Letter to the Editor

The IACOB project

XIII. Helium enrichment in O-type stars as a tracer of past binary interaction

¹ Instituto de Astrofísica de Canarias, c/Vía Láctea, s/n, E-38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
³ Argelander Institut für Astronomie, Auf dem Hügel 71, DE-53121 Bonn, Germany
⁴ Center for Computational Astrophysics, Division of Science, National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588, Japan
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, DE-53121 Bonn, Germany
⁶ LMU Munich, Universitätssternwarte, Scheinerstrasse 1, 81679 München, Germany

^⋆ Corresponding author; cms.martinezsebastian@gmail.com

Received: 15 October 2024
Accepted: 18 December 2024

Abstract

There is increasing evidence that single-star evolutionary models are unable to reproduce all of the observational properties of massive stars. Binary interaction has emerged as a key factor in the evolution of a significant fraction of massive stars. In this study, we investigate the helium (Y_He) and nitrogen (ϵ_N) surface abundances in a comprehensive sample of 180 Galactic O-type stars with projected rotational velocities of ≤150 km s⁻¹. We found a subsample (∼20% of the total, and ∼80% of the stars with Y_He ≥ 0.12) with a Y_He and ϵ_N combined pattern that is unexplainable by single-star evolution. We argue that the stars with anomalous surface abundance patterns are binary interaction products.