Episodic mass ejections from common-envelope objects

Abstract

After the initial fast spiral-in phase experienced by a common-envelope binary, the system may enter a slow, self-regulated phase, possibly lasting 100s of years, in which all the energy released by orbital decay can be efficiently transported to the surface, where it is radiated away. If the remaining envelope is to be removed during this phase, this removal must occur through some as-yet-undetermined mechanism. We carried out 1-d hydrodynamic simulations of a low-mass red giant undergoing a synthetic common-envelope event in such a slow spiral-in phase, using the stellar evolutionary code MESA. We simulated the heating of the envelope due to frictional dissipation from a binary companion's orbit in multiple configurations and investigated the response of the giant's envelope. We find that our model envelopes become dynamically unstable and develop large-amplitude pulsations, with periods in the range 3-20 years and very short growth time-scales of similar order. The shocks and associated rebounds that emerge as these pulsations grow are in some cases strong enough to dynamically eject shells of matter of up to 0.1 $\mathrm{M}_{\odot}$, $\sim 10$ % of the mass of the envelope, from the stellar surface at above escape velocity. These ejections are seen to repeat within a few decades, leading to a time-averaged mass-loss rate of order $10^{-3}$ $\mathrm{M}_{\odot} \: \mathrm{yr}^{-1}$ which is sufficiently high to represent a candidate mechanism for removing the entire envelope over the duration of the slow spiral-in phase.