Common envelope evolution of eccentric binaries

TL;DR

This study uses hydrodynamical simulations to explore how initial eccentricity affects common envelope evolution in binary stars, revealing partial circularization and higher post-CEE eccentricities than previously assumed.

Contribution

It provides new insights into the impact of initial eccentricity on CEE, challenging the common assumption of circular post-CEE orbits and highlighting its effects on transient event predictions.

Findings

Higher initial eccentricity results in higher post-CEE eccentricities.

CEE of eccentric binaries causes more dynamical mass loss.

Post-CEE eccentricities influence transient delay time distributions.

Abstract

Common envelope evolution (CEE) is believed to be an important stage in the evolution of binary/multiple stellar systems. Following this stage, the CE is thought to be ejected, leaving behind a compact binary (or a merger product). Although extensively studied, the CEE process is still little understood, and although most binaries have non-negligible eccentricity, the effect of initial eccentricity on the CEE has been little explored. Moreover, most studies assume a complete circularization of the orbit by the CE onset, while observationally such eccentricities are detected in many post-CE binaries. Here we use smoothed particle hydro-dynamical simulations (SPH) to study the evolution of initially eccentric ($0\le e\le0.95$) CE-systems. We find that initially eccentric binaries only partially circularize. In addition, higher initial eccentricity leads to a higher eccentricity following the end of the inspiral phase, with eccentricities as high as 0.18 in the most eccentric cases, and even higher if the initial peri-center of the orbit is located inside the star (e.g. following a kick into an eccentric orbit, rather than a smooth transition). CEE of more eccentric binaries leads to enhanced dynamical mass-loss of the CE compared with more circular binaries, and depends on the initial closest approach of the binary. We show that our results and the observed eccentricities of post-CE binaries suggest that the typical assumptions of circular orbits following CEE might potentially be revised. We expect post-CE eccentricities to affect the delay time distributions of various transients such as supernovae, gamma-ray bursts and gravitational-wave sources by up to tens of percents.