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

This study presents a homogeneous spectroscopic analysis of 318 Galactic O-type stars, revealing that approximately 22% exhibit surface helium enrichment likely caused by binary interactions rather than isolated stellar evolution, thereby challenging the assumption that most apparently single O-type stars are products of isolated birth.

The Gist

The Cosmic Detective Story: Why Some Giant Stars Are "Cheating" on Their Chemistry

Imagine the universe as a massive, bustling kitchen. In this kitchen, the chefs are O-type stars—the biggest, hottest, and most energetic stars in our galaxy. For decades, astronomers believed they knew exactly how these stellar chefs cooked their meals.

The standard recipe was simple: A star is born, it spins, and over time, a process called "rotational mixing" stirs the pot. This stirring brings fresh ingredients from the deep core up to the surface. If the star spins fast enough, the surface should taste a little bit different than when it was born, specifically having more helium (a chemical element).

But here's the plot twist: The paper argues that the "stirring" recipe isn't the whole story. In fact, for a huge number of these stars, the surface chemistry is being changed by something much more dramatic: binary interactions (stars interacting with a partner).

Here is the breakdown of their investigation, explained simply:

1. The Big Survey (The Census)

The researchers, part of a project called IACOB, acted like cosmic census-takers. They gathered high-quality "photos" (spectra) of 318 massive O-type stars in our galaxy. They didn't just look at them; they analyzed them with computer tools to measure exactly how much helium was on their surfaces.

They found three distinct groups of stars:

• The "Normal" Stars (60%): These stars have the standard amount of helium you'd expect from the universe's baseline recipe. They are the quiet, single stars doing their thing.
• The "Low" Stars (18%): These stars showed too little helium. The authors suspect this isn't because the stars are weird, but because they are hiding a secret. They likely have a faint, invisible companion star next to them. This companion is like a dilution agent, watering down the helium signal, making the main star look like it has less helium than it actually does.
• The "Rich" Stars (22%): This is the main discovery. These stars have way too much helium on their surfaces—far more than a single star could produce just by spinning.

2. The Mystery of the "Helium-Rich" Stars

For years, scientists thought these helium-rich stars were just fast-spinners. They believed the star was spinning so fast that it mixed its own core ingredients to the surface, like a blender.

The paper's evidence says: "No, that's not enough."

The authors compared the stars' rotation speeds to their helium levels and found a mismatch.

• The Analogy: Imagine you are looking at a group of people who have eaten a very spicy meal (high helium). You expect only the people who are running a marathon (fast spinners) to have eaten it. But you find that many people who are just walking slowly (slow rotators) have also eaten the spicy meal.
• The Conclusion: If slow walkers have the spicy meal, they didn't get it from running. They must have gotten it from someone else.

3. The Real Culprit: The "Star Swap"

The paper argues that these helium-rich stars are actually the products of binary interactions.

• The Scenario: Two stars are born close together. One star (the donor) evolves faster and starts dumping its outer layers onto its partner (the gainer).
• The Result: The "gainer" star receives a massive dose of helium-rich material from its partner. It essentially gets a "chemical makeover."
• The Evidence: The authors found that these helium-rich stars are much more likely to be Runaway stars (stars ejected from their birth clusters at high speeds). This fits the story perfectly: When a binary pair interacts, and one star explodes as a supernova, the partner is often flung out into space like a cannonball. These runaway stars are the "gainers" who survived the explosion and are now wandering the galaxy with their stolen helium.

4. Why This Matters

This study changes how we see the "family tree" of massive stars.

• Old View: Most massive stars are born alone, spin, and evolve on their own.
• New View: A significant chunk (about 22% in this sample) of stars that look like single, normal stars are actually cosmic imposters. They are the survivors of a violent past where they stole mass from a partner.

The Takeaway

The paper concludes that we can no longer assume a massive star is just a single entity evolving in isolation. If you see a star with a helium-rich surface, it's a strong hint that it has a history of interacting with a partner.

In short: The universe isn't just a solo act. It's a duet, and sometimes, one star steals the spotlight (and the helium) from the other. The authors have provided the first large-scale "forensic report" proving that binary interactions are a major player in shaping the chemical makeup of our galaxy's biggest stars.

Technical Summary

Technical Summary: Surface Helium Abundances in Galactic O-type Stars and Binary Interaction Products

Problem and Context
Since the early 1980s, the presence of helium (He) enrichment on the surfaces of Galactic O-type stars has been a persistent observational anomaly. Early spectroscopic analyses (e.g., Herrero et al. 1992) revealed that a significant fraction of these stars exhibited surface He abundances exceeding the cosmic standard, a finding that contradicted standard single-star evolutionary models which predicted that radiative envelopes should prevent core-processed material from reaching the surface. While rotational mixing was long assumed to be the dominant mechanism for this chemical contamination, accumulating evidence from smaller samples suggested that binary interaction might play an equally or more significant role. However, a comprehensive, homogeneous investigation of surface He abundances across a statistically significant sample of Galactic O-type stars had been lacking, partly due to the technical difficulties in deriving reliable He abundances and the high baseline abundance of helium which makes surface variations difficult to detect.

Methodology
This study presents the first large-scale, homogeneous spectroscopic analysis of surface He abundances ($Y_{He} = N_{He}/N_{H}$) for 318 Galactic O-type stars. The sample was drawn from the IACOB spectroscopic database, selected based on high signal-to-noise ratios (S/N > 50), high spectral resolution (R = 25,000–85,000), and the absence of peculiar spectral features or clear double-lined spectroscopic binaries.

The analysis employed the iacob-broad and iacob-gbat tools. The iacob-broad tool was used to determine projected rotational velocities ($v \sin i$) and macroturbulent broadening ($v_{mac}$). The iacob-gbat tool, utilizing an extended grid of non-LTE fastwind atmospheric models, was used to derive effective temperatures ($T_{eff}$), surface gravities ($\log g$), microturbulence ($\xi_t$), wind strength parameters, and He abundances.

Key methodological updates included:

1. Extended Grid: A new fastwind grid was computed with a reduced step size in He abundance (0.02 instead of 0.05) and a lower minimum He abundance limit (0.04) to better constrain low-abundance cases.
2. Homogeneous Analysis: The entire sample was reanalyzed with this updated grid to improve accuracy and address previous upper limits on He abundance.
3. Quality Control: A visual assessment of the best-fitting models against observed spectra was performed to assign quality flags (Q1–Q3) and identify cases requiring parameter adjustments or indicating composite spectra.
4. Validation: Results were cross-checked against literature values from four recent studies (Repolust et al. 2004; Martins et al. 2015b; Markova et al. 2018; Aschenbrenner et al. 2023) and a subsample was independently analyzed using the maui code to verify robustness.

Key Results
The study categorizes the 318 stars into three groups based on their derived He abundances relative to the cosmic standard ($Y_{He} = 0.098 \pm 0.002$):

1. He-normal (Group 1): Approximately 60% (193 stars) show He abundances consistent with the cosmic standard.
2. He-low (Group 2): Approximately 18% (56 stars) exhibit anomalously low He abundances (down to $Y_{He} \approx 0.07$). The authors argue these are likely not physical but result from flux contamination by faint, unresolved companions diluting the diagnostic lines. This group shows a higher fraction of single-lined spectroscopic binaries (SB1) compared to the normal group.
3. He-rich (Group 3): Approximately 22% (69 stars) display clear He enrichment ($Y_{He} \gtrsim 0.13$).

Analysis of He-rich Stars:

• Rotational Mixing Limitations: The distribution of He-rich stars in the $Y_{He}$ vs. $v \sin i$ diagram does not show a clear correlation expected from pure rotational mixing. Notably, a significant number of He-rich stars are slow rotators ($v \sin i < 200$ km s$^{-1}$), and even among fast rotators, He-normal stars dominate.
• Stellar Evolution Tracks: A substantial fraction of He-rich stars (approx. 47%) occupy regions in the spectroscopic Hertzsprung–Russell diagram (sHRD) where rotating single-star evolutionary models (even with high initial spin rates) fail to predict such levels of enrichment.
• Binary Interaction Evidence: The He-rich population shows a significantly higher fraction of runaway stars (48% vs. 24% for He-normal stars) and a lower fraction of detected SB1 systems (18% vs. 33%). This pattern aligns with binary evolution scenarios where mass gainers become runaways following the supernova of a companion or where the binary signature is erased by merger events or the presence of faint stripped companions.
• ON Stars: While ON stars (indicating strong Nitrogen enrichment) are found among He-rich objects, the majority of He-rich stars are not ON, suggesting that rotational mixing is not the sole driver of surface contamination.

Significance and Conclusions
The paper concludes that rotational mixing alone cannot explain the observed distribution of surface He abundances in Galactic O-type stars. Instead, the observational evidence strongly supports the hypothesis that a large fraction of He-enriched O-type stars are products of binary interaction, specifically mass-transfer episodes or mergers.

The study highlights that surface He abundance is an efficient diagnostic for identifying potential binary interaction products, even in systems that appear single or are classified as SB1. The findings emphasize the need for caution when interpreting the spectroscopic properties of apparently single O-type stars, as a significant fraction may be the outcome of binary evolution rather than isolated stellar birth. This work serves as a foundational step, utilizing the IACOB project's high-quality data, to open robust observational avenues for identifying binary interaction products, paving the way for future constraints on massive-star evolution models using upcoming large-scale spectroscopic surveys.