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 eccentricityenhancing 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.

The orbits of subdwarf-B + main-sequence binaries

“…Our results call for the study of eccentricity pumping mechanisms (e.g. Bonačić Marinović et al 2008) also in the framework of sdB binaries.…”

“…In highly eccentric systems the mass transfer through RLOF and mass loss due to stellar winds will not be constant over the orbit. Bonačić Marinović et al (2008) studied this effect in binary systems with an AGB star, and found that enhanced mass loss from the AGB star at orbital phases closer to periastron can work efficiently against the tidal circularization of the orbit. Further theoretical studies could investigate if this eccentricity enhancement mechanism can work in in the progenitors of sdB + MS binaries as well.…”

Binary properties of CH and carbon-enhanced metal-poor stars

“…The presence of these two groups in families with so vastly different metallicities as CH/CEMP-s and barium stars reinforces the reality of the observed distinction 4 . Population syntheses of binary systems, for both low (Izzard et al 2009) and solar (Bonačić Marinović & Pols 2004;Izzard et al 2010) metallicities, indicate that the observed threshold around 1000 d might correspond to the transition between the wind-accretion and RLOF channels, a hypothesis supported because most of the P < 1000 d orbits are circular, or nearly so (Fig. 6).…”

On the Eccentricity Excitation in Post-Main-Sequence Binaries

“…The ideas for such a mechanism explored in the past include the enhanced mass loss and transfer during the periastron passages (Soker 2000;Bonačić-Marinović et al 2008) and kicks received by the WD at birth (Izzard et al 2010). Eccentric binary MSPs were also suggested to originate from the dynamical evolution in a triple stellar system (Portegies Zwart et al 2011), accretion-induced collapse (Freire & Tauris 2014), and quark nova (Jiang et al 2015).…”