Hydrodynamic moving-mesh simulations of the common envelope phase in binary stellar systems

TL;DR

This study uses advanced hydrodynamic moving-mesh simulations to explore the complex dynamics of the common envelope phase in binary star systems, revealing turbulence and partial envelope ejection.

Contribution

It introduces high-resolution moving-mesh simulations of the CE phase, capturing turbulence onset and detailed flow features not previously modeled.

Findings

Spiral shocks transfer energy and angular momentum.

Large-scale flow instabilities lead to turbulence.

Only 8% of the envelope is ejected in simulations.

Abstract

The common envelope (CE) phase is an important stage in binary stellar evolution. It is needed to explain many close binary stellar systems, such as cataclysmic variables, Type Ia supernova progenitors, or X-ray binaries. To form the resulting close binary, the initial orbit has to shrink, thereby transferring energy to the primary giant's envelope that is hence ejected. The details of this interaction, however, are still not understood. Here, we present new hydrodynamic simulations of the dynamical spiral-in forming a CE system. We apply the moving-mesh code AREPO to follow the interaction of a $1M_\odot$ compact star with a $2M_\odot$ red giant possessing a $0.4M_\odot$ core. The nearly Lagrangian scheme combines advantages of smoothed particle hydrodynamics and traditional grid-based hydrodynamic codes and allows us to capture also small flow features at high spatial resolution. Our simulations reproduce the initial transfer of energy and angular momentum from the binary core to the envelope by spiral shocks seen in previous studies, but after about 20 orbits a new phenomenon is observed. Large-scale flow instabilities are triggered by shear flows between adjacent shock layers. These indicate the onset of turbulent convection in the common envelope, thus altering the transport of energy on longer time scales. At the end of our simulation, only 8% of the envelope mass is ejected. The failure to unbind the envelope completely may be caused by processes on thermal time scales or unresolved microphysics.