Thermal-timescale accretion does not always yield critical rotation in mass gainers

TL;DR

This study shows that stars accreting mass in binary systems often do not reach critical rotation after accretion, as thermal relaxation causes their spin rates to decrease, challenging previous assumptions about rapid rotation in such stars.

Contribution

The paper introduces detailed models demonstrating that thermal relaxation reduces stellar spin rates post-accretion, providing a more nuanced understanding of rotation in mass gainers.

Findings

Surface to critical angular velocity decreases during thermal contraction.

Reduction in fractional critical rotation is stronger with inefficient internal angular momentum transport.

Results from single-star models are consistent with binary evolution simulations.

Abstract

Binary evolution plays a central role in producing rapidly rotating stars. Previous studies have shown that mass gainers in binaries can reach critical rotation after accreting only modest amounts of material, particularly during thermal-timescale Case B mass transfer, where tidal spin-down is ineffective due to wide orbits. However, such rapid accretion often drives the mass gainer out of thermal equilibrium, and its subsequent spin evolution during thermal relaxation has not been analysed in depth. In this study, we construct a suite of accreting detailed single-star models with different accretion prescriptions, which inflate and spin up to critical rotation during the accretion. After the accretion has ended, the models relax thermally and deflate. We find that the ratio of surface to critical angular velocity decreases to subcritical values during thermal contraction, with the magnitude of this decrease correlating with the degree of thermal disequilibrium at the end of accretion. This reduction in fractional critical rotation is even stronger when internal angular momentum transport is inefficient. Detailed binary models show the same trend, indicating that the results from our toy single-star models also apply to real binary evolution. Our results highlight that binary mass transfer does not always produce critically rotating stars, but instead may yield a wide range of spin rates depending on the mass transfer and accretion history. Our findings offer new insights into the rotational properties of mass gainers in binaries, stellar merger products, and newly formed massive stars following accretion.