Informing the Cataclysmic Variable Donor Sequence from Gaia DR2 Color-Magnitude and Inferred Variability Metrics

Abstract

Short-period cataclysmic variables (spCVs), with orbital periods below the period gap (P orb < 2 hr), offer insight into the evolutionary models of CVs and can serve as strong emitters of detectable gravitational waves (GWs) for next-generation space-based GW observatories. To identify new spCV candidates, we crossmatch a catalog of known CVs with periods from 70 min to 8 hr to sources with well-measured parallaxes in the Gaia second data release (DR2). We uncover and fit a surprisingly (apparently) monotonic relationship between the color-absolute-magnitude diagram (CMD) position and P orb of these CVs, revealed in DR2. To supplement the CMD-P orb relation we also develop a method for identifying sources with large photometric variability, a characteristic trait of spCVs. Though Gaia is inherently a time-domain survey, the DR2 contains only a small fraction of sources with photometric light curves. Using such light curves, however, we construct a machine-learned regression model to predict physically informative variability metrics for sources in the CMD locus of known spCVs based solely on time-averaged observational covariates present in DR2. Using this approach we identify 3,253 candidate spCVs, of which ∼ 95% are previously unknown. Inspection of archival SDSS spectra of these candidates suggests that >82% are likely to be spCVs. This is a noticeably higher recovery rate than the typical recovery (∼30%) in previous light-curve searches, which bias toward flaring and active systems. We obtain optical spectra of nine previously uncharacterized systems with the Shane telescope at Lick Observatory and confirm that all objects are CV systems. We measure P orb for seven systems using archival Gaia and Palomar Transient Factory light curves, all of which are spCVs and three of which do not have previous P orb measurements. We use the CMD-P orb relation to infer the detectability of these systems to the upcoming LISA mission, and find that six of them may be coherent LISA verification binaries, with an estimated SNR > 5 in the 4 yr mission. This paper demonstrates that the time-averaged Gaia catalog is a powerful tool in the methodical discovery and characterization of even semi-stochastic time-varying objects, making it complementary to missions like ZTF, TESS, and the Vera Rubin LSST in the efficient search for rare and unusual variable systems.

White dwarf–main-sequence binaries from Gaia EDR3: the unresolved 100 pc volume-limited sample

“…Giammichele et al 2012;Holberg et al 2016). Comprehensive samples of accreting white dwarfs in cataclysmic variables (Abril et al 2020;Pala et al 2020;Abrahams et al 2020), extremely low-mass white dwarfs (Pelisoli & Vos 2019), hot subdwarfs (Geier et al 2019), white dwarfs with infrared excess typical of circumstellar debris disks (Rebassa-Mansergas et al 2019b;Xu et al 2020), and white dwarfs that are members of common proper motion pairs El-Badry et al 2021), have also been obtained from analysing the Gaia data. More recently, Inight et al (2021) have illustrated the importance of defining a volume-limited sample of close white dwarf binaries.…”

LAMOST J0140355+392651: An evolved cataclysmic variable donor transitioning to become an extremely low mass white dwarf

“…Curiously, the CMD position of J0140 is quite similar to that of "normal" CVs with the same period (e.g. Abril et al 2020;Abrahams et al 2020). Most CVs with 𝑃 orb ∼ 4 hours, however, are completely dominated by the accretion disk in the optical, even in quiescence.…”

A Systematic Search for Short-period Close White Dwarf Binary Candidates Based on Gaia EDR3 Catalog and Zwicky Transient Facility Data

“…We adopt the "variable amplitude" proposed by Deason et al (2017) based on the rms dispersion of flux. A number of studies have adopted the Gaia variable metric and successfully identified different types of variable sources, including large-amplitude variables (Mowlavi et al 2021), short-period cataclysmic variables (Abrahams et al 2020), pulsing white dwarfs (Guidry et al 2021), hot subdwarf stars (Barlow et al 2022), and the birth of extremely low-mass white dwarfs (El-Badry et al 2021b, 2021c. Following Guidry et al (2021) and El-Badry et al (2021b), we use the G-band photometries to further filter candidates with Gaia variability metric (or the variability amplitude proxy A proxy,G ; see Mowlavi et al 2021), V G : The red solid lines show the color-cut range that we defined, using the lower edge of the MS as an upper limit and the upper edge of the WD cooling sequence as a bottom limit, and using a color-cut G BP − G RP < 1.0 as a right color limit.…”