Institute of Astronomy

Pulsation-driven Ejections in Common-envelope Objects

SpeakerTalk DateTalk Series
Matthew Clayton26 July 2016Binary Stars Talks

Abstract

Common-envelope (CE) evolution remains one of the most important and least understood phases in the lives of binary star systems. Despite being a vital formation mechanism for many of the late-stage binary systems that are of greatest interest to astronomers, from double black hole and neutron star binaries to the progenitors of GRBs and SNe Ia, we still lack a theory of common envelopes which is able to predict the outcomes of this phase – whether the envelope is ejected, and the final separation of the binary if it is – with any degree of precision. A large part of the reason for this is that the difficulties associated with numerically modelling this phenomenon are formidable; the wide range of timescales that are important in the different phases of the process prevents any single 3-d or 1-d code from capturing its full dynamics. Although there has been some success in modelling fast ejections of common envelopes using 3-d hydrodynamics codes, simulations of the slow, self-regulating spiral-in phase which are restricted to 1-d have struggled to reveal the mechanisms underlying the delayed dynamical ejection of the envelope at this later stage.

I will report on hydrodynamical simulations of giant stars undergoing synthetic CE events in the self-regulated spiral-in phase, performed in 1-d using the stellar evolution code MESA, and building on the hydrostatic simulations performed by Ivanova, Justham and Podsiadlowski (2015). These simulations allow us to study the response of a giant envelope to the injection of heat expected during a CE event in different heating regimes.

In particular, I will report on the appearance of large amplitude Mira-like pulsations emerging as the giant envelope becomes increasingly dynamically unstable. In some cases these pulsations become supersonic, and the resulting shocks can lead to mass shells being dynamically ejected from the surface of the giant. These ejections may repeat on century timescales and constitute a rate of mass-loss as high as 0.001 solar masses per year. This mechanism is a promising candidate for the delayed dynamical ejection of common envelopes during the self-regulating spiral-in phase.

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