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Binarity at LOw Metallicity (BLOeM): Projected rotational velocities

This paper presents projected rotational velocities ( $v \sin i$ ) for 929 massive stars in the Small Magellanic Cloud from the BLOeM survey, revealing distinct rotational distributions across single stars, SB1, and SB2 binaries that highlight a significant velocity drop at the terminal-age main sequence, a population of low-velocity nitrogen-rich candidates potentially comprising pristine stars, long-period binaries, or merger products, and high-velocity systems consistent with tidal and mass-transfer effects.

Original authors: D. J. Lennon, S. R. Berlanas, A. Herrero, N. Britavskiy, P. L. Dufton, N. Langer, H. Jin, A. Schootemeijer, A. Menon, J. Bestenlehner, P. Crowther, J. S. Vink, J. Bodensteiner, T. Shenar, K. Deshmukh

Published 2026-03-11

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CC BY 4.0

Original authors: D. J. Lennon, S. R. Berlanas, A. Herrero, N. Britavskiy, P. L. Dufton, N. Langer, H. Jin, A. Schootemeijer, A. Menon, J. Bestenlehner, P. Crowther, J. S. Vink, J. Bodensteiner, T. Shenar, K. Deshmukh, J. Villasenor, L. Patrick, F. Najarro, A. de Koter, L. Mahy, D. M. Bowman, A. Bobrick, C. J. Evans, M. Gull, G. Holgado, Z. Katabi, J. Kubat, P. Marchant, D. Pauli, M. Pawlak, M. Renzo, D. F. Rocha, A. A. C. Sander, T. Sayada, S. Simon-Diaz, M. Stoop, R. Valli, C. Wang, X. -T. Xu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Small Magellanic Cloud (SMC) as a bustling, low-metallicity cosmic city. In this city, massive stars are the skyscrapers—huge, bright, and short-lived. For decades, astronomers have been trying to figure out how these skyscrapers are built, how they spin, and what happens when they crash into each other.

This paper, titled "Binarity at LOw Metallicity (BLOeM)," is like a massive census taken by a team of astronomers using the European Southern Observatory's giant telescopes. They looked at 929 massive stars to answer a simple but profound question: How fast are these stars spinning, and does it matter if they are alone or part of a couple?

Here is the story of their findings, broken down with some everyday analogies.

1. The Great Spin-Off: Measuring the Spin

To measure how fast a star spins, astronomers look at its "projected rotational velocity" (written as $v \sin i$ ). Think of this like watching a spinning top from the side.

If you look at a spinning top from the side, you see it blur.
If you look at it from the top (pole-on), it looks like it's standing still, even though it's spinning fast.
Because we can't always tell the angle of the star, we get a "minimum speed."

The team used a mathematical trick called the Fourier Transform (think of it as a high-tech audio equalizer that separates sounds) to analyze the light from these stars. They found that the resolution of their "camera" (spectrograph) was about 30 km/s. Anything spinning slower than that looks like it's standing still to their instruments.

2. The Three Groups of Stars

The astronomers sorted the stars into three main personality types based on their spin and their relationships:

A. The "Slow Dancers" (Single Stars & Long-Period Binaries)

Who they are: Stars that appear to be alone, or perhaps have a partner they haven't met in a long time (a very wide orbit).
The Spin: Most of these stars are surprisingly slow. Their speeds cluster around 30–60 km/s.
The Analogy: Imagine a crowded dance floor. Most people are just swaying gently to the music. The astronomers noticed a huge pile-up of slow dancers.
The Mystery: Why are they so slow?
- Some might be "pristine" stars that never sped up.
- Some might be "merger products"—stars that crashed into each other in the past and settled down.
- The Nitrogen Clue: The team found that about one-third of these slow stars are "nitrogen-rich." In the life of a star, nitrogen usually hides deep in the core. If it's on the surface, it means the star has been "stirred up" (either by spinning fast in the past or by merging). It's like finding the filling of a donut on the outside; something must have mixed it up.

B. The "Synchronized Couples" (SB2 Binaries)

Who they are: Double-star systems where we can see both stars clearly (Double-Lined Spectroscopic Binaries).
The Spin: These stars are fast, spinning at an average of 140 km/s.
The Analogy: Imagine two ice skaters holding hands and spinning around a common center. Because they are so close, the gravitational pull (tidal forces) locks them together. They spin in perfect sync, like a single, fast-spinning figure.
The Finding: These are mostly short-period binaries (they orbit each other quickly), and they are all spinning at high speeds because they are "tidally locked."

C. The "Supergiants" (The Elderly Giants)

Who they are: Massive stars that have grown old and swollen (Supergiants).
The Spin: They are almost all very slow, often too slow to measure with current tools.
The Analogy: Think of a figure skater who starts spinning fast with arms tucked in. When they open their arms wide to slow down, they spin slower. As these stars grow huge (become supergiants), their outer layers expand so much that their spin slows down to a crawl. To our telescopes, they look like they are barely moving at all.

3. The "Terminus" of the Main Sequence

The paper discovered a specific spot on the "Star Map" (Hertzsprung-Russell diagram) where the spinning behavior changes dramatically.

The Analogy: Imagine a highway. For most of the journey (the Main Sequence), cars (stars) are driving at various speeds. But then, they hit a specific exit ramp (the Terminal-Age Main Sequence).
The Observation: Just before stars leave the main sequence to become giants, their rotation speeds drop significantly. It's as if the stars hit a "brake" right before they evolve. The team identified this "brake zone" as a key marker for when a star is about to run out of its core hydrogen fuel.

4. Why This Matters

The researchers compared their findings with other surveys in the Milky Way and the Large Magellanic Cloud. They found that metallicity (how "dirty" or heavy-element-rich the star is) isn't the main driver of how fast stars spin. Instead, it's the history of the star that matters:

Did it stay single? (Slow spin).
Did it merge with a partner? (Fast spin, then slow down).
Is it in a tight binary dance? (Fast, synchronized spin).

The Big Takeaway

This paper is like a massive "speed trap" for the universe. It tells us that:

Most massive stars are surprisingly slow.
Binary stars are the speed demons, spinning fast because they are locked in a gravitational dance.
The "slow" stars are a mix of lonely stars, long-distance couples, and the aftermath of stellar crashes.
Nitrogen on the surface of slow stars suggests that even "quiet" stars might have had a violent or mixed-up past.

In short, the universe isn't just a collection of spinning tops; it's a complex ballroom where stars are either dancing solo, spinning wildly with partners, or slowing down as they age, leaving behind clues (like nitrogen) that tell the story of their entire lives.

1. Problem Statement

Understanding the rotational velocities of massive stars ( $M \gtrsim 8 M_\odot$ ) is critical for constraining stellar evolution models, star formation processes, binary interaction mechanisms (mass transfer, mergers), and the final endpoints of stars (supernovae, gamma-ray bursts). While projected rotational velocities ( $v \sin i$ ) have been extensively studied in the Milky Way and the Large Magellanic Cloud (LMC), there is a need for a comprehensive, homogeneous dataset in the Small Magellanic Cloud (SMC). The SMC offers a unique low-metallicity environment ( $Z$ ) to test how metallicity influences rotation distributions. Previous SMC studies were limited by smaller sample sizes, heterogeneous methodologies, or a lack of multi-epoch data required to robustly characterize binary systems.

2. Methodology

The authors analyzed data from the BLOeM survey, an ESO Large Programme targeting 929 massive stars across eight fields in the SMC.

Data Source: Normalized spectra from BLOeM Data Release 4 (DR4), comprising 21–23 epochs per field. Spectra were merged to create high signal-to-noise (S/N) combined spectra, corrected for radial velocity shifts relative to a reference frame.
Measurement Technique: The primary method employed was the Fourier Transform (FT) of spectral line profiles.
- The FT of the rotational broadening function yields a Bessel function with zeros characteristic of $v \sin i$ .
- The first minimum ( $f_0$ ) of the FT was used to derive $v \sin i = 0.66 / f_0$ (assuming a limb-darkening coefficient $\alpha = 0.6$ ).
- Measurements were taken from 3–10 absorption lines per star (e.g., He I $\lambda\lambda$ 4143, 4387, 4471; Si III; N II), selected based on S/N and spectral type.
- FWHM Check: Full Width at Half Maximum (FWHM) measurements were also derived via Gaussian fitting to check for broadening effects (e.g., macroturbulence, binary smearing) that might bias FT results.
Binary Handling:
- SB1 (Single-lined): Treated as single stars in the FT analysis, though secondary lines could cause profile smearing.
- SB2 (Double-lined): For 70 of the 91 identified SB2 systems, the authors performed a preliminary decomposition. They fitted double Gaussians to spectra where components were partially separated (at least 2/3 line depth) to estimate $v \sin i$ for individual components (labeled A and B).
Classification: Stars were categorized as Single, SB1, SB2, or ambiguous (SB?) based on previous BLOeM papers (Shenar et al. 2024; Villaseñor et al. 2025). Ambiguous cases were excluded from statistical demographics.

3. Key Contributions

Homogeneous Dataset: Provides the largest and most uniform sample of $v \sin i$ measurements for massive stars in the SMC to date (929 sources).
Methodological Refinement: Extends the FT approach (previously used by Bestenlehner et al. 2025 on a subset) to the full sample, addressing discrepancies between FT and Goodness-of-Fit (GOF) methods, particularly regarding binary smearing and macroturbulence.
SB2 Component Analysis: Offers the first preliminary $v \sin i$ estimates for individual components of SB2 systems in the SMC, despite the lack of full orbital solutions.
Demographic Breakdown: Separates the population into distinct evolutionary and multiplicity cohorts (Single, SB1, SB2, Supergiants, OBe) to analyze rotation distributions in the context of the Hertzsprung-Russell Diagram (HRD).

4. Key Results

A. General Distribution and Resolution Limits

The full sample exhibits a strong peak at low $v \sin i$ ( $30\text{--}60$ km s $^{-1}$ ), which is close to the instrumental resolution limit ( $\sim 30$ km s $^{-1}$ ). This indicates a significant population of stars with unresolved or very slow rotation.
Field Variations: Significant variations exist between the eight surveyed fields regarding the fraction of low $v \sin i$ stars, driven by local differences in stellar luminosity, age, and density (e.g., crowding in NGC 346).

B. Main Sequence Evolution

TAMS Identification: A distinct locus in the HRD was identified where $v \sin i$ $v sin i$ drops significantly. This corresponds to the Terminal-Age Main Sequence (TAMS).
- The drop occurs at $\log(T_{\text{eff}}) \approx 4.26$ (at $\log(L/L_\odot)=4.2$ ) to $\log(T_{\text{eff}}) \approx 4.43$ (at $\log(L/L_\odot)=5.5$ ).
- This feature is interpreted as the end of the core hydrogen-burning phase.
Main Sequence Cohorts:
- Single/SB1 Stars: Show a broad distribution with a strong peak at low $v \sin i$ ( $30\text{--}60$ km s $^{-1}$ ). A secondary, less prominent peak exists at intermediate values.
- SB2 Stars: Lack the low $v \sin i$ peak. Their distribution centers around a mean of $\sim 140$ km s $^{-1}$ . This is consistent with tidal synchronization in short-period binaries (periods mostly 1–5 days).
- High Rotators: A tail of high $v \sin i$ sources exists in Single and SB1 samples, likely spun up by mass transfer or undetected binaries.

C. Supergiants and OBe Stars

Supergiants (I/II): Generally exhibit low $v \sin i$ . Most BAF (B5–F) supergiants are unresolved at current resolution ( $v \sin i < 30$ km s $^{-1}$ ).
OBe Stars: Show a distribution similar to other studies, with a mean $v \sin i \approx 199$ km s $^{-1}$ and a standard deviation of 77 km s $^{-1}$ .

D. Nitrogen Enrichment

A preliminary analysis of the N II $\lambda 3995$ line in 43 slow-rotating ( $v \sin i \le 40$ km s $^{-1}$ ) main-sequence B-stars suggests that $\sim 30\%$ are nitrogen-rich (enhanced by $\ge 0.6$ dex).
This cohort likely represents a mix of:
1. Pristine single stars (slow initial rotation).
2. Long-period binaries (SB1s).
3. Merger products (which can spin down while mixing surface material).

5. Significance and Conclusions

Metallicity vs. Population: Comparisons with surveys in the Milky Way, LMC, and other SMC fields indicate that intrinsic stellar population differences (age distribution, mass function) drive variations in $v \sin i$ distributions more significantly than metallicity differences.
Binary Evolution: The data supports binary population synthesis models where:
- Slow rotators are largely single or long-period systems.
- Fast rotators (SB2s) are tidally locked short-period systems.
- The highest rotators are likely products of mass accretion or mergers.
Future Work: The identification of the TAMS locus via rotation provides a new diagnostic for stellar evolution. Future papers will utilize the full radial velocity data to refine orbital solutions, better constrain the nature of the slow-rotating nitrogen-rich cohort, and investigate potential runaway stars among the high-velocity outliers.

This study establishes a foundational dataset for understanding how massive stars rotate and evolve in low-metallicity environments, highlighting the critical role of binarity in shaping observed rotational velocities.