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