Low Mass Contact Binaries: Orbital Stability at Extreme Low Mass Ratios

TL;DR

This study examines how metallicity and age influence the orbital stability of low mass contact binary systems, highlighting that older, metal-poor systems are more stable and questioning the stability of known potential red nova progenitors.

Contribution

It provides a comprehensive analysis of the effects of stellar composition and age on contact binary stability, introducing new insights into the conditions leading to mergers.

Findings

Metal-poor and older systems are more stable.

Most systems with primaries of solar mass are stable.

No currently observed systems can be confidently classified as unstable.

Abstract

With the ever-increasing number of light curve solutions of contact binary systems increasing number of potential bright red nova progenitors are being reported. There remains, however, only one confirmed event. In the present study we undertake a comprehensive review of 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 ratio 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 8 potential red nova progenitors all have methodological or astrophysical concerns which lower our confidence in designating any of them as potential merger candidates.