On rapid binary mass transfer -- I. Physical model

TL;DR

This paper develops a physical model for rapid mass transfer in semi-detached binary systems, predicting significant mass loss through the L2 point at high transfer rates, affecting orbital evolution and observable emissions.

Contribution

The paper introduces a new physical model linking high mass transfer rates to L2 mass loss via a Bernoulli number criterion, explaining observed phenomena in binary systems.

Findings

Significant L2 mass loss occurs at transfer rates > few times 10^{-4} Msun/yr.

Formation of an equatorial circum-binary outflow (CBO) in high transfer rate systems.

Implications include orbital shrinking and infrared emission from disk winds.

Abstract

In some semi-detached binary systems, the donor star may transfer mass to the companion at a very high rate. We propose that, at sufficiently high mass-transfer rates such that the accretion disk around the companion becomes geometrically thick (or advection-dominated) near the disk outer radius, a large fraction of the transferred mass will be lost through the outer Lagrangian (L2) point, as a result of the excessive energy generated by viscous heating that cannot be efficiently radiated away. A physical model is constructed where the L2 mass loss fraction is given by the requirement that the remaining material in the disk has Bernoulli number equal to the L2 potential energy. Our model predicts significant L2 mass loss at mass-transfer rates exceeding a few times 10^{-4} Msun/yr. An equatorial circum-binary outflow (CBO) is formed in these systems. Implications for the orbital evolution and the observational appearance are discussed. In particular, (1) rapid angular momentum loss from the system tends to shrink the orbit and hence may increase the formation rate of mergers and gravitational-wave sources; (2) photons from the hot disk wind are reprocessed by the CBO into longer wavelength emission in the infrared bands, consistent with Spitzer observations of some ultra-luminous X-ray sources.