How Drag Force Evolves in Global Common Envelope Simulations. (arXiv:1908.06195v1 [astro-ph.SR])

How Drag Force Evolves in Global Common Envelope Simulations. (arXiv:1908.06195v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Chamandy_L/0/1/0/all/0/1">Luke Chamandy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blackman_E/0/1/0/all/0/1">Eric G. Blackman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Frank_A/0/1/0/all/0/1">Adam Frank</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Carroll_Nellenback_J/0/1/0/all/0/1">Jonathan Carroll-Nellenback</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zou_Y/0/1/0/all/0/1">Yangyuxin Zou</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tu_Y/0/1/0/all/0/1">Yisheng Tu</a>

We compute the forces, torque and rate of work on the companion-core binary
due to drag in global simulations of common envelope (CE) evolution for three
different companion masses. Our simulations help to delineate regimes when
conventional analytic drag force approximations are applicable. During and just
prior to the first periastron passage of the in-spiral phase, the drag force is
reasonably approximated by conventional analytic theory and peaks at values
proportional to the companion mass. Good agreement between global and local 3D
“wind tunnel” simulations, including similar net drag force and flow pattern,
is obtained for comparable regions of parameter space. However, subsequent to
the first periastron passage, the drag force is up to an order of magnitude
smaller than theoretical predictions, quasi-steady, and depends only weakly on
companion mass. The discrepancy is exacerbated for larger companion mass and
when the inter-particle separation reduces to the Bondi-Hoyle-Lyttleton
accretion radius, creating a turbulent thermalized region. Greater flow
symmetry during this phase leads to near balance of opposing gravitational
forces in front of and behind the companion, hence a small net drag. The
reduced drag force at late times helps explain why companion-core separations
necessary for envelope ejection are not reached by the end of limited duration
CE simulations.

We compute the forces, torque and rate of work on the companion-core binary
due to drag in global simulations of common envelope (CE) evolution for three
different companion masses. Our simulations help to delineate regimes when
conventional analytic drag force approximations are applicable. During and just
prior to the first periastron passage of the in-spiral phase, the drag force is
reasonably approximated by conventional analytic theory and peaks at values
proportional to the companion mass. Good agreement between global and local 3D
“wind tunnel” simulations, including similar net drag force and flow pattern,
is obtained for comparable regions of parameter space. However, subsequent to
the first periastron passage, the drag force is up to an order of magnitude
smaller than theoretical predictions, quasi-steady, and depends only weakly on
companion mass. The discrepancy is exacerbated for larger companion mass and
when the inter-particle separation reduces to the Bondi-Hoyle-Lyttleton
accretion radius, creating a turbulent thermalized region. Greater flow
symmetry during this phase leads to near balance of opposing gravitational
forces in front of and behind the companion, hence a small net drag. The
reduced drag force at late times helps explain why companion-core separations
necessary for envelope ejection are not reached by the end of limited duration
CE simulations.

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