The Impact of Tidal Dissipation on the Eccentric Onset of Common Envelope Phases

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 a tidal model that is accurate even for highly eccentric orbits with small pericenter distances. We find that, in many cases, tidal dissipation does not circularize the orbit before RLO. Even in systems that are tidally circularized, the donor star may be rotating sub-synchronously 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.