Interacting Binaries With Eccentric Orbits. Iii. Orbital Evolution Due to Direct Impact and Self-Accretion

Abstract

The rapid circularization and synchronization of the stellar components in an eccentric binary system at the onset of Roche lobe overflow is a fundamental assumption common to all binary stellar evolution and population synthesis codes, even though the validity of this assumption is questionable both theoretically and observationally. Here we calculate the evolution of the orbital elements of an eccentric binary through the direct three-body integration of a massive particle ejected through the inner Lagrangian point of the donor star at periastron. The trajectory of this particle leads to three possible outcomes: direct accretion onto the companion star within a single orbit, self-accretion back onto the donor star within a single orbit, or a quasi-periodic orbit around the companion star, possibly leading to the formation of a disk. We calculate the secular evolution of the binary orbit in the first two cases and conclude that direct impact accretion can increase as well as decrease the orbital semi-major axis and eccentricity, while self-accretion always decreases the orbital semi-major axis and eccentricity. In cases where mass overflow contributes to circularizing the orbit, circularization can set in on timescales as short as a few per cent of the mass transfer timescale. In cases where mass overflow increases the eccentricity, the orbital evolution is governed by competition between mass overflow and tidal torques. In the absence of tidal torques, mass overflow results in direct impact can lead to substantially subsynchronously rotating donor stars. Contrary to assumptions common in the literature, direct impact accretion furthermore does not always provide a strong sink of orbital angular momentum in close mass-transferring binaries; in fact we instead find that a significant part can be returned to the orbit during the particle orbit. The formulation presented in this paper together with our previous work can be combined with stellar and binary evolution codes to generate a better picture of the evolution of eccentric, Roche lobe overflowing binary star systems.

Double white dwarfs and LISA

“…et al , Motl et al 2007), while others suggest that much of the angular momentum can be fed back even in the absence of tidal coupling between the stars (Sepinsky et al 2010). However the best observational calibration of the space density of AM CVn stars gives a space density around a factor of 10 lower than previously assumed, and around 250 times lower than that of the detached systems, suggesting perhaps that in fact many detached DWDs do not survive mass transfer (Roelofs et al 2007).…”

An Analytic Model for Mass Transfer in Binaries with Arbitrary Eccentricity, with Applications to Triple-star Systems

“…Despite the relevance of nonzero eccentricity in mass transfer processes, binary population synthesis codes such as Star-Track (Belczynski et al 2008), BSE (Hurley et al 2002) and the updated binary c (Izzard et al 2004(Izzard et al , 2006(Izzard et al , 2009Claeys et al 2014), and SeBa (Portegies Zwart & Verbunt 1996;Toonen et al 2012), enforce circular orbits at the onset of mass transfer. Nevertheless, the problem of mass loss/mass transfer has been studied for over half a century (e.g., Huang 1956;Hadjidemetriou 1963;Kruszewski 1964;Piotrowski 1964;Matese & Whitmire 1983, 1984, and has received more recent attention in numerical studies (e.g., Regös et al 2005;Church et al 2009;Sepinsky et al 2010;Lajoie & Sills 2011a,b;van der Helm et al 2016;Bobrick et al 2017), as well as in (semi)analytical work (Sepinsky et al 2007b(Sepinsky et al , 2009(Sepinsky et al , 2010Veras et al 2011;Veras & Tout 2012;Veras et al 2013Veras et al , 2014Dosopoulou & Kalogera 2016a,b). Sepinsky et al (2007b) and Dosopoulou & Kalogera (2016b) in particular derived equations for the secular (i.e., orbit-averaged) changes of the orbital elements due to mass transfer in eccentric binaries.…”

Compact Binary Merger Rates: Comparison With Ligo/Virgo Upper Limits

“…However, it is not clear how to treat such cases. It seems that for some binary configurations RLOF may rather quickly circularize the orbit, while for others the eccentric periodic mass transfer at periastron may be a rather prolonged phase (Sepinsky et al 2010).…”