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

TL;DR

This study investigates the multiplicity of massive stars in the Small Magellanic Cloud at low metallicity, revealing stage-dependent binary fractions and implications for stellar evolution and transient phenomena.

Contribution

It provides the first comprehensive analysis of massive star multiplicity at low metallicity using multi-epoch spectroscopy, highlighting evolutionary stage differences.

Findings

O-type stars have a 70% close-binary fraction.

Early B-type dwarfs/giants reach ~80% binary fraction.

B0-B3 supergiants and A-F supergiants show lower binary fractions.

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.