Rapid Mass Transfer in Binary Systems

Abstract

The conditions for rapid mass transfer is investigated in binary stars.Previous theoretical calculations and observations of binary imply theexistence of several different timescales for mass transfer: nuclear,thermal, or dynamical. Since the mass transfer rates differ by severalorders of magnitude, it is important to know which timescales arerelevant to different systems. Dynamical timescale mass transfer isthought to cause substantial decreases in the orbital period throughmass and angular momentum losses. Thermal timescale mass transfer isthought to transform the appearance of the binary as mass exchangeoccurs in a short time. Binaries currently transferring mass are doingso on the longest, nuclear, timescale. The characteristics of binarieswhich divide the three timescales are estimated by calculating theresponse of potential mass donors in two idealized limits: an adiabaticresponse, where the entropy profile does not change with mass loss, anda thermal response. Where thermal relaxation is allowed but nuclearburning is not. A comparison of the changing surface radius, to theRoche lobe radius implies the critical mass ratios for dynamically andthermally unstable mass transfer. These calcualtions are performed herefor donors between 0.25 and 20 solar masses which would fill their Rochelobes before helium burning.