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

TL;DR

This study presents the largest homogeneous assessment to date of surface helium abundances in Galactic O-type stars (318 objects) using IACOB spectroscopic data and FASTWIND-based analyses. It finds that about 78% of stars have helium abundances consistent with a cosmic reference, while around 22% show enrichment indicative of binary-interaction products, with running and SB1 statistics supporting this interpretation. The results challenge single-star rotational mixing as the sole driver of surface He enrichment, instead highlighting binary evolution, mass transfer, and mergers as dominant processes, especially for He-rich, rapidly rotating, or runaway stars. The work demonstrates the power of large spectroscopic surveys to identify binary-interaction products and provides observational constraints for binary-evolution models, with future work planned to incorporate nitrogen abundances and orbital information, and to capitalize on forthcoming surveys like WEAVE and 4MOST.

Abstract

The presence of massive O-type stars with surfaces enriched by CNO-cycle products has been known since the early 1980s. 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. We aim to carry out a large-scale investigation of surface helium (He) abundances in Galactic O-type stars, based on the results from the analysis of high-quality spectroscopic data from the IACOB project. We perform a homogeneous spectroscopic analysis of 318 Galactic O-type stars with the IACOB-BROAD and FASTWIND/IACOB-GBAT tools, deriving rotational velocities, atmospheric parameters, and He 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. We present homogeneously determined surface He abundances (YHe=N(He)/N(H)) for the so far largest, statistically significant sample of Galactic O-type stars. About 78% of the stars show He abundances consistent with the previously proposed cosmic abundance standard of YHe=0.098$\pm$0.002. The remaining 22% display clear He enrichment (YHe>0.13). We also provide observational evidence indicating that most of these He-enriched stars are likely the products of binary interaction. 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.

Figures (6)

• Figure 1: He abundance distributions for the three groups of stars introduced in Sect. \ref{['distHe']}. Vertical dotted line indicates the present-day cosmic reference value provided by NievaPrzybilla2012. Black dot and horizontal line indicate mean and standard deviation associated with Group 1 stars.
• Figure 2: Comparison of He abundances for a sample of 67 stars in common with the literature and 48 stars independently analyzed for this work using the code maui. Open symbols refer to stars for which we have provided a Q2 or Q3 quality flag to the fastwind fits (see Appendix \ref{['App_tables']}). The 1-to-1 relation and 25% tolerance region are shown as dashed and dotted lines, respectively. The cross at the top left corner indicates the typical uncertainties in the Y$_{\rm He}$ estimates.
• Figure 3: Distribution of the 318 Galactic O-type stars in a modified Hunter diagram, using He abundance instead of nitrogen on the y-axis. The fractions of He-rich stars in the slow- and fast-rotating subsamples are indicated, assuming $v$ sin $i$ = 200 km s$^{-1}$ as the dividing threshold. ON, SB1 and RW stars are indicated with red small circles, blue crosses, and grey circles, respectively.
• Figure 4: Distribution of our sample of 318 Galactic O-type stars in a sHRD separated by the three He abundance groups described in Sect. \ref{['distHe']}. Evolutionary tracks from Ekstroem2012 for an initial spin rate $v_{\rm ini}$/$v_{\rm crit}$ = 0.4 are depicted for reference purposes, highlighting in green the sections of the tracks where the He surface abundance reaches over 1.3 the initial abundance. Symbols are the same as in Fig. \ref{['YHevsvsini']}.
• Figure 5: Distribution in the sHRD of the birth location of mass gainers as predicted by the binary evolution computations by Jin2025. The full sample of gainers is depicted with gray colors, while those with enriched He abundances are highlighted with colors separating the mass-transfer cases A (red), B (orange) and AB (blue). Single star evolutionary tracks computed with the MESA stellar evolution code by HJin2024 and used as basis for the binary evolution computation are also depicted for reference purposes. Open circles show the location of the He-rich group of O-type stars from our study.