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The Gaia Cataclysmic Variable hook

Figure 1. Left: zoom on the Gaia DR3 low extinction Hertzsprung-Russell diagram, using A0 < 0.1 mag according to the Lallement et al. 2022 extinction map and parallax uncertainty smaller than 10%. A prominent feature is seen at M_G~4 and G_BP-G_RP~0.2 going from the main sequence to the hot subdwarf clump up to G_BP-G_RP~0 than going parallel to the main sequence. Right: Same Hertzsprung-Russell diagram as grey background overlaid with Gaia DR3 variable stars classified as ‘CV’ (cataclysmic variables, orange circles) and ‘SDB’ (hot subdwarfs, blue triangles). Credit: “Cataclysmic Variables and the disc instability model in the Gaia DR3 colour-magnitude diagram” by Dubus G. and Babusiaux C.

While updating the Gaia low extinction Hertzsprung-Russell diagrams (HRD) to the Gaia DR3 data and EDR3 based extinction map of Lallement et al. 2022, we noticed a strange hook of stars at the absolute Gaia magnitude M_G~4, going from the main-sequence toward the hot sub-dwarf clump up to the colour G_BP-G_RP~0 and than moving down parallel to the main sequence. Looking at the position of Gaia variable stars on the Hertzsprung-Russell diagram, we found that this hook is populated by Cataclysmic Variable (CV) stars with a contamination with hot subdwarfs (SDB) in the upper part.

Cataclysmic Variable stars are semidetached binaries composed of a white dwarf orbiting a lower mass K or M star. The companion star fills its Roche lobe, with material lost forming an accretion disc around the white dwarf. Such discs are stable at high mass accretion rates (novae like type CV). We compared the Cataclysmic Variable stars location in the Hertzsprung-Russell diagram to disc instability model predictions and find that the hook corresponds to the region of the Hertzsprung-Russell diagram where stable systems are located. The detail of this analysis is described in the paper “Cataclysmic Variables and the disc instability model in the Gaia DR3 colour-magnitude diagram” by Dubus G. and Babusiaux C.

Story written by Carine Babusiaux and Guillaume Dubus.

Credits: ESA/Gaia/DPAC, Carine Babusiaux, Guillaume Dubus

[Published: 02/04/2024]