Informing the Cataclysmic Variable Sequence from Gaia Data: The Orbital-period-Color-Absolute-magnitude Relationship

Abstract

The orbital-period (P ${}_{o}rb$) gap in the population of cataclysmic variables (CVs) informs the theoretical narrative of CV evolution, yet a complete understanding of the driving angular momentum loss mechanisms above and below this gap remains elusive. Here we identify, for standard CVs, a new, apparently monotonic relationship between quiescent color (G ${}_{B}P$-G ${}_{R}P$), absolute magnitude (M ${}_G$), and P ${}_{o}rb$ (between 70 minutes and 8 hr) revealed in Gaia DR2 and EDR3. We show that P ${}_{o}rb$ increases in the color-absolute-magnitude diagram roughly orthogonally to the white dwarf and main sequences. We find the orbital-period-color-absolute-magnitude relationship to be stable across different CV subtypes: dwarf novae, intermediate polars, polars, and novalike systems. We place our findings in context with the known semiempirical donor sequence for CVs and find a dependence between color and M ${}_G$ for a given P ${}_{o}rb$ specifically for dwarf novae and intermediate polars above the period gap. These relations have the potential to inform a more complete picture of CV evolution.