The IACOB project sixteen: Surface helium abundances in Galactic O-type stars: indications for identifying binary interaction products

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dzp6-2026-02-02_13_22_03-diaz-et-al-2026.pdf

The IACOB project sixteen. Surface helium abundances in Galactic O-type stars: indications for identifying binary interaction products

ABSTRACT

Context. The presence of massive O-type stars with surfaces enriched by CNO-cycle products has been known since the early nineteen eighties. For many years, internal rotational mixing was assumed to be the dominant mechanism responsible for this chemical contamination. However, accumulating evidence now suggests that binary interaction - particularly mass-transfer episodes - may play an equally important, if not dominant, role.

Aims. To carry out a large-scale investigation of surface helium abundances in Galactic O-type stars, based on the results from the analysis of high-quality spectroscopic data from the IACOB project.

Methods. We perform a homogeneous spectroscopic analysis of three hundred eighteen Galactic O-type stars with the IACOB-BROAD and FASTWIND/IACOB-GBAT tools, deriving rotational velocities, atmospheric parameters, and helium abundances. We also account for the influence of binarity, runaway status, and parameter degeneracies (e.g., microturbulence, wind properties, diagnostic lines, and companion contamination) on the abundance determinations.

Results. We present homogeneously determined surface helium abundances Y sub H e equals N sub H e divided by N sub H for the so far largest, statistically significant sample of Galactic O-type stars. About seventy-eight percent of the stars show helium abundances consistent with the previously proposed cosmic abundance standard of Y sub H e equals zero point nine eight plus or minus zero point zero zero two. The remaining twenty-two percent display clear helium enrichment Y sub H e is greater than or equal to zero point one three. We also provide observational evidence indicating that most of these helium-enriched stars are likely the products of binary interaction.

Conclusions. Our study highlights how large spectroscopic surveys are gradually opening robust observational avenues to identify the products of massive binary interaction. It also emphasizes the need for caution when interpreting the spectroscopic properties of apparently single O-type stars. A significant fraction may in fact be the outcome of binary evolution rather than isolated stellar birth.

One. Introduction

Two. Sample description

Three. Methodology

Four. Results

Four point one. General distribution of He abundances

Four point two. Comparison with results from the literature

Four point three. The Hunter and spectroscopic H-R diagrams

Four point four. Statistical properties of the runaway and SB one samples

Five. Discussion

Five point two. The helium-rich sample in the context of single star evolution

Five point three. Some insights from binary evolution

Five point four. The YHe -v sini diagram

Five point five. Further insights from runaway stars and single-line systems.

Five point six. The O-type star sample.

Six. Conclusions and future prospects

Appendix C: Tables

Overview

The study examines surface helium abundances in Galactic O-type stars, suggesting that binary interaction plays a significant role in helium enrichment, challenging previous assumptions that internal mixing was the dominant mechanism. It highlights the importance of spectroscopic surveys in identifying binary interaction products among massive stars.

Key Points

1Surface helium abundances were determined for 318 Galactic O-type stars
278% of stars showed helium abundances consistent with the cosmic abundance standard
322% of stars displayed clear helium enrichment likely due to binary interactions
4The study emphasizes caution in interpreting the spectroscopic properties of single O-type stars
5The findings suggest a significant number of O-type stars may result from binary evolution.

Details

Authors: Diaz et al.
Category: Physical Sciences

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