THE RESPONSE OF MAIN-SEQUENCE STARS WITHIN A COMMON ENVELOPE

The response of a 1.25 M. main-sequence secondary star to the conditions within the envelope of a red giant primary star is investigated. These unusual circumstances may occur during the common-envelope phase of binary evolution. Model calculations for a range of initial orbital separations of the binary system and masses of the red giant reveal that the mass accretion rates range from 10(-5) to 10(-1) M. yr-1 and are limited by the Eddington value. As a consequence of these high rates and the high entropies characteristic of the accreted matter, the stellar radius of the secondary can expand by more than an order of magnitude after the accretion of less than 0.1 M.. Upon filling its Roche lobe, the secondary transfers the accreted mass back to the common envelope with a typical net gain of less than approximately 0.01 M.. The mass loss rates during this latter phase can exceed 0.1 M. yr-1. The orbital decay of the binary system is rapid and the duration of the common-envelope phase is approximately 400-4000 yr. Since the net amount of accreted material is small, the composition of the secondary is not expected to differ significantly from that corresponding to the original stellar surface. Once the common envelope is ejected, the thermal relaxation time scale of the disturbed outer envelope layers of the secondary is found to be approximately 10(4) yr. Consequently, the effect of the common-envelope phase on the properties of the main-sequence-like star in the aftermath of the common-envelope phase is minimal.