The Response of Giant Stars Upon Dynamical Timescale Mass Loss

Details The Response of Giant Stars Upon Dynamical Timescale Mass Loss The Response of Giant Stars Upon Dynamical Timescale Mass Loss

2011-11-17

arxiv.org/abs/1111.4202v3

Astronomy and Astrophysics

Astrophysics

Solar and Stellar Astrophysics

8 pages, 11 figures, re-submitted to ApJ after referee's report

Scientific paper

Mass transfer in close binaries significantly alters the evolution of both stellar components. Depending on the system parameters and the donor's radius, mass transfer can be dynamically unstable and lead to runaway evolution. Until recently, the standard picture was that giant stars, having convective envelopes, expand when they lose mass, in which case mass transfer is in most cases unstable. This description is not accurate as it is based on invalid assumptions. Based on one-dimensional simulations for donors on both the red and the asymptotic giant branches, with mass loss rates going from $10^{-3}$ up to 2\msun/yr, we show that in the case of dynamical timescale mass loss, the evolution of mass-losing giant stars is not adiabatic. The superadiabatic outer layer of the giant's envelope has a local thermal timescale comparable to the dynamical timescale induced by mass loss. Therefore, this layer has enough time to readjust. Moreover, the giant star is driven out of hydrostatic equilibrium and energy can now be stored into its kinetic form. As a result, we show that giant donors, in most cases, do not expand. If the mass loss rate is high enough, the superadiabacity of the outer layer is lost progressively and a radiative zone forms due to a combination of thermal readjustment and dynamical effects mentioned above.

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