Speaking with one voice: simulations and observations discuss the common envelope $alpha$ parameter. (arXiv:1902.02039v1 [astro-ph.SR])

Speaking with one voice: simulations and observations discuss the common envelope $alpha$ parameter. (arXiv:1902.02039v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Iaconi_R/0/1/0/all/0/1">Roberto Iaconi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Marco_O/0/1/0/all/0/1">Orsola De Marco</a>

We present a comparative study between the results of most hydrodynamic
simulations of the common envelope binary interaction to date and observations
of post common envelope binaries. The comparison is carried out by considering
the main properties of observations and simulations in various planes. The goal
to evaluate whether this dataset indicates the existence of a formula that may
predict final separations of post-common envelope systems as a function of
pre-common envelope parameters. Some of our conclusions are not surprising
while others are more subtle. We find that: (i) A good fraction of the
simulations and most post-RGB observations indicate a common value of
$alpha_{rm CE}$, of the order of 50% (ii) This value does not describe,
however, the existence of small separations, post-common envelope binaries
descending from low mass AGB giants. The dearth of common envelope simulations
with low mass AGB stars leaves us with no insight as to why this is the case.
(iii) Some simulations deviate dramatically from the trend, with final
separations that are both too large or too small. (iv) The limited resolution
of some of the simulations may have resulted in a too wide a final separation.
(v) Observations with separations larger than $sim$10~rs, tend to have high
mass primaries and/or high companion masses and likely go through a relatively
long phase of unstable Roche lobe mass transfer followed by a weakened common
envelope (or with no common envelope at all). Common envelope simulations
cannot easily reproduce these systems. (vi) The addition of recombination
energy to the simulations does not systematically alter the final separation of
the simulations.

We present a comparative study between the results of most hydrodynamic
simulations of the common envelope binary interaction to date and observations
of post common envelope binaries. The comparison is carried out by considering
the main properties of observations and simulations in various planes. The goal
to evaluate whether this dataset indicates the existence of a formula that may
predict final separations of post-common envelope systems as a function of
pre-common envelope parameters. Some of our conclusions are not surprising
while others are more subtle. We find that: (i) A good fraction of the
simulations and most post-RGB observations indicate a common value of
$alpha_{rm CE}$, of the order of 50% (ii) This value does not describe,
however, the existence of small separations, post-common envelope binaries
descending from low mass AGB giants. The dearth of common envelope simulations
with low mass AGB stars leaves us with no insight as to why this is the case.
(iii) Some simulations deviate dramatically from the trend, with final
separations that are both too large or too small. (iv) The limited resolution
of some of the simulations may have resulted in a too wide a final separation.
(v) Observations with separations larger than $sim$10~rs, tend to have high
mass primaries and/or high companion masses and likely go through a relatively
long phase of unstable Roche lobe mass transfer followed by a weakened common
envelope (or with no common envelope at all). Common envelope simulations
cannot easily reproduce these systems. (vi) The addition of recombination
energy to the simulations does not systematically alter the final separation of
the simulations.

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