Evolution of accretor stars in massive binaries: broader implications from modeling {\zeta} Ophiuchi

TL;DR

This study models the evolution of massive binary stars undergoing mass transfer, revealing how accretion affects the star's rotation, composition, and structure, with implications for understanding runaway stars like { extzeta} Ophiuchi.

Contribution

It provides detailed models of accretor stars in binaries, highlighting differences from single star models and linking to observed properties of { extzeta} Ophiuchi.

Findings

Accretor stars retain higher core spin at main sequence end.

Surface pollution by CNO-processed material observed.

Mass accretion impacts future evolution and compact object spin.

Abstract

Most massive stars are born in binaries close enough for mass transfer episodes. These modify the appearance, structure, and future evolution of both stars. We compute the evolution of a 100-day period binary consisting initially of a 25 M star and a 17 M star, which experiences stable mass transfer. We focus on the impact of mass accretion on the surface composition, internal rotation, and structure of the accretor. To anchor our models, we show that our accretor broadly reproduces the properties of {\zeta} Ophiuchi, which has long been proposed to have accreted mass before being ejected as a runaway star when the companion exploded. We compare our accretor to models of single rotating stars and find that the later and stronger spin-up provided by mass accretion produces significant differences. Specifically, the core of the accretor retains higher spin at the end of the main sequence, and a convective layer develops that changes its density profile. Moreover, the surface of the accretor star is polluted by CNO-processed material donated by the companion. Our models show effects of mass accretion in binaries that are not captured in single rotating stellar models. This possibly impacts the further evolution (either in a binary or as single stars), the final collapse, and the resulting spin of the compact object.