Runaway Coalescence at the Onset of Common Envelope Episodes

TL;DR

This paper models the unstable mass transfer in binary star systems leading to stellar coalescence, revealing how orbital decay and mass ejection evolve, which helps explain observed precursor emissions in luminous red nova transients.

Contribution

It introduces a hydrodynamic simulation of binary coalescence, detailing the transition from precursor emission to outburst and characterizing outflow morphology and mass transfer dynamics.

Findings

Orbital decay can be reconstructed from mass and angular momentum fluxes.

Outflow morphology changes from high-entropy streams to broad fans as orbit tightens.

Mass ejection rate increases significantly during the plunge of the accretor.

Abstract

Luminous red nova transients, presumably from stellar coalescence, exhibit long-term precursor emission over hundreds of binary orbits, leading to impulsive outbursts with durations similar to a single orbital period. In an effort to understand these signatures, we present and analyze a hydrodynamic model of unstable mass transfer from a giant-star donor onto a more compact accretor in a binary system. Our simulation begins with mass transfer at the Roche limit separation and traces a phase of runaway decay leading up to the plunge of the accretor within the envelope of the donor. We characterize the fluxes of mass and angular momentum through the system and show that the orbital evolution can be reconstructed from measurements of these quantities. The morphology of outflow from the binary changes significantly as the binary orbit tightens. At wide separations, a thin stream of relatively high-entropy gas trails from the outer Lagrange points. As the orbit tightens, the orbital motion desynchronizes from the donor's rotation, and low-entropy ejecta trace a broad fan of largely ballistic trajectories. An order-of-magnitude increase in mass ejection rate accompanies the plunge of the accretor with the envelope of the donor. We argue that this transition marks the precursor-to-outburst transition observed in stellar coalescence transients.