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The binary landscape of massive stars at low $Z$: Insights from the BLOeM Campaign

J. I. Villaseñor, H. Sana, J. Bodensteiner, N. Britavskiy, L. R. Patrick, T. Shenar, the BLOeM Collaboration

TL;DR

At $Z=0.2 Z_{ m solar}$ BLOeM conducts a homogeneous, nine-epoch census of 929 OB–AF stars in the SMC to quantify intrinsic close-binary fractions. The study uses forward modelling of detection biases and an MCMC analysis of DeltaRV to derive orbital-property distributions, yielding high intrinsic multiplicities for O-type and early B stars, but lower values for B-type supergiants and A/F supergiants. OBe stars appear to be predominantly post-interaction binaries, while BAF supergiants show unexpectedly low multiplicities that challenge simple evolutionary scenarios. The results have far-reaching implications for massive-star evolution, transient production, and ionising feedback in the early Universe, and the continued data release with the full 25 epochs will further refine orbital distributions and environmental effects.

Abstract

We present an overview of our recent results from the BLOeM campaign in the Small Magellanic Cloud ($Z=0.2\,{\rm Z}_{\odot}$). Using nine-epoch VLT/FLAMES spectroscopy, we investigated the multiplicity of 929 massive stars. Our findings reveal contrasting binary properties across evolutionary stages: O-type stars show an intrinsic close-binary fraction of $70\%$, and early B-type dwarfs/giants reach ${\sim80}\%$, exceeding higher-metallicity samples. In contrast, B0-B3 supergiants drop to ${\sim40}\%$, and A-F supergiants to ${\sim8}\%$; intrinsic variability likely inflates the latter, so the true multiplicity may be lower. OBe stars display distinct binary properties consistent with a post-interaction origin. These results have profound implications for massive-star evolution at low metallicity, including the production of exotic transients, gravitational-wave progenitors, and ionising radiation in the early Universe.

The binary landscape of massive stars at low $Z$: Insights from the BLOeM Campaign

TL;DR

At BLOeM conducts a homogeneous, nine-epoch census of 929 OB–AF stars in the SMC to quantify intrinsic close-binary fractions. The study uses forward modelling of detection biases and an MCMC analysis of DeltaRV to derive orbital-property distributions, yielding high intrinsic multiplicities for O-type and early B stars, but lower values for B-type supergiants and A/F supergiants. OBe stars appear to be predominantly post-interaction binaries, while BAF supergiants show unexpectedly low multiplicities that challenge simple evolutionary scenarios. The results have far-reaching implications for massive-star evolution, transient production, and ionising feedback in the early Universe, and the continued data release with the full 25 epochs will further refine orbital distributions and environmental effects.

Abstract

We present an overview of our recent results from the BLOeM campaign in the Small Magellanic Cloud (). Using nine-epoch VLT/FLAMES spectroscopy, we investigated the multiplicity of 929 massive stars. Our findings reveal contrasting binary properties across evolutionary stages: O-type stars show an intrinsic close-binary fraction of , and early B-type dwarfs/giants reach , exceeding higher-metallicity samples. In contrast, B0-B3 supergiants drop to , and A-F supergiants to ; intrinsic variability likely inflates the latter, so the true multiplicity may be lower. OBe stars display distinct binary properties consistent with a post-interaction origin. These results have profound implications for massive-star evolution at low metallicity, including the production of exotic transients, gravitational-wave progenitors, and ionising radiation in the early Universe.

Paper Structure

This paper contains 9 sections, 5 figures.

Table of Contents

  1. Introduction
  2. The BLOeM campaign
  3. Binary fraction at SMC metallicity
  4. O-type stars
  5. B-type dwarfs and giants
  6. The OBe stars
  7. Early B supergiants and bright giants
  8. The BAF supergiants
  9. Conclusions

Figures (5)

  • Figure 1: Hertzsprung-Russell diagram of the BLOeM sample in the SMC, colour-coded by subsample (legend). From villasenor+25.
  • Figure 2: Intrinsic close-binary fraction versus metallicity for O and B stars in the MW, LMC, and SMC, including BSGs and the APOGEE solar-type trend for comparison. Adapted from villasenor+25 and sana+25.
  • Figure 3: Binary detection probability $p_{\rm detect}$ as a function of period and mass ratio for a 10$\,{\rm M}_\odot$ Be star. Overplotted are literature systems, see details in bodensteiner+25.
  • Figure 4: $\Delta$RV distribution for early BSGs by spectroscopic status. Y axis limited to 15; first two bins: 150 and 24. From britavskiy+25.
  • Figure 5: Orbital period versus mass ratio parameter space for simulated samples. Contours show detection probability ($p_{\rm detect}=99,90,50,10\%$). Dashed lines mark the minimum allowed period for three representative cases. From patrick+25.