout the remainder of this paper we adopt a nomenclature of disks
“showing calcium (Ca II) gas in emission” and “non-calcium emission (non-Ca
II)” disks to distinguish the dusty debris disks that have previously been
observed to show a gaseous component in emission at the calcium II triplet in
the near-infrared, since gaseous and dusty debris are believed to coexist in
all of these planetary debris disks.

Among a sample of 35 non-Ca II dusty white dwarfs observed by the Wide-field
Infrared Survey Explorer (WISE), Swan et al. (2019) found that the majority
exhibit significant photometric variability in the near-infrared at 3.4
$\mu$m. Ensemble variability was again observed by Swan et al. (2020) among a
sample of over 40 white dwarf debris disks with Spitzer at 3.4 $\mu$m and 4.5
$\mu$m.Within this sample, a few Ca II gas disk systems have shown
exaggerated, high-amplitude variability (Swan et al., 2020). This includes
factor-of-two brightenings in the near-infrared seen towards WD 0145+234 (Wang
et al., 2019, Figure 1 Panel a), interpreted as an expansion of the emitting
area of the dust disk due to collisions (Swan et al., 2021), which in turn
should liberate gas. It also includes dimmings, like those observed with
Spitzer towards WD J0959$-$0200 (Xu & Jura, 2014), SDSS J1228+1040 (Xu et al.,
2018), and GD 56 (Farihi et al., 2018). Rogers et al. (2020) did not observe
ensemble variability in the $J$-, $H$-, and $K$-bands over a three-year
baseline among dusty white dwarfs.

Figure 1: WISE light curves for $W1$ (3.4 $\mu$m, blue squares) and $W2$ (4.6
$\mu$m, red circles) representing the four most common trends seen in our
sample. (a) Significant, stochastic variability, such as the outbursting Ca II
gas disk towards the previously studied WD 0145+234 (Swan et al., 2021). (b) A
flat light curve representing the expectation for an isolated, unchanging
white dwarf (LAWD 52). (c) A light curve showing a strong alternating pattern
in both the $W1$ and $W2$ bands caused by the different scanning angles of the
PSF from each 6-month WISE observing season, which tends to be exacerbated
when there are nearby stars to the target (PG 1411+219). The solid- and open-
faced points indicate the two clusters used to calculate
$\chi^{2}_{\mathrm{alt}}$ (see text). (d) A secular brightening or dimming
trend caused by the proper motion of the white dwarf (GJ 3770) moving into (or
away from) another star. The dotted black lines at 2012 demarcate the
transition from cryogenic to NEOWISE mission data; we only analyze the NEOWISE
epochs for our variability rankings. Blue dashed and red dashed-dotted lines
show the fits used to calculate $\chi^{2}_{W1}$ and $\chi^{2}_{W2}$,
respectively (see Section 3 for definitions of the annotated $\chi^{2}$
values). Uncertainties are shown but are generally smaller than each symbol.
The average $W1$ uncertainties are plotted in the upper right corner of each
panel.

We explore here the 3.4-$\mu$m variability of all 21 published Ca II gas disk
systems in ensemble for the first time. We aim to test whether they exhibit
stochastic variability when bright enough to be confidently detected in WISE.
We generate light curves using a new aperture photometry pipeline that
corrects for the proper motions of targets as it operates on the unWISE images
from the NEOWISE era (2013–2023). We are motivated to test if empirical
3.4-$\mu$m variability can be used as an engine for discovering more disks
showing Ca II emission around white dwarfs.

In Section 2 we describe our light curve generation pipeline and follow-up
observations. In Section 3 we detail our variability ranking and validation
method. In Section 4 we report our catalog of 3.4-$\mu$m variables and
characterize the Ca II disks relative to non-Ca II disks. We end by discussing
the consequences of our work in Section 5. Our catalog of 3.4-$\mu$m variables
is published in Appendix A.

Figure 2: Top: ASAS-SN light curve (Shappee et al., 2014; Kochanek et al.,
2017; Hart et al., 2023) of the cataclysmic variable KR Aur (Gaia DR3
3436435910858051200). Observations shown as orange diamonds were taken to
within 3 days of the WISE epochs, while the remaining open-faced points show
the full $g$\- and $V$-band light curve. Bottom: Our WISE light curve of KR
Aur showing a correlation between the optical and near-infrared variability.

##  2 Observations

###  2.1 New unWISE Light Curve Extractions

Our study utilizes images from the unWISE individual-image
catalog111https://catalog.unwise.me/catalogs.html captured by the all-sky
survey of the Wide-field Infrared Survey Explorer (WISE, Wright et al., 2010),
including the initial cryogenic mission and the ongoing NEOWISE mission
(Mainzer et al., 2011, 2014). The unWISE catalog features coadded images of
typically 12$\times$7.7 s exposures (Meisner et al., 2018), boosting
photometry reliability to magnitudes of $W1\sim 18$ mag (Schlafly et al.,
2019). The images are 2048$\times$2048 pixels at a scale of 2.75$\arcsec$ per
pixel (Lang, 2014), although the typical image resolution from the PSF is
roughly 6.1″ and 6.3″ in the $W1$ (3.4 $\mu$m) and $W2$ (4.6 $\mu$m) channels,
respectively. For a thorough description of the unWISE catalog see Schlafly et
al. (2019), and for a review of the image processing and generation see Lang
(2014) and Meisner et al. (2017a, b). We use the unWISE images from the 2023
NEOWISE Data Release.

Our pipeline operates on the single coadds for each epoch, treating the $W1$
and $W2$ images separately. Another version of this pipeline uses difference
imaging for transient work in WISE (Tran et al., 2024; Masterson et al.,
2024). Here instead we perform simple aperture photometry, using a fixed
circular 2-pixel-radius aperture (equivalently 5.5″) centered on the
coordinates of the target to match the WISE PSF. We also draw an annulus with
an inner radius of 5 pixels and width of 3 pixels for background subtraction.
Importantly, we propagate the coordinates of each target using the Gaia DR3
measured right ascension, declination, and proper motions to the epoch of the
WISE observation, rather than doing source detection from epoch-to-epoch like
unTimely (Meisner et al., 2023). Representative light curves are shown in
Figure 1. The proper motions of field stars are not included, nor are the
parallaxes of any sources. We also generate light curves extracted by PSF
photometry on the unWISE images using the same procedures.

We estimate uncertainties on the photometry using the uncertainty images
provided by WISE and further processed by unWISE that map uncertainties to the
pixel-level (Lang, 2014). Objects varying on timescales of seconds generate
greater uncertainties due to the larger image-to-image deviations. A full
detailed account of this pipeline will be presented in a forthcoming
publication, K. De et al. (in prep).

###  2.2 Input White Dwarf Catalog

We construct a sample for our study that includes all white dwarfs currently
known to harbor gaseous debris disks and various control samples to assess the
variability of the Ca II gas disks. The 21 Ca II gas disk systems are sourced
from Table 3 of Gentile Fusillo et al. (2021a). Our first control sample is
one of 45 white dwarfs with known infrared excesses due to dusty debris disks
that have not shown calcium triplet emission: 28 from the Montreal White Dwarf
Database (Dufour et al., 2017), seven systems not in MWDD from Rogers et al.
(2020), and 11 more unique systems from Wang et al. (2023). We would go on to
find two additional known dusty disks in the Gaia-catWISE input described
below after ranking all systems by their variability: KPD 0420+5203 (Gaia DR3
271992414775824640) (Hoard et al., 2013; Barber et al., 2016) and MCT
0420$-$7310 (Gaia DR3 4653404070862114176) (Hoard et al., 2013; Dennihy et
al., 2017), yielding an input of 47 objects.

Our second control is a sample of 14 known cataclysmic variables (CVs) taken
from the American Association of Variable Star
Observers222https://targettool.aavso.org/TargetTool/default/index with Gaia
G-band magnitudes of $13\leq G\leq 18$ mag, three recently discovered polars
(Guidry et al., 2021; Rodriguez et al., 2023a) and one AM CVn (Rodriguez et
al., 2023b) from the Zwicky Transient Facility. The CV sub-sample is meant to
be a test to the pipeline, as CVs are known to show both optical and IR
outbursts (e.g., Petrosky et al., 2021); correlated optical and infrared
variability of KR Aur, a known CV, is shown in Figure 2.

We build a third sample from a cross-match between the Gaia eDR3 catalog of
white dwarfs (Gentile Fusillo et al., 2021b) and the catWISE catalog of WISE-
detected sources (Eisenhardt et al., 2020). We intend to search for highly
variable white dwarfs within this sample to identify candidate gas disks, as
well as contextualize the variability of the known debris disks against that
of isolated white dwarfs. We first cut the Gaia white dwarf catalog to 100 pc
and impose $P_{\mathrm{WD}}$ $>0.75$ to define a sample of high-confidence,
isolated white dwarfs, eliminating most binary sources like CVs or unresolved
WD+MS, post-common-envelope binaries. We require all sources be brighter than
$G_{\rm RP}<17.0$ mag, and cross-match with catWISE, requiring the $W2$
detections to have a signal-to-noise greater than 5.0 and the $W1$ magnitudes
be within 1.5 mag of the Gaia $RP$ magnitude. This yields a final source
catalog of 3215 white dwarfs, including multiple duplicates from the above Ca
II and non-Ca II disks and CV sub-samples. Aggregating all these samples, we
build an input catalog of 3299 white dwarfs.

###  2.3 Near-Infrared Spectroscopic Follow-up

####  2.3.1 LDT+DeVeny

Our primary motivation for ranking white dwarfs by their WISE variability is
to build a catalog of candidate remnant planetary systems, especially those
with the highest potential to host collisionally active debris disks. We began
a preliminary follow-up campaign to test if these candidates show emission at
the calcium triplet using the DeVeny Spectrograph (Bida et al., 2014) at the
4.3-m Lowell Discovery Telescope (LDT) at Happy Jack, Arizona, USA.

For our LDT+DeVeny observations we use a 1.0″ slit paired with the DV8 grating
(831 line/mm) centered at a blaze wavelength of 8000 Å with the GG 570 order-
blocking filter, yielding a resolution of roughly $0.8$ Å across $7200-8800$
Å. We publish observations here from 2023 June 20 and 2024 February 5. We
provide a log of our LDT+DeVeny observations in Table 5 in Appendix B.

We reduced our spectra using the PypeIt reduction pipeline (Prochaska et al.,
2020), performing a telluric correction directly on our observations using a
third-order polynomial model to estimate the continuum.

####  2.3.2 SOAR+Goodman Follow-up

We also followed up some of our southern candidates using the red camera of
the Goodman Spectrograph (Clemens et al., 2004) mounted on the 4.1-m Southern
Astrophysical Research Telescope (SOAR) at Cerro Pachón, Chile on 2023 August
13 and 2023 August 18. Building on the observations of Ca II gas disks done by
Dennihy et al. (2018, 2020b), we use the 1200 line/mm grating with the 1.0″
slit and GG495 order-blocking filter, giving us spectral coverage from roughly
$7570-8750\,$Å with a resolution of roughly $0.6$ Å. Alongside the LDT+DeVeny
spectra, a log of our SOAR observations can be found in Table 5 in Appendix B.

We also reduce our SOAR spectra using PypeIt, again performing a telluric
correction on our spectra anchored to a third-order polynomial over the region
observed.

##  3 Variability Detection and Validation

We first process each WISE light curve through quality filtering before
assessing it for variability, handling the $W1$ and $W2$ observations
independently. We reject erroneous measurements (zero flux detections) and
require both the $W1$ and $W2$ flux uncertainties be $>1\,\mu$Jy. We further
perform a $3\,\sigma$ clip to the flux uncertainties to exclude spuriously
small values that would skew a $\chi^{2}$ calculation. Finally we require the
signal-to-noise ratio of each epoch be S/N$>$3.0. We then convert the WISE
Vega magnitudes to fluxes in Janskys using the published conversions:
$F_{\nu,0,W1}=309.540$ Jy, $F_{\nu,0,W2}=171.787$ Jy (Jarrett et al., 2011;
Wright et al., 2010). All analysis of the WISE light curves is hereafter
conducted in flux space.

Presuming an isolated white dwarf should be constant in infrared flux, we
assess the degree of variability for each object by the goodness of linear
(first-order polynomial) fits to each light curve. Keeping the $W1$ and $W2$
light curves independent, we fit lines to the NEOWISE-only ($2013-2023$)
portion of each light curve, allowing the slopes and offsets to vary.
Calibrations for the first few cryogenic epochs collected from $2009-2011$ can
cause flux offsets from the NEOWISE data (Meisner et al., 2017a). For every
object we record the reduced $\chi^{2}$ of each fit to each band, hereafter
referred to as $\chi^{2}_{W1}$ and $\chi^{2}_{W2}$, respectively. We do the
same to for an additional series of linear fits where the slope is fixed to be
zero. We designate these “flat” line fits to the $W1$ light curves as
$\chi^{2}_{\mathrm{flat}}$.

A common artifact seen in the light curves from WISE is an alternating set of
fluxes due to the rotation of the point-spread function (PSF) from the
different scanning angles of the spacecraft from epoch-to-epoch (Eisenhardt et
al., 2020). Panel c of Figure 1 shows an example of this variation,
alternating with the typical source revisit timescale of roughly 6 months.
This causes the source to exhibit both large $\chi^{2}_{W1}$ and
$\chi^{2}_{\mathrm{flat}}$ while not necessarily being intrinsically
astrophysically variable.

We define a second goodness-of-fit metric that analyzes each “high” and “low”
season independently. We use a one-dimensional $k$-means clustering
algorithm333https://github.com/dstein64/kmeans1d to sort each light curve
temporally, dividing the timestamps by the median separation of the whole
light curve, taking the modulo of twice that median revisit timescale to
convert the light curve points into two clusters in phase space. The cluster
with the higher average flux is classified as the “high” cluster and the other
the “low.” We fit an additional pair of lines to each cluster for each filter.
We calculate an effective reduced $\chi^{2}$ for these “high-and-low” seasons
as
$\chi^{2}_{\mathrm{alt}}=(\chi^{2}_{\mathrm{high}}+\chi^{2}_{\mathrm{low}})/\sqrt{2}$.
The comparison of $\chi^{2}_{\mathrm{alt}}-\chi^{2}_{W1}$ is shown in the top
panel of Figure 3. We de-prioritize objects with
$\chi^{2}_{\mathrm{alt}}-\chi^{2}_{W1}<0$, as they are more likely to have
flux changes due to the WISE scanning angle rather than some intrinsic
variability, as exhibited in Panel c of Figure 1.

Figure 3: Top: Excess Gaia $RP$ flux vs. the difference between the
“alternating” $\chi^{2}$ (see Panel c of Figure 1) and the $\chi^{2}_{W1}$
fits. Sources with $\chi^{2}_{\mathrm{alt}}-\chi^{2}_{W1}<0$ are likely to
have high $\chi^{2}_{W1}$ caused by the alternating pattern due to the
biannual variation in the WISE PSF scanning angle, rather than astrophysical
variability. Sources with 1.415 times more flux in the Gaia $RP$ channel in a
30″ cone than their own flux are frequently found in crowded fields, leading
to false-positive variability detections due to source crowding of the large
WISE PSF and proper motions like that in Panel d of Figure 1 due to proper
motions. We inflate y-axis values by $10^{-3}$ so that sources with zero
nearby flux are on-axis. Bottom: Our catalog of 163 3.4-$\mu$m variables
ranked by variability. Catalog inclusion requires
$\chi^{2}_{\mathrm{alt}}-\chi^{2}_{W1}>0$, $G_{\rm RP}$ excess flux
$<1.415\times$ the target flux, and $\chi^{2}_{W1}$ or
$\chi^{2}_{\mathrm{flat}}$ $>$ 2.0 (see text). The x-axis values are the
maximum value of $\chi^{2}_{W1}$ or $\chi^{2}_{\mathrm{flat}}$ for each
target. Objects with strong apparent contamination from visual inspection with
WISE View and/or still show a seasonal alternating pattern (see the Flag
column in Table 2) are shown as open-faced (only found for Gaia-catWISE white
dwarfs). The vertical dash-dotted line at $\chi^{2}=3$ marks our definition
for high-amplitude variability. In all panels, we distinguish known disks with
calcium emission as cyan stars, white dwarfs with dusty disks without calcium
emission as orange squares, and known CVs as green triangles.

We further attempted to quantify contamination by comparing the flux of our
target in the Gaia $G_{\rm RP}$ bandpass to the sum of all flux from objects
within a 30″ radius of the target. We find that the visually classified
contaminated objects suffer when the nearby sources sum to $>1.415\times$ the
$G_{\rm RP}$ flux of the target. We selected the factor-of-1.415 threshold
from a 1-d $k$-means sorting of the total $G_{\rm RP}$ fluxes from the 30″
cones into two clusters. The centroid of the cluster of lowest fluxes was
1.415. The top panel of Figure 3 shows objects marked as “likely crowded”
based on this analysis, and we discontinue consideration of these crowded
objects. We refer to this parameter as “$G_{\rm RP}$ Excess” in the tables in
Appendix A.

Our quality filtering removes 883 likely alternating sources (those with
$\chi^{2}_{\mathrm{alt}}$ $-$ $\chi^{2}_{W1}$ $<0$) and removes all likely
crowded sources (827 total) through our cut on the Gaia $G_{\rm RP}$ fluxes,
leaving 1590 objects in total, or about 48% of the original sample. Although
non-Gaussian, if these 1590 objects were normally distributed in
$\chi^{2}_{W1}$-space, the $3\,\sigma$ limit would be $\chi^{2}_{W1}$ $=2.3$.

We build a catalog variable white dwarfs as seen by WISE by ranking our sample
by $\chi^{2}_{W1}$, since signal-to-noise ratio is on average higher in the
$W1$ band. We report $\chi^{2}_{W2}$ values in the tables of Appendix A, but
do not incorporate them into our variability analysis. Having removed likely
alternating and crowded sources, we impose a third cut to yield a final
sample, keeping sources only with a $\chi^{2}_{W1}$ $>2.0$ or
$\chi^{2}_{\mathrm{flat}}$ $>2.0$ to include sources that are showing
potentially real, secular variations. Hereafter we abbreviate this ambiguity
to simply $\chi^{2}$. A cut at $\chi^{2}>2.0$ keeps the top 89th percentile of
the “non-alternating” and “uncrowded” sample. This final catalog, shown in the
bottom panel of Figure 3 and published in Table 2, contains a total of 163
unique sources.

We assess the fidelity of the unWISE images for all these candidate variables
and special samples (all the known gas disks, known infrared excess systems,
known CVs). The unWISE images have a relatively large ($>$6″) PSF full-width-
at-half-maximum (Wright et al., 2010), making source contamination another
major issue (Dennihy et al., 2020a). We visually vetted our catalog of
variables using the WISE View tool444http://byw.tools/wiseview to inspect each
unWISE image of each candidate centered on its J2000 coordinates. We look in
particular for the proper motion of targets bringing them towards or away from
a nearby field source in a manner correlated with the computed light curve. We
mark these objects as contaminated in Figure 3 and Table 2. Panel d of Figure
1 shows an example of a secular brightening of a white dwarf moving into a
bright background source. We also denote objects that still show a seasonal
alternating pattern as contaminated in Figure 3 and Table 2.

Table 1: Demographic Evolution of our Sample from the Original Input to our
Catalog of Variables.

Sample | Original Input | Pass Filtering | S/N $>10$ | Variable ($\chi^{2}$ $>2$) | Uncontaminated | Highly Variable ($\chi^{2}$ $>3$)
---|---|---|---|---|---|---
Disks with Ca II Emission | 21 | 13 | 11 | 6 | 6 | 6
Disks without Ca II Emission | 47 | 29 | 17 | 9 | 9 | 6
Cataclysmic Variables | 18 | 8 | 8 | 8 | 8 | 8
Gaia-catWISE | 3213 | 1540 | 900 | 140 | 81 | 10
Total | 3299 | 1590 | 936 | 163 | 104 | 30

Note. — Description of columns: From left to right we record the original
number of our input, the amount that pass our quality filtering, the amount
that have signal-to-noise$>$10 in our $W1$ light curves, the amount we find
variable ($\chi^{2}$ $>$2), the amount of variables that do not appear
contaminated upon closer examination, and finally the amount within our
catalog that show high-amplitude variability ($\chi^{2}$ $>$3).

##  4 Variability Analysis

Here we report the results of our variability ranking. In Section 4.1 we
breakdown our catalog of 3.4-$\mu$m variables. We reserve Section 4.2 for
analyzing the Ca II and non-Ca II debris disk systems in our catalog. We
showcase the results of our near-infrared spectroscopic follow-up in Section
4.3.

###  4.1 Catalog of 3.4-$\mu$m Variables

We build our catalog of candidate variables by selecting sources with either
$\chi^{2}_{W1}$ $>2.0$ or $\chi^{2}_{\mathrm{flat}}$ $>2.0$ (top 11% most
variable), which includes 163 unique white dwarfs. We break down in Table 1
the demographics of our catalog relative to the original inputs. Our catalog
is tabulated in full in Table 2. Among these 163 we find 104 uncontaminated
sources that we believe to be high-confidence variables at 3.4 $\mu$m, 84 of
which are published as near-infrared variables for the first time based on our
literature search.

The known CVs are by far the most variable in our catalog: all eight are
within the top 90th or higher percentile of our variable catalog, and all have
$\chi^{2}$ $>20$. None of these eight show evidence for having contaminated
photometry. The Ca II gas disk systems also show exceptionally large-amplitude
variability. We defer characterization of their variability and that of the
dusty disks to Section 4.2.

We search for optical variability within our catalog to offer some
characterization. We report the excess_flux_factor (abbreviated as EFF in the
tables of Appendix A) for all these 3.4-$\mu$m candidate variables in Table 2,
a metric that reliably quantifies empirical variability from Gaia photometry
(Gentile Fusillo et al., 2021b). We also use astroquery (Ginsburg et al.,
2019) to download any 20-second- and 2-min-cadence PDCSAP light curves
captured by the Transiting Exoplanet Survey Satellite (TESS, Ricker et al.
2015) of our candidate variables. We report the maximum period and associated
amplitude from the 2-min light curves if it is significant to a false-alarm
threshold of 0.1% in Table 2. We use the tool TESS_localize to confirm with
$>$99% likelihood that the signal is coming from our often-crowded white
dwarfs (Higgins & Bell, 2023).

Beyond the eight previously known CVs, we recover 10 additional known variable
white dwarfs in our final sample. This includes the known AM CVn G 61$-$29
(Gaia DR3 3938156295111047680, Nather et al. 1981), eight pulsating white
dwarfs (see Table 2), and one spotted magnetic white dwarf (WD 1953$-$011,
Gaia DR3 4235280071072332672, Brinkworth et al., 2005; Valyavin et al., 2011).
Like the CV KR Aur shown in Figure 2, these recoveries further confirm that
our method returns real astrophysical variables.

At optical wavelengths, 34 of the 163 sources in our catalog are flagged in
Gaia DR3 as photometrically variable (Eyer et al., 2023). Within these 34 are
all eight CVs, one infrared excess system that is a known ZZ Ceti (Gaia DR3
1641326979142898048), and the gas disk system WD 0145+234. While WD 0145+234
has both a negative varindex (Guidry et al., 2021) and an
excess_flux_error$<$1 (Gentile Fusillo et al., 2021b), suggesting it is not
variable over long baselines, the flagging by Eyer et al. (2023) indicates it
may show optical variability on some timescale.

Most of the white dwarfs in our catalog of 3.4-$\mu$m variables do not show
coherent periodic variability in the optical as seen by TESS. We find only 13
of our 163 sources in our catalog to show a significant period intrinsic to
white dwarf as determined by TESS_localize. We stress these fractions cannot
be compared exactly, given the differences in cadence and baselines. Seven of
these 13 are known pulsating white dwarfs, including KX Dra (Gaia DR3
1641326979142898048, Vauclair et al., 2000) and WD 1150$-$153 (Gaia DR3
3571559292842744960, Gianninas et al., 2006), a known non-Ca II gas debris
disk, and four are known CVs. One of these 13 objects is Gaia DR3
3150770626615542784, which shows a significant periodicity at $200.31\,$s at
an amplitude of 0.69% over its 20-second light curves in Sectors $44-46$.
Vincent et al. (2020) targeted this object because of its position in the ZZ
Ceti instability but did not observe it to vary. We believe these TESS
observations confirm it as a new pulsating white dwarf. Three of these 13
appear to show contaminated WISE light curves (see below). We highlight all of
these TESS variables in Table 2 by including the period and associated
amplitude of variability. Lastly, we reject that WD 0420+520 is a candidate
eclipsing binary (Prša et al., 2022). Our TESS_localize analysis points
towards Gaia DR3 271992483495298688 as the source of the apparent variability.

Figure 4: WISE light curves of the white dwarfs with known Ca II gas disks in
our variability catalog (all discovered in Melis et al., 2020, unless noted
otherwise): a LAMOST J0510+2315, b WD 0842+572, c SDSS J0347+1624, d WD
1622+587 (and Dennihy et al., 2020b), e WD J0529$-$3401 (Gentile Fusillo et
al., 2021a). Each shows extreme variability: all have $\chi^{2}>6$ and peak-
to-peak flux changes in $W1$ $>20$%. None of these systems have had their
infrared light curves characterized before. We show Gaia J0611$-$6931 (and
Dennihy et al., 2020b) in Panel F. Despite its exclusion from our catalog for
possible crowding, we do not see evidence for obvious contamination in WISE
View. Its disk may be undergoing a real brightening. WD 0145+234 (Figure 1
Panel A) is the only Ca II gas disk in our variable catalog not shown here.

Based on visual inspection of the unWISE images using WISE View, we flag 35 of
the 163 variables in our catalog as possibly contaminated by a nearby infrared
source (21%). These 35 (which are made up of sources only from our Gaia-
catWISE crossmatch and includes no debris disk systems) appear to show
variability correlated with the proper motion of the white dwarf or a nearby
field star as seen in WISE View. We still publish these “contaminated” objects
as candidate 3.4-$\mu$m variables, but flag them in Table 2 due to the
subjectivity of this exercise.

Finally, we note our catalog of variables is still subject to further crowding
and contamination due to the large WISE PSF. At least 31 of 163 objects still
probably show an alternating pattern in their light curves (seven of which
also have contaminated photometry). This is likely in part due to our
clustering and sorting technique sometimes failing to identify the “high” and
“low” seasons generated by the alternating PSF scanning angle of WISE. This
technique particularly fails to filter out potentially false-positive
variability due to multiple observations being taken during the same epoch.
For example, WD 0028$-$474 (Gaia DR3 4978793541987799040) and WD 2118$-$388
(Gaia DR3 6583325635088476544) have two flux measurements per epoch for most
epochs. In each case, the two concurrent measurements lie on two distinct flux
baselines, forming “high” and “low” tracks at all epochs, rather than
alternating from epoch-to-epoch like in Panel b of Figure 1. Neither object
shows evidence for rapid variability in TESS. It is unclear what is
responsible for the apparent variability seen towards these two systems.
Subsequent NEOWISE observing fields do tend to overlap by about 3′ at all
ecliptic latitudes (Meisner et al., 2019), generating multiple observations at
the same epoch with possible differences in flux due to the PSF scanning angle
varying as the spacecraft scans from field-to-field (Schlafly et al., 2019).
These two objects are not located at extreme ecliptic latitudes (both lie
between $-55^{\circ}<\beta<-35^{\circ}$). Both the non-contaminated and
contaminated subsets of our catalog of variables appear to be distributed
across all ecliptic latitudes.

In the tables in the Appendix we include the score statistic calculated in the
Gaia DR3
allwise_neighbourhood555https://gaia.aip.de/metadata/gaiadr3/allwise_neighbourhood/
table (Marrese et al., 2017, 2019, 2022) as an additional Figure of Merit
(FoM) for users to vet the sources in our catalog. We do not use the FoM to
flag objects due to its incompleteness (41/163 objects in our catalog are not
included in the Gaia-AllWISE neighborhood crossmatch analysis). Dennihy et al.
(2020a) found white dwarf targets with S/N $>$ 10 in $W1$ and FoM $>$ 4 are
unlikely to be confused with nearby field sources.

To grow the number of known white dwarf debris disks showing a gaseous
component, we recommend prioritizing near-infrared spectroscopic follow-up
like that in Section 4.3 of the 104 uncontaminated high-confidence variables
in our catalog, especially the 84 new discoveries among these, down-weighting
the sources we flag as contaminated. Since we are interested in generating a
sample of candidate strongly variable debris disks, we have not aimed for
completeness in our catalog.

###  4.2 Characterizing the Variability of Debris Disks with Detected Gaseous
Calcium II Emission

In addition to the CVs, the Ca II gas disks appear among the most variable at
3.4 $\mu$m. All six out of six gas disks in our catalog of variables have
$\chi^{2}$ $>6$ (Figure 4, Figure 1a). We publish five out of these six as
near-infrared variables for the first time (WD 0145+234 was characterized by
Swan et al. 2021).

The six Ca II gas disks in our variable catalog do not include cases like Gaia
J0611$-$6931 (Panel F of Figure 4). Gaia J0611$-$6931, which might be
accreting a disrupted water-rich body (Rogers et al., 2024b), is excluded due
to contamination from field crowding, but otherwise does not show obvious
evidence for contamination in WISE View, suggesting the apparent brightening
towards it could be real and inherent to its debris disk. Likewise, Gaia
J2100+2122 and SDSS J2133+2428 show high-amplitude variability ($\chi^{2}$
$>6$, see Table 3) and are excluded for having an excess of $G_{\rm RP}$ flux
due to nearby field stars. Both also do not show evidence for contamination in
WISE View.

There are seven Ca II gas disk systems that pass our quality filtering and do
not exhibit significant variability ($\chi^{2}$ $<2$). Six of these (WD
0842+231, SDSS J0959$-$0200, WD 1041+092, SDSS J1228+1040, HE 1349$-$2305,
SDSS J1617+1620) were reported to exhibit significant variability in at least
one Spitzer IRAC channel ($3.4\,\mu$m or $4.5\,\mu$m) by Swan et al. (2020),
albeit over significantly different light curve baselines ranging from
hundreds of days up to about 10 years, and with better sensitivity than
provided by WISE. The near-infrared variability towards WD J1930$-$5028 (Gaia
DR3 6646693887514073984), the remaining system, has not been assessed due to
the recency of the discovery of calcium triplet emission from its debris disk
(Gentile Fusillo et al., 2021a). Two of these six systems in Swan et al.
(2020) have median S/N$<$$10$ in our $W1$ light curves, whereas all six
variable gas disks in our variables catalog have S/N$>$$10$ in $W1$. This
suggests signal-to-noise in our light curves impacts our determination of
variability in some cases, especially when considering the reduced sensitivity
in NEOWISE compared to Spitzer+IRAC. When we have robust detections
(S/N$>$$10$) we find 6/11 gas disks show variability.

Our catalog of variables includes nine white dwarfs with known infrared
excesses due to dusty debris disks with no gaseous emission at the calcium
triplet yet detected. While all nine appear to show genuine astrophysical
variability, it tends to be at a lower level compared to debris disks with
gaseous Ca II emission: only six show $\chi^{2}$ $>3$ (Figure 3). The most
extreme of these is GD 56 (Gaia DR3 3251748915515143296), which has $\chi^{2}$
$=40.0$. We show the light curve of GD 56 in Panel 9 of Figure 7, where the
$W1$ and $W2$ fluxes linearly decay until about the year 2020, after which
they stabilizes to constant values in both channels. The decaying near-
infrared flux towards GD 56 was first reported by Farihi et al. (2018). Later
analysis by Swan et al. (2021) found this trend to be consistent with the
$t^{-1}$ decay expected from a collisional cascade (Dominik & Decin, 2003). We
report near-infrared variability towards the non-Ca II debris disks of MCT
0420$-$7310, KPD 0420+5203, and US 2966 for the first time.

There are 20 non-Ca II disk systems that pass our quality filtering but do not
show significant variability ($\chi^{2}$ $<2$). 11 of these 20 systems are
included in the survey by Swan et al. (2020), who report significant
variability in Spitzer IRAC photometry towards eight. Only four of these eight
are reliably bright enough (S/N$>10$) for us to detect variability, helping
explain the why some are not recovered. A total of 8/20 of these non-variable
($\chi^{2}$ $<2$) non-Ca II disks show S/N$>$$10$ in our $W1$ light curves,
yielding a variability rate of 9/17.

We find Ca II (55%) and non-Ca II (53%) disks are variable at roughly the same
rates, except disks with observed Ca II gas in emission nearly always show
larger flux variations when they are detected to vary. This variability is
observed across decade-long baselines with roughly 6 month sampling in our
$W1$ NEOWISE light curves. We do not find commensurate optical variability
towards any of these systems in TESS, save the known pulsating white dwarfs EC
11507$-$519 (0.8% pulsation amplitude in $V$-band, Gianninas et al. 2006;
Figure 8 Panel 25) and KX Dra (0.5% pulsation amplitude in TESS, Table 2;
Figure 8 Panel 35) that also harbor dusty disks. Only KX Dra shows a Gaia
excess_flux_error consistent with optical variability. This underlines that
the variability we detect towards these white dwarfs in NEOWISE is intrinsic
to their dusty disks.

In Appendix C we show light curves for all 21 published Ca II gas disk systems
in Figure 6 and the 47 non-Ca II disks across Figure 7 and Figure 8. We report
our variability parameters for the Ca II disks in Table 3 and for the non-Ca
II disks in Table 4.

Figure 5:  Near-infrared spectroscopic follow-up of seven sources from our
catalog centered on the calcium triplet region (rest wavelengths marked as
dashed grey vertical lines). While we do not detect Ca II gas in emission, we
confirm one new remnant planetary system: we observe broad Ca II absorption
towards WD J0058$-$5638 (top panel). Sources are marked with the observatory
and UT date of observation.

###  4.3 Preliminary Searches for Circumstellar Calcium II Emission

We followed up seven objects within our catalog of variables using LDT+DeVeny
and SOAR+Goodman as described in Section 2.3, the results of which we show in
Figure 5. While we fail to discover new systems showing emission at the
calcium triplet, we notably confirm one remnant planetary system, marked by Ca
II absorption. The six others show only continua in our wavelength range
covering at least $7600-8750\,$Å.

WD J0058$-$5638 (Gaia DR3 4907742035447425024) shows broad absorption at each
line of the triplet likely of photospheric origin (S/N$=38.5$). We prioritized
WD J0058$-$5638 in our follow-up based on its high degree of variability
($\chi^{2}$ $=12.8$) and red catWISE color (Figure 3). While no higher-
resolution spectrum of WD J0058$-$5638 is published, Vincent et al. (2024)
find its Gaia XP spectra indicate it is spectral type DZ with a probability of
1.0. Photometric fits using He-atmosphere models find it is a 9970 K, 0.65
$M_{\odot}$ white dwarf (Gentile Fusillo et al., 2021b).

Also included in our follow-up are MCT 0420$-$7310 (Gaia DR3
4653404070862114176) and KPD 0420+5203 (Gaia DR3 271992414775824640), two
systems known to harbor dusty disks. Both show only continua at the calcium
triplet region. Our spectra of MCT 0420$-$7310 (S/N$=$38.9) and KPD 0420+5203
(S/N$=$47.6) are the first published in this region, and suggest their debris
disks do not show gaseous calcium in emission at our detection limits.

##  5 Discussion and Conclusions

We produce a catalog of 163 unique white dwarfs that appear to show
astrophysical variability at $3.4\,\mu$m in $W1$ NEOWISE light curves with
coverage from 2013–2023 sampled roughly every six months. We generate these
light curves using a novel aperture photometry pipeline operating on the
unWISE images that corrects for the proper motion of the target. We pass an
original input of 3299 white dwarfs, including all 21 published white dwarf Ca
II gas disks and most published non-Ca II disks, through quality filtering to
eliminate known systematics in effort to include only high-fidelity sources.
We further vet the of images of the sources in our catalog of 163 variables,
finding 104 are high-confidence variables at 3.4 $\mu$m, 84 of which are
published as variable for the first time.

We find that among sources with uncontaminated photometry and sufficient
signal-to-noise (median S/N$>$$10$ in our $W1$ light curves), 6/11 Ca II gas
debris disks and 9/17 of non-Ca II gas disks are variable over baselines of
about $9-10$ years at $\approx$$6$-month sampling. Due to the substantially
dramatic variability shown by the Ca II gas disks, we hypothesize that
targeted searches for white dwarfs exhibiting variability at wavelengths
$\lambda\gtrsim 3.4\,\mu$m represents a promising avenue for discovering more
white dwarf debris disks showing gaseous emission. We encourage spectroscopic
follow-up of the catalog in Table 2 to search for emission at the calcium II
triplet indicative of a gaseous debris disk. This is especially motivated by
our discovery of one new remnant planetary system, evidenced by calcium
triplet absorption towards the white dwarf (Figure 5), though we have not yet
discovered a new gas disk in our preliminary follow-up campaign.

Our findings here are in agreement with the work from Swan et al. (2019) and
Swan et al. (2020), who found that most debris disks around white dwarfs are
variable at 3.4 $\mu$m, with the Ca II gas disks showing the highest flux
variations compared to their non-Ca II counterparts. We also tend to see the
degree of variability grow with the light curve baseline, as was observed with
Spitzer (Swan et al., 2020). Where there is overlap, all the Ca II and non-Ca
II debris disks both in our catalog of variables are also found to be variable
by Swan et al. (2020). We identify at least five new variable Ca II gas disks:
LAMOST J0510+2315, SDSS J0347+1624, WD 1622+587, and WD J0529$-$3401 (Figure
4). We report near-infrared variability towards the non-Ca II debris disks of
MCT 0420$-$7310, KPD 0420+5203, and US 2966 for the first time.

Only 10 of our 104 high-confidence 3.4-$\mu$m variables show periodic optical
variability in TESS. Notwithstanding the differences in sampling and baselines
and the determination of variability (empirically based on anomalous scatter
in NEOWISE versus significant periodicities in TESS), this suggests that the
variability we detect towards these debris disks systems is not intrinsic to
the white dwarf photosphere, but rather arises from the dusty components of
the disks.

By considering both $\chi^{2}_{W1}$ and $\chi^{2}_{\mathrm{flat}}$ in our
variability ranking, we find at least one more debris disk showing a secular
linear trend in flux. The disk around WD J0529$-$3401 (Panel e of Figure 4)
shows a secular decay reminiscent of that towards GD 56 (Panel 9 of Figure 7).
Such a decay would be consistent with that expected from a brightening induced
by a collisional cascade (Swan et al., 2024, 2021; Dominik & Decin, 2003),
albeit over a timescale of at least 10 years. Coupled with the absence of
optical variations we find, the heightened 3.4-$\mu$m variability of the disks
showing Ca II emission in our catalog relative to the non-Ca II disks seems to
lends further credence to the notion that the photometric variability of these
disks is driven by collisions (Farihi et al., 2018; Swan et al., 2019; Malamud
et al., 2021), and that the gaseous disks may be the most the most
collisionally active at this moment (Swan et al., 2020).

Lacking in our analysis is an assessment of the variability seen in our
$W2$-band (4.6 $\mu$m) light curves. We only analyze the $W1$ light curves due
to their on-average higher signal-to-noise. Still, the most variable debris
disks tend to show significant variability in $W2$ as well (Figure 4). SDSS
J1228+1040, which is not in our catalog of variables and does not appear
contaminated, appears to shows more significant flux variations in $W2$.
Indeed, Xu et al. (2018) found SDSS J1228+1040 shows greater variability at
$4.5\,\mu$m in Spitzer IRAC photometry. We do tabulate calculated
$\chi^{2}_{W2}$ values in Appendix A. We also do not analyze our PSF-
photometry generated light curves since they appear to suffer source detection
failures at the lowest signal-to-noise levels, but publish them as well and
encourage investigation.

We also plot the WISE cryogenic epochs in our light curves but exclude these
fluxes from our variability ranking. Their inclusion would tend to agree with
the finding from Swan et al. (2020) that the degree of variability of white
dwarf debris disks grows with the light curve baseline (Figure 4). However,
there are systematic offsets in flux between the cryogenic and NEOWISE
missions (Meisner et al., 2017a) that make this an unreliable comparison.

Unlike their hosts’ photospheres, the future for the study of remnant
planetary systems around white dwarfs is bright in the infrared. The Near-
Earth Object Surveyor, the successor to WISE and NEOWISE, is planned to launch
as early as 2028 and survey the sky at 4.6 and 8 $\mu$m (Mainzer et al.,
2023). SPHEREx will compliment NEO Surveyor with spectra spanning
$0.75-5\,\mu$m covering the entire sky (Doré et al., 2018). Large, deep
spectroscopic surveys like DESI and SDSS-V should discover more gaseous debris
disks from their emission at the near-infrared calcium triplet, with target
selection propelled by the discovery of hundreds of thousands high-probability
white dwarf candidates with Gaia (Gentile Fusillo et al., 2021b). JWST is
already characterizing gaseous debris disks (Swan et al., 2024), with surely
more on the way. We expect that near-infrared variability will aid in
efficiently identifying the most compelling targets for the next generation of
flagship observatories like JWST.

##  6 Acknowledgments

We thank the anonymous referee for their constructive comments and suggestions
that improved this paper.

This material is based upon work supported by the National Aeronautics and
Space Administration under Grant No. 80NSSC23K1068 issued through the Science
Mission Directorate. Support for this work was in part provided by NASA TESS
Cycle 4 grant 80NSSC22K0737 and Cycle 6 grant 80NSSC24K0878. JAG is supported
by the National Science Foundation Graduate Research Fellowship Program under
Grant No. 2234657. JAG recognizes Siyi Xu (许bsmi偲gbsn艺, Gemini Observatory)
for numerous fruitful discussions that improved this study and the
interpretation of these results. K. D. was supported by NASA through the NASA
Hubble Fellowship grant #HST-HF2-51477.001 awarded by the Space Telescope
Science Institute, which is operated by the Association of Universities for
Research in Astronomy, Inc., for NASA, under contract NAS5-26555. This
research was improved by discussions at the KITP Program “White Dwarfsas
Probes of the Evolution of Planets, Stars, the Milky Way and the Expanding
Universe” supported by National Science Foundation under grant No. NSF
PHY-1748958.

This publication makes use of data products from the Wide-field Infrared
Survey Explorer, which is a joint project of the University of California, Los
Angeles, and the Jet Propulsion Laboratory/California Institute of Technology,
funded by the National Aeronautics and Space Administration. This publication
also makes use of data products from NEOWISE, which is a project of the Jet
Propulsion Laboratory/California Institute of Technology, funded by the
Planetary Science Division of the National Aeronautics and Space
Administration.

These results make use of the Lowell Discovery Telescope at Lowell
Observatory. Lowell is a private, nonprofit institution dedicated to
astrophysical research and public appreciation of astronomy and operates the
LDT in partnership with Boston University, the University of Maryland, the
University of Toledo, Northern Arizona University, and Yale. We particularly
thank LDT telescope operators Ishara Nisley and Cecilia Siqueiros.

Our work is based on observations obtained at the Southern Astrophysical
Research (SOAR) telescope, which is a joint project of the Ministério da
Ciência, Tecnologia e Inovações (MCTI/LNA) do Brasil, the US National Science
Foundation’s NOIRLab, the University of North Carolina at Chapel Hill (UNC),
and Michigan State University (MSU). We particularly thank Sean Points and
Felipe Navarete for observing on our behalf and Jay Elias for facilitating our
observing plan.

This work has made use of data from the European Space Agency (ESA) mission
Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing
and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement.

This paper includes data collected by the TESS mission. Funding for the TESS
mission is provided by the NASA’s Science Mission Directorate.

##  Appendix A Variability Rank Tables

Here we show various tables summarizing our variability ranking and validation
for the various inputs in our sample. In Table 2 we show these parameters for
our catalog of variables, which includes all 104 high-confidence variables and
the 59 likely contaminated sources. In Table 3 we show these parameters for
the 21 previously published white dwarf debris disks that show gaseous calcium
in emission. In Table 4 we show these parameters for the 47 disks in our study
that do not show gaseous emission.

=1.5cm

Table 2: Variability Parameters for our Catalog of 163 Variable White Dwarfs

Gaia DR3 Source ID | R.A. | Decl. | Sample | Flag | $\chi^{2}_{W1}$ | $\chi^{2}_{\mathrm{alt}}$ | $\chi^{2}_{\mathrm{flat}}$ | $\chi^{2}_{W2}$ | S/N |  $G_{\rm RP}$ Excess | $G_{\rm RP}$ | $W1$ | $W1-W2$ | Spectral | $T_{\mathrm{eff}}$ | $M_{\rm WD}$ | EFF | FoM | TESS
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---
| (J2000) | (J2000) |  |  |  |  |  |  | $W1$ |  | (mag) | (mag) | (mag) | Type | (K) | ($M_{\odot}$) |  |  | (hr, %)
2312821266302222720 | 0.66720 |  $-$34.22766 | GC | * | 1.3 | 1.9 | 2.2 |  | 5.1 | 0.000 | 14.5 | 20.2 |  $-$0.3 | DC [1] | 6460 | 0.73 | 1.8 | 9.3 |
537563127588521600 | 1.28048 | 73.21927 | GC | * | 2.5 | 3.1 | 2.6 | 1.0 | 43.1 | 0.231 | 14.3 | 17.8 |  $-$0.8 | DBZA [2] | 14160 | 0.66 | 1.4 | 7.6 |
4978793541987799040 | 7.69668 |  $-$47.21010 | GC | 2 | 47.1 | 68.1 | 46.5 | 4.0 | 18.9 | 0.000 | 15.2 | 19.0 |  $-$0.8 | DA [3] | 13600 | 0.31 | 0.3 | 9.0 |
381396329995329408 | 9.98592 | 42.49150 | GC | * | 2.1 | 2.8 | 2.3 | 0.0 | 9.4 | 0.295 | 16.4 | 19.5 |  $-$0.8 | DA [4] | 9990 | 0.55 | 0.0 | 7.6 |
414603986339418624 | 12.18956 | 50.12801 | GC | * | 2.4 | 3.7 | 2.3 | 0.9 | 18.5 | 0.361 | 16.1 | 18.7 |  $-$0.8 | DA [5] | 10130 | 0.76 | 0.0 |  |
4907742035447425024 | 14.56430 |  $-$56.63616 | GC | * | 12.0 | 18.9 | 12.8 | 4.9 | 14.3 | 0.560 | 16.9 | 19.1 |  $-$0.2 | DZ [5] | 10250 | 0.74 | 0.7 | 8.5 |
5026963661794939520 | 17.15014 |  $-$32.62872 | IR, GC |  | 2.2 | 2.6 | 2.5 | 0.9 | 28.9 | 1.110 | 15.5 | 18.1 |  $-$0.7 | DAZ [3] | 16050 | 0.63 | 0.0 |  |
4913589203924379776 | 18.08809 |  $-$56.24098 | IR, GC |  | 2.6 | 4.1 | 2.7 | 1.1 | 20.4 | 0.000 | 15.9 | 18.8 |  $-$0.3 | DBAZ [6] | 18790 | 0.59 | 0.0 | 8.7 |
321856332242222336 | 18.88398 | 37.62655 | CV |  | 137.5 | 203.3 | 156.3 | 12.8 | 23.5 | 0.000 | 17.2 | 18.3 |  $-$0.5 | CV [7] |  |  |  | 7.7 |
409956900443661312 | 19.72531 | 52.45381 | GC | * | 1.5 | 2.5 | 2.1 | 0.5 | 15.5 | 0.475 | 15.6 | 18.9 |  $-$0.7 | DA [3] | 10760 | 0.60 | 0.7 | 5.1 |
4704847230628098048 | 21.63848 |  $-$65.94308 | GC | * | 2.5 | 4.0 | 2.4 | 0.9 | 19.7 | 0.572 | 15.6 | 19.0 |  $-$0.9 | DA [8] | 13400 | 0.76 | 1.2 | 6.7 |
2480731779699826176 | 22.59595 |  $-$4.79939 | GC | 1 | 1.2 | 1.9 | 3.4 | 0.9 | 12.1 | 0.355 | 15.0 | 19.0 |  $-$0.7 | DA [3] | 16280 | 0.57 | 0.0 | 9.0 |
4686195252815125888 | 22.74033 |  $-$73.83004 | GC | 1 | 1.9 | 2.4 | 14.6 | 1.0 | 28.7 | 0.566 | 16.4 | 18.7 |  $-$0.6 | DA [9] | 7820 | 0.52 | 0.0 |  |
4912427947846613760 | 23.99450 |  $-$54.73145 | GC | * | 2.1 | 2.1 | 2.3 | 1.1 | 27.5 | 0.360 | 15.4 | 18.5 |  $-$0.7 | DA [5] | 10310 | 0.61 | 1.3 | 8.0 |
572487740053132288 | 25.36576 | 83.58301 | GC | * | 2.1 | 2.4 | 2.7 | 1.6 | 98.0 | 0.013 | 13.3 | 16.8 |  $-$0.7 | DA [3] | 19000 | 0.65 | 0.3 | 10.4 |
5135466183642594304 | 26.84096 |  $-$21.94761 | GC | [a] | 1.4 | 2.1 | 2.4 | 0.9 | 65.6 | 0.000 | 15.2 | 16.9 |  $-$0.5 | DA [3] | 11080 | 0.60 | 7.8 | 8.9 | 0.12, 0.5
291057843317534464 | 26.97841 | 23.66211 | Ca II, GC |  | 422.6 | 690.7 | 482.4 | 254.9 | 54.3 | 0.021 | 14.1 | 16.9 |  $-$0.4 | DA [10] | 12980 | 0.66 | 0.8 | 5.5 |
2574761734934286848 | 28.77645 | 12.14059 | GC | * | 2.2 | 3.4 | 2.1 |  | 6.6 | 0.000 | 17.0 | 19.8 |  $-$0.4 | DA [5] | 7810 | 0.61 | 0.0 | 6.9 |
4970215770740383616 | 33.45361 |  $-$33.75834 | GC | 1 | 1.7 | 1.8 | 10.8 | 0.9 | 25.4 | 0.555 | 15.1 | 18.3 |  $-$0.7 | DAZ [1] | 4640 | 0.37 | 27.4 | 1.4 |
5063539946887524864 | 37.11329 |  $-$32.70939 | GC | * | 2.1 | 2.4 | 2.1 | 1.1 | 34.4 | 0.011 | 14.1 | 17.9 |  $-$0.7 | DA [3] | 22280 | 0.63 | 0.3 | 8.0 |
4945893096067016960 | 39.19143 |  $-$44.87680 | GC | * | 2.1 | 2.6 | 2.0 |  | 6.7 | 0.000 | 16.6 | 20.2 |  $-$0.8 | DA [5] | 15690 | 0.77 | 1.9 | 6.2 |
548423278811480960 | 40.38260 | 74.71518 | GC | 1 | 1.3 | 2.1 | 3.7 | 0.9 | 15.5 | 0.505 | 15.8 | 19.1 |  $-$0.7 | DA [3] | 9130 | 0.60 | 0.0 | 9.3 |
5052391208218637440 | 43.82827 |  $-$32.11233 | GC | * | 2.2 | 2.5 | 2.2 |  | 6.6 | 0.000 | 17.0 | 19.9 |  $-$0.7 | DA [5] | 6770 | 0.83 | 1.5 | 8.2 |
5078074837769743744 | 45.65272 |  $-$23.03090 | GC | * | 2.5 | 3.5 | 2.5 | 0.0 | 8.4 | 0.105 | 15.9 | 19.6 |  $-$0.7 | DA [3] | 22880 | 0.86 | 1.2 | 7.9 |
4733604373137759616 | 48.32741 |  $-$56.12639 | GC | 1,[b] | 1.3 | 1.7 | 11.9 | 1.0 | 52.4 | 0.636 | 14.6 | 17.9 |  $-$0.8 | DA [11] | 10980 | 0.60 | 6.1 |  | 0.22, 0.2
4847443748331819136 | 49.20349 |  $-$44.62911 | GC | * | 1.8 | 2.6 | 2.5 | 0.8 | 22.4 | 0.200 | 17.0 | 18.8 |  $-$0.8 | DA [5] | 5590 | 0.64 | 0.8 |  |
5098597840775610112 | 50.37700 |  $-$22.45211 | GC | * | 2.7 | 2.8 | 2.7 | 0.0 | 7.8 | 0.038 | 16.7 | 19.7 |  $-$0.6 | DA [5] | 5210 | 0.38 | 13.0 | 4.2 |
5053839127592396032 | 52.21176 |  $-$32.64194 | GC | 1 | 1.2 | 1.3 | 5.0 | 0.9 | 13.0 | 1.399 | 16.6 | 19.2 |  $-$0.7 | DC [1] | 4640 | 0.45 | 1.1 | 8.5 |
63126196662620416 | 55.84636 | 19.97044 | GC | * | 2.0 | 2.6 | 2.0 | 0.6 | 27.8 | 0.487 | 15.3 | 18.0 |  $-$0.6 | DA [12] | 7110 | 0.87 | 0.0 | 8.9 |
43629828277884160 | 56.90288 | 16.40270 | Ca II | * | 12.3 | 17.7 | 12.1 | 0.5 | 11.2 | 0.940 | 16.7 | 19.2 |  $-$0.4 | DAZ [13] | 17710 | 0.52 | 0.0 | 6.9 |
570086853335107200 | 57.49233 | 82.90492 | GC | * | 2.2 | 3.1 | 2.1 | 0.4 | 12.8 | 0.153 | 16.2 | 19.6 |  $-$0.8 | DA [5] | 13430 | 0.61 | 1.2 | 6.0 |
5094843764483219712 | 58.24161 |  $-$19.93029 | GC | * | 2.2 | 3.0 | 2.1 | 0.5 | 8.8 | 0.000 | 16.5 | 19.6 |  $-$1.0 | DA [5] | 10250 | 0.59 | 2.6 | 7.1 |
276192136878054656 | 61.52921 | 54.52561 | GC | * | 3.6 | 4.5 | 3.4 | 0.3 | 8.9 | 0.945 | 15.5 | 19.5 |  $-$0.8 | DA [3] | 17010 | 0.63 | 0.9 |  |
3251748915515143296 | 62.75904 |  $-$3.97294 | IR |  | 4.9 | 6.8 | 40.0 | 3.3 | 56.2 | 0.268 | 15.7 | 17.1 | 0.2 | DA [3] | 14900 | 0.61 | 0.1 | 10.7 |
229143725086190336 | 64.52301 | 42.18394 | GC | 1 | 1.3 | 2.0 | 3.3 | 1.9 | 23.1 | 0.989 | 16.4 | 18.3 |  $-$0.7 | DA [14] | 5050 | 0.49 | 0.2 |  |
4653404070862114176 | 64.90760 |  $-$73.06236 | IR, GC | * | 2.4 | 3.9 | 3.5 | 1.3 | 59.6 | 0.150 | 15.7 | 17.7 |  $-$0.1 | DA [3] | 17790 | 0.58 | 0.0 | 9.6 |
232990572675079296 | 65.37243 | 46.13271 | GC | 1 | 1.2 | 1.2 | 2.2 | 1.1 | 41.0 | 0.996 | 14.9 | 17.6 |  $-$0.8 | DA [14] | 7810 | 0.59 | 0.0 | 7.0 |
271992414775824640 | 66.06547 | 52.16962 | IR, GC | * | 3.1 | 4.9 | 17.4 | 2.2 | 40.1 | 0.876 | 15.2 | 17.6 |  $-$0.3 | DA [3] | 22170 | 0.66 | 0.9 | 8.9 |
3186021141200137472 | 69.44754 |  $-$8.81962 | GC | 2 | 6.8 | 7.4 | 23.1 | 1.3 | 14.2 | 0.073 | 13.2 | 19.1 |  $-$0.8 | DQ [15] | 6720 | 0.66 | 1.8 | 9.7 |
4867104257483687040 | 71.11395 |  $-$35.29586 | GC | * | 1.5 | 2.3 | 2.3 | 0.1 | 10.9 | 0.138 | 15.7 | 19.5 |  $-$0.9 | DBA [16] | 13870 | 0.75 | 0.0 | 8.1 |
4893995356962398208 | 72.26348 |  $-$24.21089 | GC | * | 1.6 | 1.8 | 2.0 | 1.2 | 22.6 | 1.228 | 16.7 | 18.4 |  $-$0.6 | DA [11] | 4870 | 0.56 | 0.0 |  |
4776747972460494208 | 73.81052 |  $-$54.69595 | GC | * | 1.9 | 2.1 | 4.7 | 1.5 | 55.2 | 0.291 | 14.5 | 17.9 |  $-$0.8 | DA [17] | 17360 | 0.55 | 0.0 | 5.9 |
3212803625248377088 | 76.55461 |  $-$4.13535 | CV |  | 21.3 | 31.7 | 20.2 | 5.3 | 26.4 | 0.091 | 16.7 | 18.2 |  $-$0.3 | CV [18] |  |  | 51.6 | 10.0 |
3415788525598117248 | 77.50894 | 23.26151 | Ca II, GC | * | 41.9 | 65.9 | 49.5 | 16.0 | 41.8 | 0.796 | 15.2 | 17.4 |  $-$0.1 | DAZ [19] | 21250 | 0.73 | 0.0 | 9.0 |
4823042355496728832 | 82.30968 |  $-$34.01892 | Ca II | * | 2.0 | 2.2 | 6.4 | 0.8 | 28.5 | 0.230 | 17.6 | 18.4 | 0.1 | DAZ [20] | 17190 | 0.46 | 0.0 |  |
4805197659735412224 | 86.36308 |  $-$39.59992 | GC | * | 1.4 | 1.5 | 2.2 | 1.0 | 31.4 | 0.085 | 16.3 | 18.4 |  $-$0.7 | DA [5] | 5220 | 0.27 | 0.0 | 8.9 |
3429193393407592576 | 87.69361 | 26.20574 | GC | * | 2.5 | 3.3 | 2.4 | 0.8 | 9.7 | 0.822 | 15.6 | 19.2 |  $-$0.8 | DA [21] | 21210 | 0.57 | 0.0 |  |
3218697767783768320 | 87.83117 |  $-$0.17253 | GC | * | 2.2 | 2.6 | 2.6 | 1.2 | 59.0 | 0.015 | 14.0 | 17.0 |  $-$0.7 | DQP [1] | 6180 | 0.72 | 4.5 | 9.5 |
3022568258011201024 | 88.03243 |  $-$5.41682 | CV |  | 1216.6 | 1640.3 | 1152.6 | 526.6 | 81.3 | 0.113 | 13.5 | 15.3 |  $-$0.5 | CV [22] |  |  |  | 10.1 |
4650712810002383104 | 89.51038 |  $-$72.48012 | GC | 2 | 2.9 | 5.5 | 2.6 | 1.2 | 14.8 | 1.284 | 16.7 | 19.5 |  $-$0.8 | DC [1] | 6850 | 0.83 | 0.0 | 7.1 |
1114054838014073984 | 91.81668 | 73.53325 | GC | 1 | 1.1 | 1.5 | 5.5 | 0.8 | 10.1 | 0.049 | 16.6 | 19.6 |  $-$0.5 | DA [14] | 6520 | 0.92 | 1.3 | 6.6 |
943770757800160384 | 101.90802 | 37.51585 | GC | 1 | 0.2 | 0.2 | 6.0 |  | 6.3 | 0.063 | 12.3 | 19.8 |  $-$0.7 | DA [3] | 21390 | 0.71 | 0.2 | 10.5 |
950478049312082688 | 104.43818 | 40.12384 | GC | 2 | 2.3 | 2.9 | 2.3 | 0.0 | 11.3 | 0.061 | 16.6 | 19.3 |  $-$0.9 | DA [14] | 6580 | 0.46 | 0.6 | 7.8 |
5536077746353130240 | 113.40764 |  $-$42.89924 | GC | 3 | 1.7 | 2.4 | 15.8 | 1.2 | 14.0 | 0.514 | 14.3 | 19.2 |  $-$0.7 | DA [10] | 15020 | 0.70 | 0.3 |  |
5295512298775133824 | 115.47008 |  $-$57.14576 | GC | * | 2.1 | 2.1 | 2.0 | 2.0 | 24.4 | 0.891 | 15.3 | 18.8 |  $-$0.7 | DA [3] | 19540 | 0.47 | 0.0 |  |
3150770626615542784 | 117.92301 | 11.34175 | GC | * | 2.1 | 3.1 | 2.1 | 0.7 | 8.0 | 0.690 | 16.4 | 19.6 |  $-$0.9 | DA [23] | 11810 | 0.56 | 0.8 | 6.1 | 0.06, 0.7
5210507882302442368 | 121.86563 |  $-$76.53352 | CV |  | 367.0 | 520.4 | 346.0 | 245.6 | 98.9 | 0.626 | 15.1 | 16.3 |  $-$0.4 | CV [24] |  |  | 111.8 | 2.2 |
1091051096255456384 | 123.11779 | 62.60623 | CV |  | 499.9 | 956.4 | 489.1 | 288.5 | 92.0 | 0.028 | 14.3 | 15.9 |  $-$0.4 | CV [25] |  |  |  | 10.0 |
908364559240386944 | 125.00536 | 38.57653 | GC | * | 1.9 | 2.5 | 2.2 |  | 5.9 | 0.084 | 16.2 | 19.9 |  $-$0.9 | DA [10] | 7640 | 0.62 | 0.0 | 5.6 |
911639111025648512 | 130.03196 | 40.25104 | GC | [c] | 1.4 | 2.2 | 2.1 | 1.2 | 17.1 | 0.000 | 15.6 | 18.8 |  $-$0.5 | DA [3] | 11570 | 0.61 | 3.4 |  | 0.14, 0.9
5639391810273308416 | 130.38422 |  $-$32.94248 | GC | 1 | 51.2 | 76.7 | 49.9 | 6.9 | 20.6 | 0.089 | 11.7 | 18.4 |  $-$0.8 | DA [3] | 9080 | 0.47 | 0.9 | 7.3 |
5734737438536674432 | 130.70190 |  $-$13.78700 | GC | * | 2.5 | 2.9 | 2.5 | 1.1 | 41.3 | 0.218 | 15.1 | 17.7 |  $-$0.2 | DZ [3] | 5520 | 0.75 | 4.4 | 10.3 |
5522446550952428288 | 131.23160 |  $-$44.14232 | GC | 1 | 2.0 | 2.3 | 2.8 | 1.8 | 35.8 | 0.386 | 15.2 | 17.7 |  $-$0.8 | DA [5] | 9190 | 0.42 | 0.0 |  |
910341618586847232 | 131.35268 | 38.03232 | GC | * | 2.6 | 3.0 | 2.5 | 0.6 | 19.4 | 0.044 | 15.7 | 18.6 |  $-$0.7 | DA [3] | 8000 | 0.58 | 0.0 | 8.0 |
1037258898615762048 | 131.51030 | 57.05795 | Ca II | * | 25.2 | 37.1 | 26.1 | 9.0 | 95.4 | 0.090 | 16.9 | 16.4 | 0.0 | DA [3] | 15410 | 0.57 | 0.1 | 8.0 |
5220278009486912896 | 131.94610 |  $-$73.21382 | GC | 1 | 0.9 | 1.9 | 2.2 | 0.5 | 26.4 | 0.443 | 15.4 | 18.9 |  $-$1.0 | DA [3] | 17030 | 0.54 | 0.5 |  |
720353602808918016 | 134.37693 | 40.27029 | GC | * | 2.0 | 2.3 | 2.3 | 1.1 | 15.2 | 0.248 | 15.0 | 18.9 |  $-$1.0 | DA [3] | 22600 | 0.61 | 0.4 | 9.1 |
5622626147732121856 | 134.79668 |  $-$36.79189 | GC | 1 | 3.0 | 4.0 | 2.9 | 1.4 | 17.6 | 0.364 | 15.4 | 18.8 |  $-$0.8 | DA [26] | 9610 | 0.58 | 0.4 | 2.5 |
712888090655562624 | 134.81103 | 32.95338 | GC | 2 | 2.8 | 3.4 | 2.8 | 0.9 | 17.0 | 0.068 | 15.0 | 18.7 |  $-$0.7 | DQ [3] | 10370 | 1.05 | 2.4 | 8.9 |
711744456827031680 | 135.67776 | 31.34542 | GC | 3 | 2.7 | 4.0 | 2.9 | 0.9 | 15.3 | 0.000 | 15.1 | 18.6 |  $-$0.8 | DA [3] | 9840 | 0.74 | 0.1 |  |
1123700235048742016 | 136.38352 | 73.24860 | GC | 2 | 2.2 | 2.5 | 2.1 | 0.2 | 9.4 | 1.059 | 16.1 | 19.8 |  $-$0.5 | DC [14] | 5130 | 0.57 | 0.7 | 9.7 |
5635344954642200576 | 136.66879 |  $-$30.20627 | GC | 3 | 3.2 | 3.6 | 3.9 | 0.6 | 19.3 | 0.335 | 15.7 | 18.6 |  $-$0.6 | DA [5] | 9580 | 0.59 | 0.0 | 1.8 |
692134843040270080 | 136.89278 | 27.65095 | GC | * | 2.2 | 2.7 | 2.1 | 1.6 | 28.1 | 0.953 | 16.2 | 18.1 |  $-$0.8 | DA [21] | 14660 | 0.81 | 0.0 |  |
591040864898749312 | 139.01121 | 10.18630 | GC | 2 | 2.3 | 2.7 | 2.2 | 1.2 | 15.9 | 0.258 | 15.6 | 18.9 |  $-$0.9 | DQZ [27] | 9460 | 0.78 | 0.0 | 8.9 |
3843957354387777152 | 140.73361 | 1.05282 | GC | 1 | 1.1 | 1.4 | 4.3 | 1.6 | 34.6 | 0.269 | 16.1 | 17.9 |  $-$0.7 | DAZ [28] | 6060 | 0.57 | 0.0 |  |
5669427512997660800 | 153.00767 |  $-$18.72591 | GC | * | 6.7 | 7.6 | 6.4 | 0.3 | 9.1 | 0.000 | 15.0 | 19.5 |  $-$0.5 | DZ [2] | 6520 | 0.75 | 0.7 | 9.8 |
3830623164560911872 | 154.02864 |  $-$1.32143 | GC | 2 | 1.5 | 1.7 | 2.6 | 1.0 | 8.1 | 0.677 | 15.0 | 19.6 |  $-$0.1 | DA [3] | 7760 | 0.29 | 0.4 | 10.2 |
5446683430923603840 | 155.40966 |  $-$34.80381 | GC | * | 2.1 | 3.2 | 2.0 | 0.0 | 6.3 | 0.559 | 16.3 | 20.0 |  $-$1.3 | DA [26] | 14210 | 0.57 | 1.2 | 6.1 |
3882547566823086848 | 157.22873 | 11.09741 | GC | 1 | 1.5 | 1.8 | 2.3 | 0.9 | 19.7 | 1.252 | 15.7 | 18.5 |  $-$0.5 | DA [3] | 14710 | 0.62 | 0.2 |  |
1076941716370493696 | 159.24298 | 71.18298 | GC | 1 | 3.7 | 5.9 | 3.9 | 0.0 | 6.8 | 1.202 | 15.9 | 20.2 |  $-$0.5 | DC [29] | 4690 | 0.68 | 0.2 | 2.2 |
5394487930327131904 | 160.94445 |  $-$39.11038 | GC | * | 2.0 | 2.9 | 1.9 | 0.0 | 10.8 | 0.717 | 15.4 | 19.4 |  $-$0.7 | DA [5] | 25710 | 0.63 | 2.1 |  |
3555923756458320000 | 162.83398 |  $-$18.17569 | GC | * | 2.2 | 3.0 | 2.0 |  | 6.9 | 0.601 | 16.9 | 19.9 |  $-$0.7 | DA [5] | 9150 | 0.60 | 0.0 | 4.6 |
3763445409285757824 | 164.39636 |  $-$7.52310 | GC | 3 | 8.1 | 13.2 | 105.4 | 1.2 | 13.4 | 0.196 | 14.0 | 18.9 |  $-$0.6 | DC [30] | 7520 | 0.87 | 0.5 |  |
861050512312844672 | 166.92737 | 59.97490 | GC | * | 2.6 | 2.9 | 2.5 | 1.1 | 52.6 | 0.003 | 14.0 | 17.7 |  $-$0.7 | DA [3] | 18310 | 0.69 | 0.0 | 10.1 |
3810099989754827136 | 168.90400 | 0.55475 | GC | * | 2.5 | 2.9 | 2.4 | 0.4 | 10.4 | 0.000 | 16.7 | 19.1 |  $-$0.8 | DA [3] | 5050 | 0.39 | 0.0 | 9.0 |
3817262208497857024 | 169.16826 | 6.45074 | GC | * | 2.5 | 2.7 | 2.6 | 0.2 | 8.9 | 0.000 | 16.2 | 19.5 |  $-$0.7 | DA [3] | 6260 | 0.54 | 0.0 | 8.6 |
3790040465258127616 | 169.56276 |  $-$3.23488 | GC | 1 | 4.5 | 7.2 | 13.1 | 0.0 | 6.9 | 0.000 | 15.2 | 19.9 |  $-$0.7 | DQ [30] | 9900 | 0.70 | 0.0 |  |
3540018805367474816 | 175.48576 |  $-$23.34921 | GC | * | 2.2 | 2.4 | 2.1 |  | 7.1 | 0.433 | 16.5 | 19.8 |  $-$0.7 | DA [3] | 13410 | 0.60 | 0.5 |  |
1074721664954807040 | 175.91038 | 71.68904 | CV |  | 411.4 | 623.4 | 405.7 | 352.3 | 135.4 | 0.019 | 14.4 | 15.2 |  $-$0.4 | CV [31] |  |  |  | 11.2 | 3.97, 3.0
4019789359821201536 | 176.99441 | 28.53223 | IR | * | 1.9 | 3.1 | 2.6 | 0.9 | 10.1 | 0.245 | 17.5 | 19.4 | 0.1 | DA [3] | 11990 | 0.58 | 0.3 | 8.3 |
3571559292842744960 | 178.31358 |  $-$15.61014 | IR, GC | [d] | 4.6 | 6.6 | 4.3 | 3.2 | 40.5 | 0.046 | 16.0 | 17.7 | 0.2 | DA [3] | 11770 | 0.62 | 0.1 | 9.4 |
1129251428738666880 | 178.67848 | 79.24235 | GC | 2 | 2.8 | 3.0 | 2.8 | 0.2 | 13.1 | 0.000 | 15.8 | 19.6 |  $-$0.8 | DA [3] | 10470 | 0.57 | 0.4 | 8.4 |
3466326581136347520 | 181.69834 |  $-$32.57586 | GC | 2 | 2.2 | 2.3 | 3.1 | 0.5 | 13.5 | 0.054 | 15.8 | 19.1 |  $-$0.6 | DA [3] | 19970 | 0.61 | 0.0 |  |
3576269871237617024 | 184.79672 |  $-$12.89442 | GC | 1 | 3.2 | 5.0 | 8.2 | 1.9 | 25.3 | 0.257 | 16.4 | 18.2 |  $-$0.4 | DA [5] | 8310 | 0.58 | 0.0 |  |
1543723709905857792 | 188.73446 | 47.62593 | GC | * | 2.7 | 3.0 | 2.7 | 1.4 | 33.4 | 0.000 | 14.6 | 18.2 |  $-$0.8 | DA [3] | 15230 | 0.59 | 0.4 | 9.7 |
6127333286605955072 | 189.70606 |  $-$49.80006 | GC | [e] | 2.2 | 2.6 | 2.3 | 0.8 | 27.6 | 0.764 | 13.8 | 18.1 |  $-$0.6 | DA [3] | 11350 | 1.04 | 0.9 | 9.0 | 0.16, 0.1
1540371883066128768 | 191.39846 | 42.64019 | GC | 2 | 2.0 | 3.0 | 1.9 |  | 8.2 | 0.058 | 17.0 | 19.9 |  $-$0.7 | DA [27] | 9540 | 0.80 | 0.0 |  |
3938156295111047680 | 196.42659 | 18.01771 | GC |  | 5.1 | 7.0 | 5.3 | 1.7 | 14.6 | 0.100 | 15.9 | 18.9 |  $-$0.0 | CV [32] | 11530 | 0.53 | 10.1 | 8.7 | 0.78, 1.1
3743646365965636352 | 198.42570 | 13.68691 | GC | * | 2.3 | 3.6 | 2.3 |  | 5.7 | 0.119 | 16.9 | 20.1 |  $-$0.4 | DC [33] | 9710 | 0.91 | 0.0 | 6.6 |
3506567328028533120 | 199.08138 |  $-$20.12557 | GC | 1 | 1.1 | 1.3 | 4.5 | 0.3 | 9.5 | 0.549 | 16.4 | 19.2 |  $-$0.9 | DZ [34] | 5230 | 0.83 | 19.6 |  |
3630035787972473600 | 202.55677 |  $-$8.57485 | GC | 3 | 108.0 | 163.9 | 108.8 | 14.8 | 45.4 | 0.044 | 12.5 | 17.1 |  $-$0.5 | DA [3] | 14560 | 0.59 | 0.3 | 4.2 |
1470081444731625728 | 202.79520 | 34.14484 | GC | * | 3.2 | 4.8 | 3.1 | 0.6 | 18.1 | 0.041 | 15.3 | 18.8 |  $-$0.8 | DA [3] | 16720 | 0.55 | 0.6 | 8.5 |
3714914271705535360 | 205.34082 | 5.01273 | GC | 2 | 1.7 | 1.8 | 2.3 | 1.6 | 17.0 | 0.050 | 15.5 | 18.7 |  $-$0.4 | DC [28] | 4020 | 0.44 | 0.0 | 10.0 |
1444161214019904768 | 207.63476 | 24.57086 | GC | * | 2.4 | 3.9 | 2.3 | 0.0 | 8.3 | 0.000 | 16.2 | 19.6 |  $-$0.9 | DA [23] | 13140 | 0.67 | 0.4 | 7.6 |
3644717669817337856 | 210.64444 |  $-$4.91747 | GC | 3 | 1.2 | 1.2 | 2.6 | 0.7 | 15.2 | 1.270 | 16.4 | 18.7 |  $-$0.9 | DA [5] | 7730 | 0.55 | 0.0 |  |
6217739938701063808 | 221.84281 |  $-$30.58990 | GC | * | 2.2 | 5.1 | 1.9 |  | 4.0 | 0.365 | 16.9 | 20.4 |  $-$0.6 | DA [35] | 7100 | 0.63 | 0.0 | 7.1 |
6306842338087328512 | 225.01548 |  $-$16.06410 | GC | * | 1.8 | 6.3 | 2.1 |  | 7.8 | 0.103 | 16.9 | 19.6 |  $-$0.7 | DA [5] | 7150 | 0.60 | 0.0 | 5.6 |
1164185283975493120 | 233.32823 | 7.29944 | GC | * | 1.8 | 2.6 | 2.1 |  | 6.4 | 0.000 | 16.6 | 20.0 |  $-$1.3 | DA [23] | 10260 | 0.54 | 0.7 | 5.6 |
1641326979142898048 | 235.43704 | 64.89805 | IR, GC | [f] | 1.9 | 2.5 | 3.2 | 1.6 | 65.7 | 0.188 | 15.6 | 17.8 | 0.2 | DAV [36] | 11600 | 0.63 | 10.2 | 10.5 | 0.19, 0.5
1598771000065137536 | 237.14875 | 57.14068 | GC | 2 | 2.1 | 2.3 | 2.2 | 0.9 | 16.7 | 0.000 | 16.9 | 19.4 |  $-$0.7 | DC [37] | 5910 | 0.75 | 0.9 | 8.2 |
1696309737421796352 | 237.84308 | 71.75329 | CV |  | 39.8 | 54.1 | 42.1 | 6.8 | 45.2 | 0.000 | 16.5 | 18.2 |  $-$0.5 | CV [38] |  |  |  | 6.2 | 1.63, 0.7
4406733207018066944 | 241.47717 |  $-$0.46974 | GC | * | 2.3 | 3.4 | 2.2 |  | 8.2 | 0.527 | 16.5 | 19.6 |  $-$0.7 | DA [3] | 9350 | 0.59 | 0.1 | 7.7 |
1429618420396285952 | 243.31921 | 55.35721 | GC | * | 2.1 | 2.9 | 2.4 | 1.3 | 29.0 | 0.041 | 16.0 | 18.7 |  $-$0.3 | DA [3] | 11980 | 0.66 | 0.6 | 8.9 |
1623866184737702912 | 245.74852 | 58.67525 | Ca II | * | 9.8 | 14.8 | 9.2 | 2.9 | 20.9 | 0.152 | 17.0 | 19.2 |  $-$0.2 | DB [3] | 18790 | 0.48 | 1.5 |  |
1653044367185115264 | 247.16083 | 70.88936 | GC | * | 3.2 | 3.4 | 2.5 |  | 4.9 | 0.000 | 16.9 | 20.9 |  $-$0.5 | DA [14] | 4900 | 0.57 | 0.3 | 8.4 |
4563152548683139968 | 251.58233 | 19.76768 | GC | 2 | 3.1 | 5.1 | 2.9 | 0.9 | 18.5 | 0.242 | 15.1 | 18.8 |  $-$0.8 | DB [3] | 15260 | 0.61 | 0.0 | 9.2 |
1705751041906319872 | 251.78541 | 77.21271 | GC | * | 2.4 | 3.9 | 2.4 |  | 9.8 | 0.026 | 16.6 | 20.1 |  $-$1.0 | DA [23] | 12740 | 0.57 | 1.1 | 5.5 |
4379328051494006784 | 253.15663 |  $-$1.23234 | GC | * | 2.0 | 2.6 | 2.1 | 0.2 | 7.8 | 0.468 | 16.6 | 19.7 |  $-$0.6 | DA [3] | 5540 | 0.75 | 0.0 | 5.9 |
4565048312887877888 | 254.29109 | 21.44685 | GC | 2 | 4.3 | 5.4 | 4.4 | 0.7 | 7.8 | 0.199 | 13.9 | 19.8 |  $-$0.7 | DA [3] | 9200 | 0.60 | 0.0 | 10.1 |
1638563322306634368 | 258.27492 | 69.52375 | GC | 2 | 67.1 | 68.2 | 59.7 | 0.6 | 62.6 | 0.000 | 13.4 | 17.7 |  $-$0.7 | DA [3] | 15730 | 0.61 | 0.0 | 10.4 |
1348275102768198912 | 261.87223 | 42.08119 | GC | * | 1.5 | 1.7 | 2.6 | 0.7 | 16.5 | 0.042 | 16.5 | 19.3 |  $-$0.8 | DA [5] | 9580 | 0.65 | 0.0 |  |
5913195150770419840 | 262.59637 |  $-$57.92381 | GC | * | 2.5 | 4.0 | 2.4 |  | 7.5 | 0.911 | 15.4 | 19.6 |  $-$1.0 | DBA [39] | 14060 | 0.61 | 0.4 | 6.5 |
1706631093589103872 | 263.40091 | 79.82129 | GC | 1 | 1.1 | 1.8 | 2.4 | 1.3 | 28.1 | 0.346 | 16.5 | 18.9 |  $-$0.7 | DC [14] | 5760 | 0.84 | 1.0 |  |
4557279335526790528 | 265.13949 | 23.34029 | GC | * | 3.1 | 3.5 | 3.2 |  | 6.4 | 0.000 | 16.7 | 20.2 |  $-$0.5 | DC [5] | 7170 | 0.75 | 0.0 | 7.7 |
5920900901901635968 | 266.90216 |  $-$54.60866 | GC | 1 | 1.3 | 2.0 | 4.7 | 1.5 | 28.6 | 1.139 | 15.2 | 18.0 |  $-$0.6 | DC [1] | 4350 | 0.47 | 0.8 |  |
1417200020676904704 | 269.49775 | 54.68952 | GC | 2 | 2.7 | 3.8 | 2.5 | 1.1 | 10.7 | 0.316 | 16.8 | 19.8 |  $-$0.7 | DA [14] | 7670 | 0.56 | 0.0 |  |
4470233817461336704 | 271.84740 | 3.95050 | GC | 2 | 2.6 | 2.6 | 2.7 | 0.9 | 33.1 | 0.582 | 14.5 | 17.8 |  $-$0.8 | DA [3] | 10220 | 0.64 | 0.1 |  |
4497414466452138496 | 274.27702 | 13.47361 | GC | 3 | 5.6 | 5.8 | 5.8 | 0.1 | 9.7 | 0.818 | 15.0 | 19.5 |  $-$0.5 | DA [3] | 4870 | 0.48 | 0.3 |  |
2158285185808357504 | 275.33333 | 61.01872 | GC | 1 | 1.2 | 1.3 | 4.5 | 1.7 | 6.6 | 0.157 | 14.7 | 20.3 |  $-$0.6 | DA [3] | 4820 | 0.46 | 0.0 | 3.5 |
2256410856215182464 | 278.82519 | 64.35491 | GC | * | 2.2 | 2.3 | 2.0 | 0.6 | 25.9 | 0.045 | 16.7 | 18.7 |  $-$0.7 | DC [1] | 4900 | 0.58 | 1.1 | 9.5 |
2156469995191226112 | 279.53127 | 60.36160 | GC | * | 2.9 | 3.2 | 2.8 | 0.6 | 40.7 | 0.024 | 15.6 | 18.4 |  $-$0.7 | DA [5] | 8990 | 0.52 | 0.0 |  |
2127566548919332608 | 291.61211 | 46.33753 | GC | 2 | 2.3 | 3.0 | 2.2 | 0.8 | 29.1 | 0.826 | 15.6 | 18.6 |  $-$0.9 | DA [40] | 8140 | 0.72 | 0.4 | 7.7 | 4.66, 2.4
6740493675455190784 | 296.22801 |  $-$36.05303 | GC | * | 1.9 | 2.7 | 2.0 | 1.1 | 42.0 | 0.126 | 14.1 | 17.5 |  $-$0.7 | DA [26] | 13080 | 0.67 | 0.8 | 9.9 |
2264107613704439808 | 296.25519 | 73.27496 | GC | * | 1.3 | 1.3 | 2.2 | 1.5 | 21.7 | 1.387 | 16.5 | 19.0 |  $-$0.9 | DA [5] | 8330 | 0.56 | 0.0 |  |
2263690864438162944 | 297.69110 | 71.92769 | GC | * | 2.0 | 2.7 | 2.0 | 0.9 | 14.2 | 1.258 | 16.7 | 19.5 |  $-$0.8 | DA [3] | 11870 | 0.72 | 0.0 |  |
4235280071072332672 | 299.12178 |  $-$1.04241 | GC | 1 | 63.3 | 96.7 | 592.3 | 7.8 | 35.8 | 0.519 | 13.3 | 17.6 |  $-$0.8 | DAH [1] | 7790 | 0.70 | 12.0 |  |
6427468556182061312 | 302.09929 |  $-$66.07714 | GC | * | 2.2 | 2.8 | 2.4 | 1.4 | 14.2 | 0.208 | 16.1 | 18.9 |  $-$0.2 | DA [17] | 21380 | 0.72 | 1.1 |  |
4223980836743666176 | 304.46180 |  $-$1.44747 | GC | * | 2.2 | 3.5 | 2.1 |  | 7.2 | 1.211 | 15.7 | 19.7 |  $-$0.7 | DA [5] | 16260 | 0.57 | 0.4 | 7.8 |
1831553382794173824 | 308.59100 | 25.06382 | GC | 1 | 1.3 | 1.7 | 16.7 | 1.2 | 15.4 | 0.072 | 11.7 | 18.8 |  $-$0.7 | DA [37] | 20140 | 0.59 | 0.3 | 10.3 |
6471102606408911360 | 309.57045 |  $-$53.07371 | GC | * | 2.0 | 2.0 | 2.6 | 1.2 | 28.7 | 0.103 | 14.4 | 18.2 |  $-$0.7 | DB [3] | 15720 | 0.40 | 0.1 | 6.7 |
6683212727417918848 | 309.67282 |  $-$36.82044 | GC | * | 1.9 | 2.4 | 2.3 | 0.9 | 28.4 | 0.215 | 14.8 | 18.0 |  $-$0.8 | DA [3] | 9490 | 0.62 | 0.0 | 9.2 |
6681773947733560192 | 310.95503 |  $-$39.05501 | GC | 1,[g] | 3.1 | 3.7 | 2.9 | 1.5 | 34.1 | 1.187 | 13.7 | 17.6 |  $-$0.1 | DA [3] | 11220 | 0.63 | 6.7 | 0.3 | 0.17, 0.3
6794613321399870080 | 312.37830 |  $-$30.08413 | GC | * | 2.9 | 4.5 | 2.9 | 0.2 | 13.3 | 0.971 | 15.4 | 18.9 |  $-$0.8 | DBA [39] | 12030 | 0.70 | 0.0 | 6.5 |
2221406155496413952 | 317.46853 | 65.12276 | GC | * | 2.1 | 3.6 | 2.3 | 1.1 | 21.8 | 0.144 | 15.5 | 19.0 |  $-$0.8 | DAH [26] | 21400 | 0.88 | 0.0 | 0.4 |
2690763205257368832 | 317.94327 | 1.34841 | GC | 2 | 2.2 | 3.0 | 2.6 |  | 8.1 | 0.813 | 15.4 | 19.4 |  $-$0.7 | DA [14] | 15730 | 0.58 | 0.3 | 8.2 |
6896767366186700416 | 318.03855 |  $-$8.82688 | CV |  | 35.1 | 41.7 | 34.5 | 8.4 | 51.2 | 0.000 | 16.4 | 17.3 |  $-$0.3 | CV [41] |  |  | 34.3 | 10.3 | 1.54, 7.1
6583325635088476544 | 320.52351 |  $-$38.64296 | GC | 2 | 182.1 | 272.5 | 180.1 | 16.7 | 24.0 | 0.059 | 15.7 | 18.7 |  $-$0.7 | DC [1] | 5100 | 0.47 | 0.0 | 7.9 |
2220815923910913920 | 320.63282 | 66.01214 | GC | 2 | 2.0 | 2.4 | 2.0 | 0.9 | 13.7 | 0.434 | 15.8 | 19.4 |  $-$0.7 | DB [5] | 11250 | 0.74 | 0.0 |  |
1785659488269249408 | 320.77531 | 19.28408 | GC | * | 2.7 | 3.2 | 2.6 |  | 6.8 | 0.511 | 16.7 | 19.9 |  $-$0.9 | DA [23] | 9840 | 0.63 | 0.0 | 5.2 |
6828110179364684672 | 321.60953 |  $-$22.73151 | GC | * | 2.3 | 3.0 | 2.1 | 0.0 | 10.2 | 0.057 | 15.4 | 19.2 |  $-$0.9 | DA [3] | 15020 | 0.69 | 0.0 | 8.0 |
6830482410058056192 | 325.89003 |  $-$19.20734 | GC | * | 2.1 | 3.1 | 2.4 | 0.0 | 7.1 | 0.000 | 16.9 | 19.7 |  $-$0.5 | DA [5] | 7460 | 0.53 | 0.0 | 7.0 |
2673699059671833984 | 328.42542 |  $-$2.24063 | GC | * | 2.1 | 3.4 | 2.1 | 0.3 | 9.9 | 0.081 | 17.0 | 19.4 |  $-$0.9 | DA [5] | 6620 | 0.56 | 0.0 | 2.3 |
2676566272464334720 | 329.69637 |  $-$2.67345 | GC | 2 | 2.2 | 2.9 | 2.1 | 1.3 | 19.1 | 0.000 | 16.5 | 18.6 |  $-$0.7 | DC [1] | 4730 | 0.48 | 0.0 | 9.2 |
6613289285448236288 | 331.15850 |  $-$31.45382 | GC | 1 | 1.8 | 2.4 | 5.4 | 1.6 | 11.5 | 0.068 | 16.2 | 19.1 |  $-$0.2 | DA [11] | 4810 | 0.53 | 0.1 | 0.2 |
2595728287804350720 | 336.07265 |  $-$16.26310 | IR, GC |  | 1.6 | 2.3 | 3.4 | 0.7 | 19.5 | 0.663 | 15.9 | 18.5 |  $-$0.4 | DAZ [6] | 9900 | 0.67 | 0.0 | 8.2 |
2737918849495239936 | 337.11568 | 17.90141 | GC | * | 2.4 | 3.7 | 2.4 |  | 8.5 | 0.280 | 16.3 | 19.6 |  $-$1.0 | DA [23] | 14650 | 0.59 | 0.7 | 7.0 |
2623101763649861632 | 337.14432 |  $-$6.20543 | GC | * | 2.2 | 3.3 | 2.2 | 1.4 | 12.9 | 0.000 | 16.3 | 19.0 |  $-$0.9 | DA [5] | 6940 | 0.54 | 0.6 | 8.1 |
2842650153836732928 | 346.57374 | 24.53547 | GC | [h] | 2.2 | 3.4 | 2.1 | 1.2 | 20.3 | 0.000 | 15.3 | 18.5 |  $-$0.7 | DA [3] | 11430 | 0.60 | 10.0 | 8.8 | 0.22, 1.7
2407167579853954688 | 348.87669 |  $-$14.66824 | GC | 2 | 2.9 | 4.2 | 2.7 | 0.5 | 20.5 | 0.079 | 15.3 | 18.4 |  $-$0.8 | DA [35] | 8850 | 0.84 | 0.0 | 7.9 |
2393546245693497728 | 351.32675 |  $-$17.86607 | GC | * | 2.4 | 3.1 | 2.3 |  | 7.6 | 0.000 | 15.6 | 19.6 |  $-$1.1 | DA [3] | 22030 | 0.58 | 0.6 | 7.0 |
6485572518732377856 | 355.75224 |  $-$64.79386 | GC | 1 | 2.0 | 2.9 | 2.0 | 1.8 | 15.4 | 0.019 | 16.5 | 19.0 |  $-$0.7 | DC [1] | 5790 | 0.58 | 0.5 | 0.8 |
2763719512612656000 | 356.55201 | 11.98061 | GC | * | 2.0 | 3.2 | 2.0 | 0.2 | 10.9 | 0.374 | 16.7 | 19.3 |  $-$0.8 | DC [14] | 6210 | 0.78 | 0.0 | 7.7 |
6525205175564946816 | 356.55229 |  $-$47.24147 | GC | * | 1.4 | 2.1 | 2.0 | 0.7 | 16.3 | 0.458 | 15.5 | 19.0 |  $-$0.8 | DA [5] | 12500 | 0.64 | 0.0 | 7.7 |

Note. — Description of columns: Legend for the Sample column: Ca II are
objects with previously identified Ca II gas emission (Table 3), IR refers to
non-Ca II debris disk systems (Table 4), CV are known cataclysmic variables,
and GC are objects from our Gaia-catWISE crossmatch. Objects flagged with
asterisks are believed to be newly discovered near-infrared variables; they
are not known CVs nor are they published in Swan et al. (2019, 2020, 2021). We
also flag known ZZ Cetis: [a] (Fontaine et al., 2003), [b] (Romero et al.,
2022), [c] (Vauclair et al., 1997), [d] (Gianninas et al., 2006), [e] (Kanaan
et al., 1992), [f] (Vauclair et al., 2000), [g] (Romero et al., 2019), [h]
(Vauclair et al., 1987). We define $G_{\rm RP}$ Excess as the ratio of the
summed flux in the Gaia $RP$ bandpass in a 30″ cone around the target white
dwarf to target white dwarf’s $RP$ flux. Flag legend: 1=contaminated as seen
in WISE View, 2=false-positive variability due to a seasonal alternating
pattern, 3=contaminated both as seen in WISE View and from alternating. S/NW1
is the median signal-to-noise ratio of our $W1$ light curves. All magnitudes
are reported here on the AB scale. catWISE $W1-W2$ colors are also on the AB
scale. References for the tabulated spectral types are as follows: [1] O’Brien
et al. (2024), [2] Coutu et al. (2019), [3] McCook & Sion (1999), [4] Guo et
al. (2015), [5] Vincent et al. (2024), [6] Koester et al. (2005), [7]
Meinunger (1984), [8] Kilkenny et al. (2016), [9] Subasavage et al. (2022),
[10] Gianninas et al. (2011), [11] O’Brien et al. (2023), [12] Limoges et al.
(2013), [13] Dennihy et al. (2020b), [14] Limoges et al. (2015), [15] Koester
et al. (1982), [16] Bergeron et al. (2021), [17] O’Donoghue et al. (2013),
[18] Bateson et al. (1991), [19] Melis et al. (2020), [20] Gentile Fusillo et
al. (2021a), [21] Zhang et al. (2013), [22] Howarth (1978), [23] Kilic et al.
(2020), [24] Warner (1974), [25] Udalski (1990), [26] Raddi et al. (2017),
[27] Kleinman et al. (2013), [28] Sayres et al. (2012), [29] Sion et al.
(2014), [30] Bergeron et al. (2001), [31] Šimon (2000), [32] Burbidge &
Strittmatter (1971), [33] Caron et al. (2023), [34] Bergeron et al. (1997),
[35] Kawka & Vennes (2012), [36] Kilic et al. (2012), [37] McCleery et al.
(2020), [38] Kato et al. (2000), [39] Bergeron et al. (2011), [40] Tremblay et
al. (2020), [41] Harrison et al. (2009). $T_{\mathrm{eff}}$ and $M_{\rm WD}$
are derived from Gaia photometry assuming a hydrogen-dominated atmosphere
(Gentile Fusillo et al., 2021b). EFF=excess_flux_error (Gentile Fusillo et
al., 2021b). FoM is the Figure of Merit metric computed from the Gaia-AllWISE
neighbor crossmatch analysis (Marrese et al., 2017, 2019, 2022). In the TESS
column we report the significant periodicities in TESS that we attributed
intrinsic to the white dwarf using TESS_localize in hours and the associated
amplitude in %.

=1.0cm

Table 3: Variability Parameters for the 21 Published White Dwarf Debris Disks
That Show Calcium II Gas in Emission

Figure 6 | Gaia DR3 Source ID | Alt. Name | R.A. | Decl. | Flag | $\chi^{2}_{W1}$ | $\chi^{2}_{\mathrm{alt}}$ | $\chi^{2}_{\mathrm{flat}}$ | $\chi^{2}_{W2}$ | S/N |  $G_{\rm RP}$ Excess | $G_{\rm RP}$ | $W1$ | $W1-W2$ | $T_{\mathrm{eff}}$ | $M_{\rm WD}$ | EFF | FoM
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---
|  |  | (J2000) | (J2000) |  |  |  |  |  | $W1$ |  | (mag) | (mag) | (mag) | (K) | ($M_{\odot}$) |  |
a | 2860923998433585664 | SDSS J0006+2858 | 1.64464 | 28.97960 | 1 | 0.3 | 0.3 | 0.3 | 1.0 | 5.3 | 237.861 | 16.5 | 18.1 |  $-$0.0 | 21980 | 0.55 | 1.7 |
b | 291057843317534464 | WD 0145+234 | 26.97841 | 23.66211 |  | 422.6 | 690.7 | 482.4 | 254.9 | 54.3 | 0.021 | 14.1 | 16.9 |  $-$0.4 | 12980 | 0.66 | 0.8 | 5.5
c | 2489275328645218560 | SDSS J0234$-$0406 | 38.56461 |  $-$4.10258 | 1 | 1.2 | 1.8 | 1.3 | 0.6 | 12.5 | 3.039 | 16.5 | 19.1 |  $-$0.5 | 13100 | 0.63 | 0.7 | 8.3
d | 43629828277884160 | SDSS J0347+1624 | 56.90288 | 16.40270 | * | 12.3 | 17.7 | 12.1 | 0.5 | 11.2 | 0.940 | 16.7 | 19.2 |  $-$0.4 | 17710 | 0.52 | 0.0 | 6.9
e | 3415788525598117248 | LAMOST J0510+2315 | 77.50894 | 23.26151 | * | 41.9 | 65.9 | 49.5 | 16.0 | 41.8 | 0.796 | 15.2 | 17.4 |  $-$0.1 | 21250 | 0.73 | 0.0 | 9.0
f | 4823042355496728832 | WD J0529–3401 | 82.30968 |  $-$34.01892 | * | 2.0 | 2.2 | 6.4 | 0.8 | 28.5 | 0.230 | 17.6 | 18.4 | 0.1 | 17190 | 0.46 | 0.0 |
g | 5279484614703730944 | Gaia J0611−6931 | 92.88208 |  $-$69.51726 | 1 | 45.6 | 129.8 | 93.1 | 22.6 | 120.3 | 4.803 | 16.9 | 17.1 | 0.1 | 16140 | 0.54 | 0.0 |
h | 3105360521513256832 | Gaia J0644−0352 | 101.02179 |  $-$3.86845 | 1 | 2.2 | 2.9 | 2.2 | 2.1 | 7.6 | 25.020 | 16.3 | 19.2 | 0.0 | 17380 | 0.56 | 0.0 | 7.2
i | 671450448046315520 | SDSS J0738+1835 | 114.67736 | 18.58603 | 1 | 1.1 | 1.8 | 1.0 | 0.3 | 5.3 | 6.238 | 17.8 | 20.0 |  $-$0.3 | 15740 | 0.72 | 0.1 |
j | 689352219629097856 | WD 0842+231 | 131.41326 | 22.95785 |  | 1.7 | 2.4 | 1.7 | 1.7 | 22.8 | 0.000 | 16.0 | 18.3 |  $-$0.2 | 19180 | 0.58 | 0.0 | 3.2
k | 1037258898615762048 | WD 0842+572 | 131.51030 | 57.05795 | * | 25.2 | 37.1 | 26.1 | 9.0 | 95.4 | 0.090 | 16.9 | 16.4 | 0.0 | 15410 | 0.57 | 0.1 | 8.0
l | 3829599892897720832 | SDSS J0959$-$0200 | 149.76957 |  $-$2.01323 |  | 1.0 | 1.5 | 1.0 | 0.0 | 4.7 | 0.000 | 18.2 | 20.2 |  $-$0.0 | 11330 | 0.48 | 0.0 | 6.2
m | 3869060540584643328 | WD 1041+092 | 160.92305 | 8.93287 |  | 0.8 | 1.4 | 0.8 | 0.0 | 5.1 | 0.000 | 17.4 | 20.2 |  $-$1.0 | 16060 | 0.59 | 0.0 |
n | 3904415787947492096 | SDSS J1228+1040 | 187.24973 | 10.67585 |  | 1.5 | 2.2 | 2.0 | 1.3 | 15.8 | 0.122 | 16.6 | 18.8 |  $-$0.0 | 19310 | 0.62 | 0.4 | 5.6
o | 6287259310145684608 | HE 1349$-$2305 | 208.18385 |  $-$23.33483 |  | 0.9 | 1.4 | 1.7 | 1.0 | 13.8 | 0.086 | 16.6 | 18.9 |  $-$0.4 | 16540 | 0.53 | 0.0 |
p | 4465269178854732160 | SDSS J1617+1620 | 244.32100 | 16.33960 |  | 1.4 | 2.2 | 1.7 | 1.0 | 13.2 | 0.718 | 17.2 | 19.2 |  $-$0.4 | 13220 | 0.63 | 0.0 | 8.2
q | 1623866184737702912 | WD 1622+587 | 245.74852 | 58.67525 | * | 9.8 | 14.8 | 9.2 | 2.9 | 20.9 | 0.152 | 17.0 | 19.2 |  $-$0.2 | 18790 | 0.48 | 1.5 |
r | 6646693887514073984 | WD J1930–5028 | 292.65688 |  $-$50.47136 |  | 1.0 | 1.3 | 1.0 | 0.7 | 12.3 | 0.482 | 17.1 | 19.2 | 0.0 | 13570 | 0.63 | 0.9 | 6.8
s | 1837948790953103232 | Gaia J2100+2122 | 315.14437 | 21.38247 | 2 | 6.7 | 6.2 | 6.7 | 1.9 | 53.8 | 1.772 | 15.4 | 17.3 | 0.0 | 25030 | 0.65 | 0.0 | 10.2
t | 1797494081674032512 | SDSS J2133+2428 | 323.46132 | 24.46831 | 1 | 12.9 | 23.5 | 15.1 | 4.2 | 17.2 | 1.536 | 17.5 | 18.9 |  $-$0.3 | 26320 | 0.58 | 0.0 | 9.4
u | 2600209141285265280 | ATLAS J2212–1352 | 333.01198 |  $-$13.87777 | 1 | 1.7 | 1.9 | 1.7 | 1.7 | 10.5 | 2.554 | 17.5 | 19.1 | 0.1 | 19360 | 0.53 | 0.0 | 8.7

Note. — Description of columns: Flag legend: 1=likely crowded ($G_{\rm RP}$
Excess$>$1.415), 2=likely alternating ($\chi^{2}_{\mathrm{alt}}$
$<$$\chi^{2}_{W1}$, see Section 3). Asterisks denote new near-infrared
variables. We define $G_{\rm RP}$ Excess as the ratio of the summed flux in
the Gaia $RP$ bandpass in a 30″ cone around the target white dwarf to target
white dwarf’s $RP$ flux (Section 3). S/NW1 is the median signal-to-noise ratio
of our $W1$ light curves. All magnitudes are reported here on the AB scale.
catWISE $W1-W2$ colors are also on the AB scale. $T_{\mathrm{eff}}$ and
$M_{\rm WD}$ are derived from Gaia photometry assuming a hydrogen-dominated
atmosphere (Gentile Fusillo et al., 2021b). EFF=excess_flux_error (Gentile
Fusillo et al., 2021b). FoM is the Figure of Merit metric computed from the
Gaia-AllWISE neighbor crossmatch analysis (Marrese et al., 2017, 2019, 2022).
We also guide the reader to Table 3 of Gentile Fusillo et al. (2021a) for a
summary of the gaseous species in emission towards these 21 white dwarfs.

=1.5cm

Table 4: Variability Parameters for the White Dwarf Debris Disks That Do Not
Show Calcium II Gas in Emission

Figure 7, 8 | Gaia DR3 Source ID | SIMBAD ID | R.A. | Decl. | Flag | $\chi^{2}_{W1}$ | $\chi^{2}_{\mathrm{alt}}$ | $\chi^{2}_{\mathrm{flat}}$ | $\chi^{2}_{W2}$ | S/N |  $G_{\rm RP}$ Excess | $G_{\rm RP}$ | $W1$ | $W1-W2$ | $T_{\mathrm{eff}}$ | $M_{\rm WD}$ | EFF | FoM
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---
|  |  | (J2000) | (J2000) |  |  |  |  |  | $W1$ |  | (mag) | (mag) | (mag) | (K) | ($M_{\odot}$) |  |
1 | 2859951106737135488 | PG 0010+281 | 3.33780 | 28.33884 |  | 1.0 | 1.8 | 1.0 | 1.5 | 15.6 | 0.200 | 15.9 | 18.9 |  $-$0.5 | 25150 | 0.52 | 0.0 | 5.6
2 | 5026963661794939520 | Ton S 193 | 17.15014 |  $-$32.62872 |  | 2.2 | 2.6 | 2.5 | 0.9 | 28.9 | 1.110 | 15.5 | 18.1 |  $-$0.7 | 16050 | 0.63 | 0.0 |
3 | 4913589203924379776 | JL 234 | 18.08809 |  $-$56.24098 |  | 2.6 | 4.1 | 2.7 | 1.1 | 20.4 | 0.000 | 15.9 | 18.8 |  $-$0.3 | 18790 | 0.59 | 0.0 | 8.7
4 | 95297185335797120 | Wolf 88 | 27.23665 | 19.04097 | 2 | 5.2 | 4.3 | 4.9 | 1.8 | 27.8 | 0.022 | 15.5 | 18.0 | 0.0 | 11690 | 0.77 | 0.0 | 10.2
5 | 2518936059258271232 | GALEX J015550.2+043131 | 28.95924 | 4.52516 |  | 1.5 | 2.2 | 1.4 |  | 5.7 | 0.000 | 16.9 | 20.0 |  $-$0.8 | 14110 | 0.55 | 0.0 | 6.0
6 | 547501815051141248 | EGGR 474 | 42.96365 | 73.69309 | 1 | 0.6 | 0.7 | 0.6 |  | 6.3 | 4.494 | 17.0 | 20.2 |  $-$0.6 | 8600 | 0.81 | 0.0 | 3.1
7 | 5187830356195791488 | GD 40 | 45.72127 |  $-$1.14272 |  | 1.2 | 1.7 | 1.2 | 1.3 | 22.2 | 0.000 | 15.5 | 18.4 |  $-$0.2 | 14090 | 0.60 | 0.0 | 9.1
8 | 13611477211053824 | HS 0307+0746 | 47.53816 | 7.95891 |  | 1.1 | 1.6 | 1.0 | 0.0 | 8.3 | 0.000 | 16.1 | 19.5 |  $-$0.8 | 10100 | 0.59 | 0.6 | 8.4
9 | 3251748915515143296 | GD 56 | 62.75904 |  $-$3.97294 |  | 4.9 | 6.8 | 40.0 | 3.3 | 56.2 | 0.268 | 15.7 | 17.1 | 0.2 | 14900 | 0.61 | 0.1 | 10.7
10 | 4653404070862114176 | MCT 0420$-$7310 | 64.90760 |  $-$73.06236 | * | 2.4 | 3.9 | 3.5 | 1.3 | 59.6 | 0.150 | 15.7 | 17.7 |  $-$0.1 | 17790 | 0.58 | 0.0 | 9.6
11 | 271992414775824640 | KPD 0420+5203 | 66.06547 | 52.16962 | * | 3.1 | 4.9 | 17.4 | 2.2 | 40.1 | 0.876 | 15.2 | 17.6 |  $-$0.3 | 22170 | 0.66 | 0.9 | 8.9
12 | 203931163247581184 | GD 61 | 69.66406 | 41.15899 | 1 | 1.1 | 1.3 | 1.1 | 0.9 | 21.7 | 4.625 | 14.9 | 18.1 |  $-$0.5 | 16940 | 0.63 | 0.9 |
13 | 706076062068973952 | GALEX J084244.5+294815 | 130.68559 | 29.80441 | 0 |  |  | 0.0 |  | 4.0 | 13.083 | 18.5 | 20.4 |  $-$0.2 | 13120 | 0.47 | 0.0 |
14 | 1029081452683108480 | PG 0843+517 | 131.75953 | 51.48149 |  | 0.9 | 1.5 | 0.9 | 0.6 | 17.9 | 0.000 | 16.3 | 18.9 |  $-$0.3 | 21210 | 0.51 | 0.2 | 7.4
15 | 692795546450243456 | GALEX J085028.3+271956 | 132.61811 | 27.33230 |  | 0.4 | 0.5 | 0.5 | 0.2 | 6.7 | 0.000 | 17.5 | 19.9 |  $-$0.6 | 18900 | 0.58 | 0.0 | 0.0
16 | 719422660057614080 | GALEX J090525.5+394339 | 136.35620 | 39.72756 | 0 |  |  |  |  |  | 0.269 | 18.0 |  | 0.1 | 12970 | 0.34 | 0.0 |
17 | 618139424181751552 | GALEX J092801.1+133219 | 142.00460 | 13.53869 |  | 0.7 | 0.9 | 0.7 | 0.0 | 4.5 | 0.000 | 18.1 | 20.3 | 0.1 | 22500 | 0.63 | 0.0 | 5.9
18 | 3888723386196630784 | PG 1015+161 | 154.51595 | 15.86627 |  | 0.8 | 1.0 | 0.8 | 1.0 | 18.6 | 0.032 | 15.8 | 18.6 |  $-$0.3 | 18370 | 0.60 | 0.0 | 7.4
19 | 804169064957527552 | PG 1018+411 | 155.48128 | 40.83747 | 1 | 1.4 | 2.2 | 1.4 | 0.2 | 8.7 | 4.317 | 16.6 | 19.6 |  $-$0.4 | 20750 | 0.63 | 0.0 | 8.0
20 | 3862040300575181184 | PG 1031+063 | 158.52221 | 6.04658 | 2 | 0.9 | 0.9 | 0.9 | 0.0 | 7.0 | 0.000 | 16.5 | 19.8 |  $-$1.0 | 19600 | 0.54 | 0.2 | 3.0
21 | 3808488483665268736 | GALEX J105807.8+020648 | 164.53280 | 2.11356 | 0 | 0.3 |  | 0.2 |  | 4.6 | 0.000 | 17.0 | 20.3 |  $-$0.1 | 17350 | 0.52 | 0.0 | 5.4
22 | 3810933247769901696 | GD 133 | 169.80166 | 2.34251 | 2 | 2.3 | 2.2 | 2.8 | 0.6 | 48.8 | 0.008 | 14.7 | 17.3 |  $-$0.2 | 12150 | 0.63 | 1.1 |
23 | 4019789359821201536 | US 2966 | 176.99441 | 28.53223 | * | 1.9 | 3.1 | 2.6 | 0.9 | 10.1 | 0.245 | 17.5 | 19.4 | 0.1 | 11990 | 0.58 | 0.3 | 8.3
24 | 3796414192429498880 | LBQS 1145+0145 | 177.14012 | 1.48317 |  | 0.3 | 0.5 | 0.4 |  | 5.2 | 0.000 | 17.3 | 20.1 |  $-$0.1 | 16030 | 0.66 | 5.4 | 6.8
25 | 3571559292842744960 | EC 11507$-$1519 | 178.31358 |  $-$15.61014 |  | 4.6 | 6.6 | 4.3 | 3.2 | 40.5 | 0.046 | 16.0 | 17.7 | 0.2 | 11770 | 0.62 | 0.1 | 9.4
26 | 3908649148233327232 | GALEX J122150.8+124512 | 185.46171 | 12.75370 | 1 | 1.0 | 1.7 | 1.0 | 0.4 | 8.7 | 1.687 | 18.3 | 19.5 |  $-$0.1 | 12390 | 0.69 | 1.3 |
27 | 3583181371265430656 | PG 1225$-$079 | 186.94733 |  $-$8.24388 |  | 1.8 | 2.1 | 1.7 | 2.0 | 28.4 | 0.030 | 14.7 | 18.0 |  $-$0.7 | 11720 | 0.77 | 0.0 | 8.4
28 | 1571584539980588544 | SBSS 1232+563 | 188.63615 | 56.11195 |  | 1.9 | 2.5 | 1.8 | 0.5 | 8.7 | 0.258 | 18.1 | 19.8 |  $-$0.0 | 12570 | 0.70 | 13.2 | 7.2
29 | 4011097617323921920 | GALEX J123843.9+291004 | 189.68301 | 29.16762 |  | 0.5 | 1.1 | 0.5 |  | 3.8 | 0.000 | 18.9 | 20.8 |  $-$0.1 | 14820 | 0.86 | 0.0 | 2.6
30 | 1188075433968561792 | PG 1454+173 | 224.17141 | 17.07087 |  | 1.4 | 2.0 | 1.4 |  | 5.6 | 0.647 | 17.3 | 20.2 |  $-$0.2 | 34150 | 0.76 | 0.0 |
31 | 1281989124439286912 | EGGR 298 | 224.52704 | 29.62487 | 2 | 2.4 | 2.2 | 3.9 | 0.5 | 15.0 | 0.044 | 15.2 | 19.1 |  $-$0.2 | 7340 | 0.58 | 0.0 |
32 | 6332172027974304512 | PG 1457$-$086 | 224.97079 |  $-$8.82490 | 2 | 1.3 | 1.2 | 1.4 | 0.4 | 10.9 | 0.014 | 15.9 | 19.2 |  $-$1.0 | 18330 | 0.51 | 2.1 | 6.6
33 | 1288812212565231232 | GALEX J150701.9+324545 | 226.75835 | 32.76250 | 3 | 1.4 | 1.3 | 1.4 | 0.7 | 6.4 | 1.864 | 18.5 | 20.2 |  $-$0.2 | 6960 | 0.53 | 0.1 | 0.1
34 | 1595298501827000960 | SBSS 1536+520 | 234.35719 | 51.85745 | 2 | 2.5 | 2.1 | 3.9 | 1.3 | 30.6 | 0.000 | 17.4 | 18.6 | 0.1 | 18180 | 0.51 | 0.0 | 3.6
35 | 1641326979142898048 |  KX Dra | 235.43704 | 64.89805 |  | 1.9 | 2.5 | 3.2 | 1.6 | 65.7 | 0.188 | 15.6 | 17.8 | 0.2 | 11600 | 0.63 | 10.2 | 10.5
36 | 1196531988354226560 | KUV 15519+1730 | 238.53757 | 17.35672 |  | 0.1 | 0.1 | 0.1 |  | 3.7 | 0.652 | 17.4 | 20.7 |  $-$0.8 | 15630 | 0.58 | 0.0 |
37 | 4454599238843776128 | GALEX J155720.8+091625 | 239.33658 | 9.27355 | 0 | 0.0 |  | 0.0 |  | 3.7 | 0.000 | 18.8 | 20.7 | 0.2 | 25260 | 0.51 | 1.6 | 3.3
38 | 1336442472164656000 | GD 362 | 262.89299 | 37.08909 | 3 | 3.2 | 1.3 | 3.3 | 1.1 | 31.4 | 66.096 | 16.0 | 18.3 | 0.1 | 10480 | 0.70 | 0.0 | 10.2
39 | 4287654959563143168 | GALEX J193156.8+011745 | 292.98722 | 1.29559 | 3 | 0.5 | 0.4 | 0.5 | 0.6 | 28.2 | 5.375 | 14.5 | 16.8 |  $-$0.2 | 21540 | 0.62 | 0.7 |
40 | 6462911897617050240 | LAWD 84 | 319.90222 |  $-$55.83735 | 2 | 24.8 | 1.6 | 23.3 | 1.6 | 50.7 | 0.062 | 14.2 | 17.3 |  $-$0.1 | 9520 | 0.58 | 0.0 | 10.1
41 | 1742342784582615936 | HS 2132+0941 | 323.71172 | 9.92229 |  | 0.8 | 1.2 | 0.8 | 0.6 | 12.2 | 0.054 | 16.0 | 19.2 |  $-$0.6 | 13210 | 0.59 | 0.0 | 8.0
42 | 6592315723192176896 | L 570$-$26 | 325.48985 |  $-$33.00828 | 2 | 4.5 | 1.6 | 4.9 | 1.5 | 55.9 | 0.007 | 14.2 | 17.1 |  $-$0.6 | 7550 | 0.71 | 0.3 | 10.2
43 | 2683271785860305280 | Gaia DR2 2683271785860305280 | 331.04273 | 2.55887 |  | 1.2 | 1.7 | 1.1 | 0.1 | 6.5 | 0.000 | 17.3 | 19.9 |  $-$0.1 | 15480 | 0.67 | 0.8 | 2.6
44 | 2727904257071365760 | GALEX J220934.8+122337 | 332.39521 | 12.39349 |  | 1.5 | 2.4 | 1.4 | 0.7 | 10.8 | 1.150 | 17.4 | 19.3 |  $-$0.1 | 14450 | 0.54 | 0.0 |
45 | 2595728287804350720 | PHL 5103 | 336.07265 |  $-$16.26310 |  | 1.6 | 2.3 | 3.4 | 0.7 | 19.5 | 0.663 | 15.9 | 18.5 |  $-$0.4 | 9900 | 0.67 | 0.0 | 8.2
46 | 2660358032257156736 | G 29$-$38 | 352.19849 | 5.24840 | 2 | 23.7 | 5.9 | 23.9 | 7.0 | 46.8 | 0.049 | 13.1 | 16.4 | 0.1 | 11520 | 0.63 | 8.2 | 11.5
47 | 2762605088857836288 | PG 2328+108 | 352.67362 | 11.03513 |  | 1.3 | 2.1 | 1.5 | 0.9 | 12.3 | 0.673 | 15.7 | 19.1 |  $-$0.8 | 17870 | 0.46 | 0.0 | 8.3

Note. — Description of columns: Flag legend: 1=likely crowded ($G_{\rm RP}$
Excess$>$1.415), 2=likely alternating ($\chi^{2}_{\mathrm{alt}}$
$<$$\chi^{2}_{W1}$, see Section 3), 3=both likely crowded and likely
alternating, and 0=insufficient $W1$ data to analyze. Asterisks denote new
near-infrared variables. We define $G_{\rm RP}$ Excess as the ratio of the
summed flux in the Gaia $RP$ bandpass in a 30″ cone around the target white
dwarf to target white dwarf’s $RP$ flux (Section 3). S/NW1 is the median
signal-to-noise ratio of our $W1$ light curves. All magnitudes are reported
here on the AB scale. catWISE $W1-W2$ colors are also on the AB scale.
$T_{\mathrm{eff}}$ and $M_{\rm WD}$ are derived from Gaia photometry assuming
a hydrogen-dominated atmosphere (Gentile Fusillo et al., 2021b).
EFF=excess_flux_error (Gentile Fusillo et al., 2021b). FoM is the Figure of
Merit metric computed from the Gaia-AllWISE neighbor crossmatch analysis
(Marrese et al., 2017, 2019, 2022).

##  Appendix B LDT+DeVeny and SOAR+Goodman Follow-up Spectroscopy

Table 5 shows an observing log of all of the near-infrared spectra we publish
in Figure 5. A description of our observing setups is found in Section 2.3.

Table 5: Log of Observations at LDT and SOAR

Gaia DR3 ID | WDJ Name | Instrument+Facility | UTC Date | $G_{\rm RP}$ | Exp. Time | S/N | Spectral Type | Ca II Detected?
---|---|---|---|---|---|---|---|---
|  |  |  | (mag) | (sec.) |  |  |
4907742035447425024 | WD J0058$-$5638 | SOAR+Goodman | 2023-08-13 | 16.9 | 7200 | 38.5 | DZ [1] | Absorption
5078074837769743744 | WD J0302$-$2301 | SOAR+Goodman | 2023-08-13 | 15.9 | 5400 | 61.5 | DA [2] | No
276192136878054656 | WD J0406+5431 | LDT+DeVeny | 2024-02-05 | 15.5 | 7200 | 32.9 | DA [2] | No
4653404070862114176 | WD J0419$-$7303 | SOAR+Goodman | 2023-08-13 | 15.7 | 1800 | 38.9 | DA [2] | No
271992414775824640 | WD J0424+5210 | LDT+DeVeny | 2023-10-06 | 15.2 | 7200 | 47.6 | DA [2] | No
720353602808918016 | WD J0857+4016 | LDT+DeVeny | 2024-02-05 | 15.0 | 7200 | 35.4 | DA [2] | No
6306842338087328512 | WD J1500$-$1603 | SOAR+Goodman | 2023-08-18 | 16.9 | 6600 | 21.7 | DA [1] | No

Note. — We calculate the tabulated signal-to-noise ratios (S/N) by taking the
average of the ratio of the continuum normalized flux to its standard
deviation at the wavelengths where we do not see features from the calcium
triplet. References for the tabulated spectral types are as follows: [1]
Vincent et al. (2024), [2] McCook & Sion (1999).

##  Appendix C Light Curves

Here were show our light curves of all 21 published Ca II gas disks in our
original input in Figure 6. We show the first half of the 47 non-Ca II disks
in Figure 7 and the second half in Figure 8.

Figure 6: Our WISE light curves for all 21 published white dwarfs known to
harbor Ca II gas disks (Gentile Fusillo et al., 2021a), plotted following the
convention in Figure 1. Objects are shown in ascending order of right
ascension. See Table 3 for full information on this sub-sample.  Figure 7: Our
WISE light curves for the first 23 of the non-Ca II disk systems included in
our input catalog, plotted following the convention in Figure 1. Objects are
shown in ascending order of right ascension. See Table 4 for full information
on this sub-sample.  Figure 8: Continuation of Figure 7, showing the remaining
24 non-Ca II disks. These light curves are plotted following the convention in
Figure 1. Objects are shown in ascending order of right ascension. See Table 4
for full information on this sub-sample.

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