Orbital Evolution of Equal-mass Eccentric Binaries due to a Gas Disk: Eccentric Inspirals and Circular Outspirals

We solve the equations of two-dimensional hydrodynamics describing a circumbinary disk accreting onto an eccentric, equal-mass binary. compute time rate change binary semimajor axis $a$ and eccentricity $e$ over continuous range eccentricities spanning $e=0$ to $e=0.9$. find that binaries with initial $e_0 \lesssim 0.1$ tend $e=0$, where expands. All others are attracted $e \approx 0.4$, decays. The 0.4$ attractor is caused by rapid in response from nearly origin-symmetric state precessing asymmetric state. causes rates $\dot{a}$ $\dot{e}$ steeply sign at same critical resulting attracting solution $\dot{a} = \dot{e} 0$. This does not, however, result stalled, eccentric finite-transition between states evolve beyond both directions oscillating orbital parameters drift axis. For chosen parameters, \gtrsim toward then oscillate around they shrink Because unequal mass grow equal through preferential accretion, our results applicable wide ratios. Hence, these findings merit further investigations this transition; understanding its dependence on vital for determining fate undergoing evolution disk.