Tidal Dissipation Impact on the Eccentric Onset of Common Envelope Phases in Massive Binary Star Systems

TL;DR

This study investigates how tidal dissipation affects the eccentricity of massive binary star systems at the onset of common envelope phases, revealing that many systems retain eccentricity contrary to previous assumptions.

Contribution

It introduces a coupled model of stellar evolution and tidal dissipation applicable to highly eccentric orbits, challenging the assumption of rapid orbit circularization before Roche-lobe overflow.

Findings

Many systems retain initial eccentricity at RLO.

Tidal dissipation often does not circularize orbits before RLO.

Donor stars may be subsynchronously rotating at mass transfer onset.

Abstract

Tidal dissipation due to turbulent viscosity in the convective regions of giant stars plays an important role in shaping the orbits of pre-common envelope systems. Such systems are possible sources of transients and close compact binary systems that will eventually merge and produce detectable gravitational wave signals. Most previous studies of the onset of common envelope episodes have focused on circular orbits and synchronously rotating donor stars under the assumption that tidal dissipation can quickly spin up the primary and circularize the orbit before the binary reaches Roche-lobe overflow (RLO). We test this assumption by coupling numerical models of the post main sequence stellar evolution of massive-stars with the model for tidal dissipation in convective envelopes developed in Vick & Lai (2020) $-$ a tidal model that is accurate even for highly eccentric orbits with small pericentre distances. We find that, in many cases, tidal dissipation does not circularize the orbit before RLO. For a $10~M_\odot$ ($15~M_\odot$) primary star interacting with a $1.4~M_\odot$ companion, systems with pericentre distances within 3 AU (6 AU) when the primary leaves the main sequence will retain the initial orbital eccentricity when the primary grows to the Roche radius. Even in systems that tidally circularize before RLO, the donor star may be rotating subsynchronously at the onset of mass transfer. Our results demonstrate that some possible precursors to double neutron star systems are likely eccentric at the Roche radius. The effects of pre-common envelope eccentricity on the resulting compact binary merit further study.