Disk instability model for the AM canum venaticorum stars

Abstract

We examine AM CVn stars, ultra-short-period variables thought to have helium disks, from the stand-point of the disk instability model. By calculating the vertical structure of the helium accretion disks, we have confirmed that the helium accretion disks are thermally unstable, since their thermal-equilibrium curves exhibit the characteristic S-shaped form. We apply these results to AM CVn stars. We have found that the observed large-amplitude photometric variations in AM CVn stars can be explained by the thermal-tidal instability model of helium accretion disks.