Friday, November 18, 2011

1111.4202 (Jean-Claude Passy et al.)

The Realistic Response of Giant Stars Upon Dynamical Timescale Mass Loss    [PDF]

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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