X-Shooting ULLYSES: Massive Stars at Low Metallicity: XI. Pipeline-determined Physical Properties of Magellanic Cloud OB Stars

TL;DR

This study analyzes ultraviolet and optical spectra of massive OB stars in the Magellanic Clouds to determine their physical properties, revise spectral calibrations, and compare wind characteristics at different metallicities, enhancing templates for metal-poor stars.

Contribution

It provides a pipeline-based analysis of a large spectroscopic sample of Magellanic Cloud OB stars, deriving their physical parameters and wind properties at low metallicity, and revises spectral type calibrations.

Findings

Physical parameters for 174 OB stars were determined.

Revised Teff-spectral type calibration for Magellanic Cloud stars.

LMC stars exhibit ~0.27 dex higher wind momenta than SMC stars.

Abstract

The proximity & low dust extinction of the Magellanic Clouds provides an ideal environment for metal-poor massive stars to be studied in detail. The HST ULLYSES initiative has provided exquisite ultraviolet spectroscopy of a large sample of OB stars in the Magellanic Clouds, and its legacy value has been enhanced through the acquisition of optical VLT/XShooter spectroscopy (XShootU). We aim to determine the spectral types and physical properties of 122 LMC & 103 SMC OB stars observed via XShootU. Physical parameters are required for these to serve as templates in spectral libraries of metal poor massive stars. We also aim to identify double-lined binaries and OeBe stars for which analysis requires non-standard treatment. We apply a pipeline designed to analyse large spectroscopic samples of hot luminous stars to XShootU spectroscopic datasets, together with grids of synthetic model spectra computed with the non-LTE atmospheric code fastwind at LMC & SMC metallicities. We determine physical and wind properties of 97 LMC & 77 SMC massive stars, ranging from O2 to B9 subtypes, representing the majority of the XShootU OB sample (OeBe & candidate SB2 systems are excluded). Results are broadly in agreement with previous optical spectroscopic studies, with evolutionary masses spanning 12-117 Msun in the LMC & 11-74 Msun in the SMC. We determine a revised Teff-spectral type calibration for Magellanic Cloud stars, identify stars with peculiar radial velocities, and compare wind properties of high luminosity O stars with dense winds, revealing ~0.27 dex higher wind momenta of LMC stars with respect to SMC counterparts. Incorporating the recent empirical metallicity dependence of Z^0.22 for wind velocities, this suggests a mass-loss dependence of Z^0.5 for luminous O stars. Studies incorporating ultraviolet mass-loss diagnostics are required for OB stars with weak winds and/or low luminosities.