Episodic mass ejections from common-envelope objects

Abstract

After the initial fast spiral-in phase experienced by a common-envelope binary, the system mayenter a slow, self-regulated phase, possibly lasting hundreds 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 throughsome as-yet-undetermined mechanism. We carried out 1D hydrodynamic simulations of alow-mass red giant undergoing a synthetic common-envelope event in such a slow spiral-inphase, using the stellar evolutionary code MESA. We simulated the heating of the envelopedue 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 3-20yr 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 shellsof matter of up to 0.1 M⊙, ~10 percent of the mass of the envelope, from the stellar surfaceat above escape velocity. These ejections are seen to repeat within a few decades, leading toa time-averaged mass-loss rate of the order of 10-3 M⊙ yr-1, which is sufficiently high torepresent a candidate mechanism for removing the entire envelope over the duration of theslow spiral-in phase.