The eccentric behaviour of windy binary stars

Abstract

Carbon-enhanced metal-poor stars, CH stars, barium stars and extrinsic S stars, among other classes of chemically peculiar stars, are thought to be the products of the interaction of low-and intermediate-mass binaries which occurred when the most evolved star was in the asymptotic giant branch (AGB) phase. Binary evolution models predict that because of the large sizes of AGB stars, if the initial orbital periods of such systems are shorter than a few thousand days, their orbits should have circularised due to tidal effects. However, observations of the progeny of AGB binary stars show that many of these objects have substantial eccentricities, up to e ≈ 0.9. In this work we explore the impact of wind mass transfer on the orbital parameters of AGB binary stars by performing numerical simulations in which the AGB wind is modelled using a hydrodynamical code and the dynamics of the stars is evolved using an N-body code. We find that in most models the effect of wind mass transfer will contribute to the circularisation of the orbit, but on longer timescales than tidal circularisation if e 0.4. We also find that for relatively low initial wind velocities and pseudo-synchronisation of the donor star, a structure resembling wind Roche-lobe overflow is observed as the stars approach periastron. In this case, the interaction between the gas and the star is stronger than when the initial wind velocity is high and the orbit shrinks while the eccentricity decreases. In one of our models wind interaction is found to pump the eccentricity of the orbit on a similar timescale as tidal circularisation. However, since the orbit of this model is shrinking tidal effects will become stronger during the evolution of the system. Although our study is based on a small sample of models, it offers some insight into the orbital evolution of eccentric binary stars interacting via winds. A larger grid of numerical models for different binary parameters is needed to test if a regime exists where hydrodynamical eccentricity pumping can effectively counteract tidal circularisation, and if this can explain the puzzling eccentricities of the descendants of AGB binaries.

Symbiotic Stars in the APOGEE Survey: The Case of LIN 358 and SMC N73 (LIN 445a)

“…The change in strength of these nebular features might also indicate variability in the mass accretion of the WD, which, in turn, may be a reflection of a non-circular binary orbit, since the instantaneous mass-accretion rate would be expected to vary strongly with the orbital velocity and separation of the stars. For example, the hydrodynamical simulations of AGB binaries by e.g., Saladino & Pols (2019) have shown that for high eccentricity systems one should expect significant variability in the mass accretion rate of the WD, and therefore significant variability in the nebular emission features, as observed here in the Brackett lines and found for other lines in LIN 358 spectra, like Hα (see discussion in Section 1.2). A highly eccentric orbit could also cause variable X-ray emission as the WD accretes at different rates, and a long-term change in X-ray flux has been reported for LIN 358 by Kahabka & Haberl (2006).…”

Common envelope episodes that lead to double neutron star formation

“…Bonacić Marinović, Glebbeek, & Pols (2008) propose a model which allows for tides, mass loss, and mass transfer in an eccentric orbit, physically motivated by Sirius. Following Bonacić Marinović et al (2008), Saladino & Pols (2019) carried out hydrodynamic simulations of binary stars with significant wind-driven mass loss and find that this eccentricity-enhancing mechanism is non-negligible.…”

Evolution and Mass Loss of Cool Ageing Stars: a Daedalean Story

“…(3) an accretion disk around the secondary; (4) a bipolar outflow, that can display a ripple-like structures if the AGB pulsations are included; (5) an equatorial density enhancement (EDE), with a regular (Keplerian) or complex velocity vector field; (6) and even 'spider' or 'rose-like' structures (see Figure 7; Kim & Taam 2012, Chen et al 2017, Liu et al 2017, Saladino et al 2018, Saladino & Pols 2019, Kim et al 2019, El Mellah et al 2020, Chen et al 2020.…”