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The common envelope (CE) phase is an important stage in binary stellarevolution. It is needed to explain many close binary stellar systems, such ascataclysmic variables, Type Ia supernova progenitors, or X-ray binaries. Toform the resulting close binary, the initial orbit has to shrink, therebytransferring energy to the primary giant's envelope that is hence ejected. Thedetails of this interaction, however, are still not understood. Here, wepresent new hydrodynamic simulations of the dynamical spiral-in forming a CEsystem. 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 particlehydrodynamics and traditional grid-based hydrodynamic codes and allows us tocapture also small flow features at high spatial resolution. Our simulationsreproduce the initial transfer of energy and angular momentum from the binarycore to the envelope by spiral shocks seen in previous studies, but after about20 orbits a new phenomenon is observed. Large-scale flow instabilities aretriggered by shear flows between adjacent shock layers. These indicate theonset of turbulent convection in the common envelope, thus altering thetransport of energy on longer time scales. At the end of our simulation, only8% of the envelope mass is ejected. The failure to unbind the envelopecompletely may be caused by processes on thermal time scales or unresolvedmicrophysics.

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