Jean-Claude Passy, Falk Herwig, Bill Paxton
Mass transfer in close binaries significantly alters the evolution of both
components. Depending on how the system parameters and the donor's radius vary,
mass transfer can be dynamically unstable and lead to runaway evolution. Until
now, the standard picture was that giant stars expand when they lose mass so
mass transfer in systems with such donors is in most cases unstable. In this
contribution, we show that this description is not accurate as it was based on
invalid assumptions. We run one-dimensional simulations for donors on both the
red and the asymptotic giant branches, with mass loss rates going from
$10^{-2}$ up to 2\msun/yr. We show that in the case of dynamical timescale mass
loss, mass-losing giant stars are out of hydrostatic equilibrium and their
evolution is not adiabatic as the superadiabatic outer layer of their envelope
has a local thermal timescale comparable to the dynamical timescale induced by
mass loss. Therefore, this layer has enough time to readjust and in most cases,
giant donors do not expand. If the mass loss rate is high enough, the
superadiabatic layer is consumed progressively and a radiative zone forms below
it as the opacity decreases.
View original:
http://arxiv.org/abs/1111.4202
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