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Project summary

We use one-dimensional and three-dimensional simulations to study the common envelope evolution. This phase is believed to be responsible for the formation of numerous astrophysical objects and phenomena, including planetary nebulae, Type Iae progenitors and gamma-ray bursts.
  • Passy, De Marco et al. (2012): we use three-dimensional hydrodynamical simulations to study the rapid infall phase of the common envelope (CE) interaction of a red giant branch star of mass equal to 0.88 M⊙ and a companion star of mass ranging from 0.9 down to 0.1 M⊙. We first compare the results obtained using two different numerical techniques with different resolutions, and find very good agreement overall. We then compare the outcomes of those simulations with observed systems thought to have gone through a CE. The simulations fail to reproduce those systems in the sense that most of the envelope of the donor remains bound at the end of the simulations and the final orbital separations between the donor’s remnant and the companion, ranging from 26.8 down to 5.9 R⊙, are larger than the ones observed. We suggest that this discrepancy vouches for recombination playing an essential role in the ejection of the envelope and/or significant shrinkage of the orbit happening in the subsequent phase.
  • Passy, Herwig, and Paxton (2012): we study the response of giant stars to mass loss. One-dimensional simulations of red and asymptotic giant branch stars with mass loss rates from 10−3 up to a few M⊙yr−1 show in no case any significant radius increase. The largest radius increase of 0.2% was found in the case with the lowest mass loss rate. For dynamical-timescale mass loss rates that may be encountered during a common envelope phase, the evolution is not adiabatic. The superadiabatic outer layer of the giant’s envelope has a local thermal timescale comparable to the dynamical timescale. Therefore, this layer has enough time to readjust thermally. Moreover, the giant star is driven out of hydrostatic equilibrium and evolves dynamically. In these cases no increase of the stellar radius with respect to its initial value is found. If the mass loss rate is high enough, the superadiabaticity of the outer layer is lost progressively and a radiative zone forms due to a combination of thermal and dynamical readjustment. Conditions for unstable mass transfer based on adiabatic mass loss models that predict a significant radius increase may need to be re-evaluated.