Binarity at LOw Metallicity (BLOeM): Projected rotational velocities

TL;DR

The BLOeM survey studies rotational velocities of massive stars in the SMC, revealing distinct velocity distributions linked to stellar evolution stages, binary status, and potential merger history, with implications for understanding stellar rotation and binarity at low metallicity.

Contribution

This paper presents the first comprehensive analysis of projected rotational velocities for a large sample of massive stars in the SMC, linking rotation to binarity and evolutionary stage.

Findings

Main sequence stars show a broad vsini distribution with a peak at 30-60 km/s.

SB2 systems are mostly short period binaries with higher vsini around 140 km/s.

Supergiants generally have low vsini, indicating slow rotation or unresolved spectra.

Abstract

The Binarity at LOw Metallicity (BLOeM) survey is an ESO large programme designed to obtain multi-epoch spectroscopy for 929 massive stars in the Small Magellanic Cloud (SMC). It will provide binary fractions and orbital configurations of binary systems, and search for dormant black-hole binary candidates (OB+BH). Here we present projected rotational velocities (vsini) of all sources and, using the multiplicity properties presented in previous papers, we derive the vsini distributions of apparent single stars, single lined spectroscopic (SB1) binaries, and SB2 systems. We identify a locus in the Hertzsprung-Russell diagram where rotational velocities decrease significantly; we interpret this feature as broadly corresponding to the terminal-age main sequence. The main sequence cohort is distinguished by a broad range of vsini values, but with a strong peak in the distribution in the range 30-60 km/s, close to the resolution limit of 30 km/s. Sources in this low vsini peak are distributed throughout the main sequence, and are also present in the SB1 sample, though less prominent than in the single star distribution. A preliminary analysis of the lowest vsini cohort, that includes SB1 systems, implies that roughly one third may be nitrogen rich and we speculate that this cohort is a mix of pristine single stars, long period binaries, and merger products. The SB2 systems appear to be mostly short period binaries in synchronous rotation and have vsini estimates distributed around a mean value of approximately 140 km/s . Higher vsini sources are also present in the single and SB1 systems, all of which have tail to higher vsini values, consistent with tidal and mass-transfer effects. The supergiants, with a few exceptions, have low vsini, the bulk of these systems being essentially unresolved at current spectral resolution.