RAPID MASS TRANSFER IN BINARY SYSTEMS

Abstract

This thesis investigates the conditions for rapid mass transfer in binary stars. Previous theoretical calculations and observations of binaries imply the existence of several different time scales for mass transfer: nuclear, thermal, or dynamical. Since the mass transfer rates differ by several orders of magnitude, it is important to know which time scales are relevant to different systems. Dynamical time-scale mass transfer is thought to cause substantial decreases in the orbital period through mass and angular momentum losses. Thermal time-scale mass transfer is thought to transform the appearance ofthe binary as mass exchange occurs in a short time. Binaries currently transferring mass are doing so on the longest, nuclear time scale. The characteristics of binaries which divide the three time scales are estimated by calculating the response of potential mass donors in two idealized limits: an adiabatic response, where the entropy profile does not change with mass loss, and a thermal response, where thermal relaxation is allowed but nuclear burning is not. A comparison ofthe changing surface radius, £ = d'nR/d'n3)l to the Roche-lobe radius implies the critical mass ratios for dynamically and thermally unstable mass transfer. These calculations are performed here for donors between 0.25 and 20 TtQ which would fill their Roche lobes before helium burning. The adiabatic mass-loss calculations have provided a clear relation between £ad and the donor's convective envelope mass fraction (fce), with a smaller dependence on the state of the interior. Low-mass ZAMS donors and models near the base ofthe giant branch have £ad ^> 1 change to £ad ~