The minimum mass ratio for contact close binary systems of W Ursae Majoris-type

Abstract

Extreme mass ratio close binaries of W UMa-type represent an interesting class of objects in which ∼1M⊙ main-sequence star is in contact with a significantly less massive companion (M2 ∼ 0.1M⊙). Earlier theoretical investigations of these systems found that there is a minimum mass ratio q = M2/M1 ≈ 0.085 - 0.095 (obtained for n = 3 polytrope-fully radiative primary) above which these systems are stable. If the mass ratio is lower than minimum, a tidal instability develops (Darwin's instability). This instability, which is secular, growing on a viscous dissipation timescale, eventually forces the stars to merge into a single, rapidly rotating object (such as FK Corn-type stars or blue stragglers), implying that such systems would not be observed. There appear to be, however, some W UMa-type binaries with empirically obtained q values below the theoretical limit for stability. The aim of this dissertation was to try to resolve the discrepancy between theory and observations by considering rotating polytropes. Other candidate systems for stellar mergers such as AM CVn-type stars have also been discussed in the dissertation. © 2009. The Astronomical Society of the Pacific. All rights reserved.