Response of Low-Mass Main Sequence Stars to Accretion

Abstract

When the massive component in a close binary system evolves to fill its Roche lobe, mass transfer occurs and gas is accreted onto the companion star. Recently, the response of the unevolved secondary to accretion has been studied by a number of authors, but the emphasis has been on relatively massive stars which have a radiative envelope (Ulrich and Burger 1976; Flannery and Ulrich 1977; Kippenhalm and Meyer-Hofmeister 1977; Neo et al. 1977). The results show that the mass accepting star becomes overluminous and grows in radius until rapid mass transfer ultimately brings the two stars into contact. Such changes in the structure are caused by the steep increase in the specific entropy in the outermost layers and only a small amount of mass (about a tenth of the initial mass of the star) is accreted before contact is made. Thereafter, the expansion of the common envelope will lead to mass loss from the system. It is also found that, for a given accretion rate, the radial increase is much more conspicuous for a smaller mass star. Thus, a characteristic transfer rate which will lead to an increase in radius by, say, a factor of ten is much smaller for a less massive star and becomes as small as 10-6M⊙yr-1 for a model with an initial mass Mi=0.75M⊙, as computed by Neo et al. (1977) assuming a radiative envelope. In such a low mass main sequence star, however, surface convection develops and therefore. response to the accretion is expected to be quite different from that of massive main sequence stars. The evolution of binary systems containing a low mass star is important since such systems may be progenitors of cataclysmic binaries and/or progenitors of Type I supernovae. In this paper, we will focus on the evolution of a low mass main sequence star during accretion.