Orbital eccentricities of binary systems with a former AGB star

Abstract

Context.Many binary stellar systems in which the primary star is beyond the asymptotic giant branch (AGB) evolutionary phase show significant orbital eccentricities, whereas current binary interaction models predict that their orbits are circularised. Aims.In the search for a mechanism to counteract the circularising effect of tidal interaction, we analyse how the orbital parameters in a system are modified under mass loss and mass exchange among its binary components. Methods.We propose a model for enhanced mass loss from the AGB star due to tidal interaction with its companion, which allows a smooth transition between the wind and Roche-lobe overflow mass loss regimes. We explicitly follow its effect along the orbit on the change in eccentricity and orbital semi-major axis, as well as the effect of accretion by the companion. We calculate timescales for the variation in these orbital parameters and compare them to the tidal circularisation timescale. Results.We find that in many cases, due to the enhanced mass loss of the AGB component at orbital phases closer to the periastron, the net eccentricity growth rate in one orbit is comparable to the rate of tidal circularisation. We show that with this eccentricity-enhancing mechanism it is possible to reproduce the orbital period and eccentricity of the Sirius system, which is expected to be circularised under the standard assumptions of binary interaction. We also show that this mechanism may provide an explanation for the eccentricities of most barium star systems, which are expected to be circularised due to tidal dissipation. Conclusions.By proposing a tidally enhanced model of mass loss from AGB stars, we find a mechanism that efficiently works against the tidal circularisation of the orbit. This mechanism can explain the significant eccentricities observed in binary systems containing a white dwarf and a less evolved companion, which are predicted to be circularised due to their proximity, such as Sirius and systems with barium stars.