How negative feedback and the ambient environment limit the influence of recombination in common envelope evolution

We perform 3D hydrodynamical simulations to study recombination and ionization during the common envelope (CE) phase of binary evolution, and develop techniques to track the ionic transitions in time and space. We simulate the interaction of a $2\,M_\odot$ red giant branch primary and a $1\,M_\odot$ companion modeled as a particle. We compare a run employing a tabulated equation of state (EOS) that accounts for ionization and recombination, with a run employing an ideal gas EOS. During the first half of the simulations, $\sim15$ per cent more mass is unbound in the tabulated EOS run due to the release of recombination energy, but by simulation end the difference has become negligible. We explain this as being a consequence of (i) the tabulated EOS run experiences a shallower inspiral and hence smaller orbital energy release at late times because recombination energy release expands the envelope and reduces drag, and (ii) collision and mixing between expanding envelope gas, ejecta and circumstellar ambient gas assists in unbinding the envelope, but does so less efficiently in the tabulated EOS run where some of the energy transferred to bound envelope gas is used for ionization. The rate of mass unbinding is approximately constant in the last half of the simulations and the orbital separation steadily decreases at late times. A simple linear extrapolation predicts a CE phase duration of $\sim2\,\mathrm{yr}$, after which the envelope would be unbound.Comment: Submitted to MNRA

Jets from main sequence and white dwarf companions during common envelope evolution

It has long been speculated that jet feedback from accretion onto the companion during a common envelope (CE) event could affect the orbital evolution and envelope unbinding process, but this conjecture has heretofore remained largely untested. We present global 3D hydrodynamical simulations of CE evolution (CEE) that include a jet subgrid model and compare them with an otherwise identical model without a jet. Our binary consists of a $2M_\odot$ red giant branch primary and a $1M_\odot$ or $0.5M_\odot$ main sequence or white dwarf secondary companion modeled as a point particle. We run the simulations for 10 orbits (40 days). Our jet model adds mass at a constant rate $\dot{M}_\mathrm{j}$ of order the Eddington rate, with maximum velocity $v_\mathrm{j}$ of order the escape speed, to two spherical sectors with the jet axis perpendicular to the orbital plane, and supplies kinetic energy at the rate $\sim\dot{M}_\mathrm{j} v_\mathrm{j}^2/40$. We explore the influence of the jet on orbital evolution, envelope morphology and envelope unbinding, and assess the dependence of the results on jet mass-loss rate, launch speed, companion mass, opening angle, and whether or not subgrid accretion is turned on. In line with our theoretical estimates, we find that in all cases the jet becomes choked around the time of first periastron passage. We also find that jets lead to increases in unbound mass of up to $\sim10\%$, as compared to simulations which do not include a jet.Comment: 16 pages, 8 figure