Pre-Common-Envelope Mass Loss from Coalescing Binary Systems. (arXiv:2003.01123v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+MacLeod_M/0/1/0/all/0/1">Morgan MacLeod</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Loeb_A/0/1/0/all/0/1">Abraham Loeb</a>
Binary systems undergoing unstable Roche Lobe overflow spill gas into their
circumbinary environment as their orbits decay toward coalescence. In this
paper, we use a suite of hydrodynamic models of coalescing binaries involving
an extended donor and a more compact accretor. We focus on the period of
unstable Roche Lobe overflow that ends as the accretor plunges within the
envelope of the donor at the onset of a common envelope phase. During this
stage, mass is removed from the donor and flung into the circumbinary
environment. Across a wide range of binary mass ratios, we find that the mass
expelled as the separation decreases from the Roche limit to the donor’s
original radius is of the order of 25% of the accretor’s mass. We study the
kinematics of this ejecta and its dependencies on binary properties and find
that it assembles into a toroidal circumbinary distribution. These circumbinary
tori have approximately constant specific angular momentum due to momentum
transport by spiral shocks launched from the orbiting binary. We show that an
analytic model with these torus properties captures many of the main features
of the azimuthally-averaged profiles of our hydrodynamic simulations. Our
results, in particular the simple relationship between accretor mass and
expelled mass and its spatial distribution, may be useful in interpreting
stellar coalescence transients like luminous red novae, and in initializing
hydrodynamic simulations of the subsequent common envelope phase.
Binary systems undergoing unstable Roche Lobe overflow spill gas into their
circumbinary environment as their orbits decay toward coalescence. In this
paper, we use a suite of hydrodynamic models of coalescing binaries involving
an extended donor and a more compact accretor. We focus on the period of
unstable Roche Lobe overflow that ends as the accretor plunges within the
envelope of the donor at the onset of a common envelope phase. During this
stage, mass is removed from the donor and flung into the circumbinary
environment. Across a wide range of binary mass ratios, we find that the mass
expelled as the separation decreases from the Roche limit to the donor’s
original radius is of the order of 25% of the accretor’s mass. We study the
kinematics of this ejecta and its dependencies on binary properties and find
that it assembles into a toroidal circumbinary distribution. These circumbinary
tori have approximately constant specific angular momentum due to momentum
transport by spiral shocks launched from the orbiting binary. We show that an
analytic model with these torus properties captures many of the main features
of the azimuthally-averaged profiles of our hydrodynamic simulations. Our
results, in particular the simple relationship between accretor mass and
expelled mass and its spatial distribution, may be useful in interpreting
stellar coalescence transients like luminous red novae, and in initializing
hydrodynamic simulations of the subsequent common envelope phase.
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