Eccentric binaries in retrograde discs

Abstract

ABSTRACT Modern numerical hydrodynamics tools have recently enabled detailed examinations of binaries accreting from prograde circumbinary discs. These have reframed the current understanding of binary-disc interactions and disc driven orbital evolution. We present the first full-domain grid-based hydrodynamics simulations of equal-mass, eccentric binaries accreting from retrograde circumbinary discs. We study binary eccentricities that span e = 0.0 to e = 0.8 continuously, and explore the influence of retrograde accretion on the binary orbital response, disc morphology, and observational properties. We find that, at all eccentricities, retrograde accretion shrinks the binary semimajor axis and pumps its eccentricity leading to the previously identified possibility of highly eccentric mergers. Contrary to past studies and models, we observe gravitational forces to dominate the binary’s orbital evolution as opposed to the physical accretion of mass and momentum. Retrograde accretion variability also differs strongly from prograde solutions. Preeminently, binaries with e > 0.55 reveal a unique two-period, double-peaked accretion signature that has not previously been identified. We additionally find evidence for the emergence of retrograde Lindblad resonances at large eccentricities in accordance with predictions from linear theory. Our results suggest that some astrophysical binaries for which retrograde accretion is possible will experience factors-of-a-few times faster orbital decay than in prograde discs and will have their eccentricities pumped beyond the limits found from prograde solutions. Such effects could lead to rapid inward migration for some young stellar binaries, the detection of highly eccentric LISA mergers, and the tentatively observed turnover at the low-frequency end of the gravitational wave background.