Unsteady Mass Loss in Close Binaries

Abstract

The evolution of a system of 1.50M_⊙ and 0.50M_⊙ components with initial separation 3.00R_⊙ is presented. The primary reaches its Roche lobe after 6.108 × 10⁸ yr, and the mass loss rate grows quickly to ~ 1.1 × 10⁻⁷M_⊙ yr⁻¹. The deep convective envelope of the secondary contracts in response to accretion, however, and the system avoids contact, even though the main sequence mass-radius relationship crosses the Roche relationship at 0.758M_⊙.

The growth of the convective envelope of the primary with mass loss prevents thermaltimescale mass transfer from reaching any stable value. The mass loss rate continues to increase as a result of the tendency of the growing convective envelope to expand with mass loss. A Bath-type pulsational instability in the mass transfer rate is manifested as a longer-period thermal instability in the hydrostatic treatment. Eventually mass loss proceeds on a dynamical timescale, reaching 1.5 × 10⁻⁴M_⊙yr⁻¹. This is followed by a detached phase with the mass ratio not yet reversed, and a second dynamical episode leaving a 0.6666M_⊙ + 1.3334M_⊙ binary still in a semi-detached state. The original secondary, now more massive, has a surface luminosity of 0.0538L_⊙ but a nuclear luminosity of 1540L_⊙, and a convective core of 1.076M_⊙ separated by a radiative zone of only 0.028M_⊙ from its convective envelope.

An attempt is made to follow the evolution of the secondary in the case where the stream energy, much greater than the intrinsic luminosity of the secondary, is dissipated spherically symmetrically at its surface. The outer envelope departs radically from the previous case, an optically deep radiative surface layer forming, and the convection zone splitting into two, the lower one remaining very massive. Because the mass transfer timescale is short compared with the thermal timescale of the secondary, however, it is concluded that the stream luminosity cannot significantly alter the evolution of the secondary.