Low-mass contact binaries: orbital stability at extreme low mass ratios

ABSTRACT With the ever-increasing number of light-curve solutions of contact binary systems, an increasing number of potential bright red nova progenitors are being reported. There remains, however, only one confirmed event. In this study, we undertake a comprehensive review of the orbital stability of contact binary systems, considering the effects of the stellar internal composition (metallicity) and age on the evolution of the gyration radius and its effect on the instability mass ratio of contact binaries. We find that both metallicity and age have an independent effect on orbital stability, with metal-poor and older systems being more stable. The combined effects of age and metallicity are quite profound, such that for most systems with primaries of solar mass or greater, which are halfway or more through the main-sequence lifespans have instability mass ratios at levels where the secondary component would be below the hydrogen fusion mass limit. We find that from the currently available solutions we cannot confidently assign any system as unstable. Although we identify eight potential red nova progenitors, all have methodological or astrophysical concerns, which lowers our confidence in designating any of them as potential merger candidates.