On the evolution of massive close binaries

Abstract

We discuss which fraction of the matter flowing to the companion during a Roche lobe overflow phase can actually be accreted by the secondary star. Employing new evolutionary models for massive close binaries which include the effects of rotation for both components as well as angular momentum accretion and spin-orbit coupling, we propose a physical model to calculate the accretion efficiency in Case A and B systems. We provide examples showing that both cases, high and low accretion efficiency, do occur within these models, as it seems required by observed post-mass transfer systems. Furthermore, we discuss late evolutionary stages of such binaries, with emphasis on the formation of compact objects: what are their spin rates, which systems can produce black holes, which gamma-ray bursts?

The VLT-FLAMES survey of massive stars: NGC 346-013 as a test case for massive close binary evolution

“…3.2) implies that it has been "spun up" by mass transferred from the secondary, as tidal effects in close binary systems are otherwise expected to lead to synchronization of rotation. The secondary appears to be rotating somewhat slower than expected for a synchronized system, consistent with a predicted spin-down of the mass donor during mass transfer (Langer 1998;Langer et al 2003), and has almost filled its Roche lobe, implying that despite its current lower mass it is in a more advanced evolutionary state. The primary appears somewhat underluminous for its mass (see Fig.…”

“…At the end of the rapid phase of case A transfer, "slow" transfer commences, lasting for ∼1 Myr at rates ∼10 −7 M yr −1 (Wellstein et al 2001;Langer et al 2003). An initial 16 + 15 M system would start the slow phase with an overluminous lowmass donor and a rapidly-rotating accretor with M ∼ 25 M (Wellstein et al 2001;Langer et al 2003), but a higher mass initial configuration provides a much better match: taking model 13 of Wellstein & Langer (1999) as a template, a 22+18 M system in an inital three-day orbit will exit the fast phase of mass transfer with an 11.5 M mass donor that still fills its Roche lobe, very similar to the B-type star in NGC 346-013.…”

“…An initial 16 + 15 M system would start the slow phase with an overluminous lowmass donor and a rapidly-rotating accretor with M ∼ 25 M (Wellstein et al 2001;Langer et al 2003), but a higher mass initial configuration provides a much better match: taking model 13 of Wellstein & Langer (1999) as a template, a 22+18 M system in an inital three-day orbit will exit the fast phase of mass transfer with an 11.5 M mass donor that still fills its Roche lobe, very similar to the B-type star in NGC 346-013. The model is not perfect, as at the end of the fast phase of mass transfer both the orbital period of 7.3 days and the 28.5 M mass-gainer are again inconsistent with the current state of NGC 346-013, but the model was computed assuming fully conservative mass transfer with no loss of mass or angular momentum from the system.…”

“…Within ∼10 6 years the system will return to synchronous rotation, while slow case A transfer will finish when core hydrogen burning ends in the mass donor. A second phase of rapid case AB transfer then commences with the onset of shell burning, spinning up the Otype star, still on the main sequence, back to near-critical rotation (Langer et al 2003). During this phase the mass donor will lose its remaining hydrogen envelope to leave a helium core, while the orbital period expands greatly (Wellstein et al 2001), leaving a rapidly rotating ∼20−25 M OB (super)giant that appears as an apparently-single star, with the extreme mass ratio and increasingly low luminosity of the mass donor making detection challenging.…”

Mass loss out of close binaries

“…The formalism was taken from Zahn (1977). Strong tidal interaction is calculated with f sync = 0.1, whereas f sync = 1 implies weak tides, as adopted by Langer et al (2003) and discussed in detail by Wellstein (2001). One sees in Fig.…”