The spin rates and spin evolution of the O components in WR+O binaries

TL;DR

This study investigates the spin evolution of O-type stars in WR+O binaries, revealing that mass transfer during Roche lobe overflow influences their rotation, with some stars spinning down despite accretion.

Contribution

It combines observational data with binary evolution modeling to analyze the spin-up and spin-down processes of O-type stars during mass transfer in WR+O binaries, challenging previous assumptions about common envelope evolution.

Findings

O-type stars were affected by accretion during RLOF

Many stars end RLOF with sub-critical rotation speeds

Spin-down processes may counteract spin-up during mass transfer

Abstract

Despite 50 years of extensive binary research we have to conclude that the Roche lobe overflow/mass transfer process that governs close binary evolution is still poorly understood. It is the scope of the present paper to lift a tip of the veil by studying the spin-up and spin-down processes of the O-type components of WR+O binaries. We critically analyze the available observational data of rotation speeds of the O-type components in WR+O binaries. By combining a binary evolutionary code and a formalism that describes the effects of tides in massive stars with an envelope in radiative equilibrium, we compute the corresponding rotational velocities during the Roche lobe overflow of the progenitor binaries. In all the studied WR+O binaries, we find that the O-type stars were affected by accretion of matter during the RLOF of the progenitor. This means that common envelope evolution which excludes any accretion onto the secondary O-star, has not played an important role to explain the WR+O binaries. Moreover, although it is very likely that the O-type star progenitors were spun-up by the mass transfer, many ended the RLOF/mass transfer phase with a rotational velocity that is significantly smaller than the critical rotation speed. This may indicate that during the mass transfer phase there is a spin-down process which is of the same order as, although significantly less than that of the spin-up process. We propose a Spruit-Tayler type dynamo spin-down suggested in the past to explain the rotation speeds of the mass gainers in long-period Algols.