Binarity at LOw Metallicity (BLOeM): Pipeline-Determined Physical Properties of OB Stars

TL;DR

This study analyzes the physical properties of 778 OB stars in the Small Magellanic Cloud using a pipeline applied to spectroscopic data, revealing bimodal rotational velocities and confirming stellar mass cutoffs.

Contribution

It introduces a pipeline for analyzing large spectroscopic samples of OB stars and provides new classifications and detailed physical parameters for a significant number of stars.

Findings

Bimodality in v_e sin i of single O stars.

Distinct v_e sin i distributions for single and binary OB stars.

Confirmation of the lower mass cutoff at 8 Msun in the SMC.

Abstract

We aim to determine the physical properties of OB stars from the multi-epoch VLT/FLAMES BLOeM spectroscopic survey of the Small Magellanic Cloud. We apply a pipeline designed to analyse large spectroscopic samples of OB stars to the co-added, initial 9 epochs of the BLOeM survey, utilising grids of synthetic model spectra computed with the stellar atmosphere code FASTWIND. 69 OB stars are excluded from the analysis owing to disk emission or significant contamination by secondaries in SB2 binaries. We determine physical properties of 778 OB stars, including Teff, log g, log L/Lsun and v_e sin i. There appears to be a bimodality in v_e sin i of single O stars, while v_e sin i distributions of OB stars are strikingly different for single (median 78 km/s) and binary (median 200 km/s) systems. Inferred temperatures are broadly in agreement with literature results for stars in common, plus results from a grid-based automization tool for a subset of O and early B stars, although uncertainties are larger for surface gravities. Rotational velocities are broadly in line with an independent tool applied to the same subset. We recover the anticipated lower mass cutoff at 8 Msun from the survey design using a Bayesian inference method coupled with SMC metallicity evolutionary models, with median masses of 12.6 Msun (19.8 Msun) for B-type (O-type) stars. Spectroscopic masses exceed evolutionary masses, albeit with large uncertainties in surface gravities. We also provide an updated catalogue of O stars in the SMC since half of the 159 BLOeM O stars are newly classified as O-type stars.