Cataclysmic variables and the disc instability model in the Gaia DR3 colour–magnitude diagram

Abstract

Cataclysmic variables (CVs) are semi-detached binaries composed of a white dwarf orbiting a lower-mass K or M star. We investigate whether CVs are responsible for a new intriguing feature (the `hook') that appears in the DR3 colour--magnitude Hertzsprung-Russell diagram (HRD) when selecting sources with low extinction. We also aim to understand the location of CVs in the HRD based on the predictions of the disc instability model (DIM). The DIM forms the foundation of our basic understanding of stable (novae-like) and outbursting CVs (dwarf novae). We calculated the expected behaviour of CVs in the HRD while taking into account the variable light contributed by the accretion disc, the companion, the white dwarf, and the bright spot where the Roche lobe overflow stream from the companion intersects the disc. We find that the hook feature is most likely composed of CVs. The hook corresponds to the limited region where stable (novae-like) CVs must be located in the HRD according to the DIM. Unstable systems giving rise to dwarf novae outbursts trace counterclockwise loops in the HRD. The overall behaviour is consistent with the location of the various CV subtypes in the HRD. These results can be used as a basis on which to pinpoint interesting outliers in the HRD, either due to their location or their tracks. These outliers could signal new subtypes, such as cold, stable CVs with truncated discs, or may challenge the disc instability model.