  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.
