Runaway Coalescence at the Onset of Common Envelope Episodes

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