Mass loss from convective envelopes of giant components in close binary systems

In many spectroscopic binaries, the more massive component will reach the Roche limit when it is a giant star with a deep convective envelope. Mass loss ensuing in this case is studied for a red giant initially of 7 solar masses. The Jendrzejec mode of a steady laminar oufflow of mass from the convective envelope leads to very high rates of mass transfer, up to 10-1 solar masses per year. Two additional modes of mass outflow have been investigated in order to see whether these rates could actually be lower. Convective overshoot was found negligible at all phases. Mass loss from the radiative atmosphere contributes significantly to the process at later stages, but does not affect the peak rates. Four sequences of evolutionary models have been computed. After an initial phase, a rapid phase of mass transfer sets in because the star exceeds the Roche critical lobe by up to 20% of its radius. Then the radius decreases abruptly and a slow phase of mass transfer follows, during which the rates can still be as high as 1O- 9.) . The mass-losing star is by now the less- massive component, and its Roche radius soon exceeds its photospheric radius. Mass loss then continues only from the radiative atmosphere. The process ends when core helium burning invokes contraction of the envelope. This occurs always at the same time and luminosity since the star's core develops independently of the envelope. Using the principles that the deep convective envelope has a large thermal inertia, and that the star evolves at constant core mass, the successive stages of the mass-transfer process can be explained together with deviations found for sequences with different initial conditions. The rapid phase is essentially an adiabatic phase, while the slow phase proceeds on a thermal time scale. The final outcome is an inhomogeneous helium-burning star with mass varying between 1.35 and 2.45 9M0 for the four sets of initial conditions. An even larger variety of final parameters is expected ff there is some mass loss from the system. Certain modes of this mass loss are likely to decrease the high rates of mass oufflow by one or two orders of magnitude. It is suggested that the binary shell star AX Mon can be represented by a model in the slow phase of sequence II, and that the recurrent nova T CrB may be in a rapid phase of convective mass loss. Key words: evolution - mass loss - close binaries - giant components - convective envelopes