Radiatively Cooled Binary Mass Transfer: Flow Structure, Luminosities, and L2 Outflows Across Mass Transfer Rates

Abstract

High rates of stable mass transfer (MT) occur for some binary star systems, resulting in luminous transients and circumbinary outflows (CBOs). We perform hydrodynamical simulations of a $10 \ M_\odot$ donor star and a $5\ M_\odot$ point mass accretor, incorporating approximate effects of radiative cooling. By varying the orbital separation of the system, we probe MT rates between $10^{-5}$ and $10^{-1} M_\odot$/yr. Mass flows from the donor into a disk around the accretor, with significant equatorially concentrated outflows through the outer Lagrange point L2 occurring for MT rates $\gtrsim 10^{-3} M_\odot$/yr, while the MT remains mostly conservative for lower MT rates. In all cases, any outflowing gas approximately carries the specific angular momentum of L2. The gas cooling luminosity $L$ and temperature increases with MT rate, with $L \sim 10^{5} L_\odot$ and $T \sim 10^{4}$ K for simulations featuring the strongest outflows, with contributions from both the CBO and the accretor's disk. The most luminous transients associated with mass outflows will be rare due to the high MT rate requirement, but generate significant optical emission from both near the accretor and the CBO.