On the effects of radiation on mass transfer in binary stars

TL;DR

This paper investigates how radiation influences mass transfer in binary stars, revealing that radiation can significantly enhance transfer rates and alter the critical points, especially in super-Eddington conditions, impacting binary evolution models.

Contribution

The study provides analytical and numerical models showing how radiation modifies mass transfer rates and critical points, introducing a new super-Eddington transfer mode affecting binary evolution.

Findings

Radiation causes exponential increase in mass transfer rate as Eddington factor approaches unity.

Photon tiring limit is less restrictive near L1 in super-Eddington outflows.

Super-Eddington, convectively inefficient layers can drive high mass transfer rates before Roche-lobe overflow.

Abstract

Mass transfer (MT) in binary systems is a common evolutionary process that can significantly affect the structure, evolution, and final fate of both stars. In modeling MT hydrodynamics, it is usually assumed that the critical point of the flow, where the velocity exceeds the local sound speed, coincides with the inner Lagrange point (L1). However, in massive donors where radiative pressure dominates over gas pressure and the Eddington factor $\Gamma_\text{Edd}$ can approach or exceed unity, radiation-gas coupling can shift the critical point away from L1, altering the MT rate ($\dot{M}_\text{d}$). We investigate the effects of radiation on MT using time-steady radiative hydrodynamic equations and the von Zeipel theorem. We derive analytical expressions that closely approximate $\dot{M}_\text{d}$, algebraic solutions for simplified cases, and numerical results using a realistic equation of state. Two main differences emerge relative to traditional prescriptions for $\dot{M}_\text{d}$. First, for Roche-lobe-underfilling donors with $\Gamma_\text{Edd} \lesssim 1$, radiative momentum exchange leads to an exponential increase of $\dot{M}_\text{d}$ as a function of $1-\Gamma_\text{Edd}$. We provide a simple modification of existing prescriptions that captures this effect. Second, the photon tiring limit for super-Eddington outflows is much less restrictive near L1 than in spherical stars. We suggest that donors with super-Eddington, convectively inefficient subsurface layers can drive MT with $-\dot{M}_\text{d} \gtrsim 10^{-2}\,\text{M}_\odot\,\text{yr}^{-1}$ even before Roche-lobe overflow. We characterize the conditions for this new mode of super-Eddington-boosted MT and discuss its implications for binary evolution, including potential links to nonterminal outbursts of Luminous Blue Variables.