Critically rotating stars in binaries - an unsolved problem -

TL;DR

This paper discusses the unresolved challenges in understanding how mass and angular momentum transfer affect the evolution of stars in close binary systems, highlighting the role of spin-up and angular momentum loss mechanisms.

Contribution

It introduces the first implementation of spin-up by accretion into the Eggleton stellar evolution code's TWIN version, addressing a key gap in binary star modeling.

Findings

Accreting stars in binaries can reach critical rotation rapidly.

Magnetic fields may help low-mass stars shed angular momentum.

Most binaries with periods over a few days likely evolve non-conservatively.

Abstract

In close binaries mass and angular momentum can be transferred from one star to the other during Roche-lobe overflow. The efficiency of this process is not well understood and constitutes one of the largest uncertainties in binary evolution. One of the problems lies in the transfer of angular momentum, which will spin up the accreting star. In very tight systems tidal friction can prevent reaching critical rotation, by locking the spin period to the orbital period. Accreting stars in systems with orbital periods larger than a few days reach critical rotation after accreting only a fraction of their mass, unless there is an effective mechanism to get rid of angular momentum. In low mass stars magnetic field might help. In more massive stars angular momentum loss will be accompanied by strong mass loss. This would imply that most interacting binaries with initial orbital periods larger than a few days evolve very non-conservatively. In this contribution we wish to draw attention to the unsolved problems related to mass and angular momentum transfer in binary systems. We do this by presenting the first results of an implementation of spin up by accretion into the TWIN version of the Eggleton stellar evolution code.