Enhanced Mass Transfer from Extended Shock-Supported Stellar Chromospheres

Abstract

Mass transfer (MT) in binary systems is a crucial and complex process. Even before the donor's photosphere fills its Roche lobe, matter from its outer atmosphere can reach the inner Lagrange point, thus initiating interaction in the so-called optically thin phase. Most existing models account for a very tenuous atmosphere supported by thermal gas pressure, thus predicting very little MT until the donor fills its Roche lobe. However, recent hydrodynamic simulations of giant stars suggest that shock-driven outflows can create substantially thicker "chromospheres'' extending to a few stellar radii. In this work, we derive a simple analytic model for a shock-supported chromosphere in a binary potential and calculate the corresponding MT rate. We also implement our prescription in the stellar evolution code MESA. We show that chromospheric MT rates can be orders of magnitude higher than prior estimates for optically thin MT, especially for stars high on the red giant or asymptotic giant branch. This allows for substantial accretion before Roche lobe overflow, both for systems that undergo stable mass transfer and for systems that go through common envelope evolution. Thus, for a given initial binary configuration, this may alter the final period and component masses compared to those predicted classically. Our model is similar to Wind Roche-lobe Overflow, but it is physically rather than empirically motivated. We discuss implications for various post-interaction binaries that can be tested with population synthesis studies and observed binary populations. Additionally, we have identified an inaccuracy in the current implementation of the optically-thin Ritter 1988 prescription in MESA, which we correct.