Orbital Evolution of Equal-mass Eccentric Binaries due to a Gas Disk: Eccentric Inspirals and Circular Outspirals. (arXiv:2103.09251v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+DOrazio_D/0/1/0/all/0/1">Daniel J. D'Orazio</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Duffell_P/0/1/0/all/0/1">Paul C. Duffell</a>
We solve the equations of two-dimensional hydrodynamics describing a
circumbinary disk accreting onto an eccentric, equal-mass binary. We compute
the time rate of change of the binary semimajor axis $a$ and eccentricity $e$
over a continuous range of eccentricities spanning $e=0$ to $e=0.9$. We find
that binaries with initial eccentricities $e_0 lesssim 0.1$ tend to $e=0$,
where the binary semimajor axis expands. All others are attracted to $e approx
0.4$, where the binary semimajor axis decays. The $e approx 0.4$ attractor is
caused by a rapid change in the disk response from a nearly origin-symmetric
state to a precessing asymmetric state. The state change causes the time rates
of change $dot{a}$ and $dot{e}$ to steeply change sign at the same critical
eccentricity resulting in an attracting solution where $dot{a} = dot{e} = 0$.
This does not, however, result in a stalled, eccentric binary. The
finite-transition time between disk states causes the binary eccentricity to
evolve beyond the attracting eccentricity in both directions resulting in
oscillating orbital parameters and a drift of the semimajor axis. For the
chosen disk parameters, binaries with $e_0 gtrsim 0.1$ evolve toward and then
oscillate around $e approx 0.4$ where they shrink in semimajor axis. Because
unequal mass binaries grow toward equal mass through preferential accretion,
our results are applicable to a wide range of initial binary mass ratios.
Hence, these findings merit further investigations of this disk transition;
understanding its dependence on disk parameters is vital for determining the
fate of binaries undergoing orbital evolution with a circumbinary disk.
We solve the equations of two-dimensional hydrodynamics describing a
circumbinary disk accreting onto an eccentric, equal-mass binary. We compute
the time rate of change of the binary semimajor axis $a$ and eccentricity $e$
over a continuous range of eccentricities spanning $e=0$ to $e=0.9$. We find
that binaries with initial eccentricities $e_0 lesssim 0.1$ tend to $e=0$,
where the binary semimajor axis expands. All others are attracted to $e approx
0.4$, where the binary semimajor axis decays. The $e approx 0.4$ attractor is
caused by a rapid change in the disk response from a nearly origin-symmetric
state to a precessing asymmetric state. The state change causes the time rates
of change $dot{a}$ and $dot{e}$ to steeply change sign at the same critical
eccentricity resulting in an attracting solution where $dot{a} = dot{e} = 0$.
This does not, however, result in a stalled, eccentric binary. The
finite-transition time between disk states causes the binary eccentricity to
evolve beyond the attracting eccentricity in both directions resulting in
oscillating orbital parameters and a drift of the semimajor axis. For the
chosen disk parameters, binaries with $e_0 gtrsim 0.1$ evolve toward and then
oscillate around $e approx 0.4$ where they shrink in semimajor axis. Because
unequal mass binaries grow toward equal mass through preferential accretion,
our results are applicable to a wide range of initial binary mass ratios.
Hence, these findings merit further investigations of this disk transition;
understanding its dependence on disk parameters is vital for determining the
fate of binaries undergoing orbital evolution with a circumbinary disk.
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