Steady-state Stellar Winds Driven by Recombination

Abstract

Hydrogen and helium recombination energy has been proposed as a potential driver of mass ejection in common-envelope evolution and other eruptive stellar phenomena. We investigate whether recombination can by itself launch a steady, transonic wind from near a stellar surface. Using a tabulated equation of state, we explore steady-state, adiabatic wind solutions over a broad range of stellar mass, density, and temperature. We classify a wind as recombination-driven only if the gas is gravitationally bound prior to recombination and if the released energy remains trapped until the flow becomes unbound. Only a small fraction of the solutions satisfy both conditions. In most cases, the gas is either already unbound without recombination or loses the released energy through radiative diffusion while still bound. The subset of valid solutions require outflow velocities $\gtrsim 10\,{\rm km\,s^{-1}}$ at $10\,R_\odot$, inconsistent with a wind launched from a hydrostatic star. We conclude that recombination energy alone is unlikely to produce steady stellar winds. It can, however, accelerate and unbind a pre-existing outflow generated by processes such as binary orbital decay, producing mass-loss rates of $\sim \rm M_\odot\,yr^{-1}$.