X-Shooting ULLYSES: Massive stars at low metallicity

Abstract

Context. The proximity and low dust extinction of the Magellanic Clouds provides an ideal environment for metal-poor massive stars to be studied in detail. The Hubble Space Telescope 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).Aims. We aim to determine the spectral types and physical properties of 122 LMC and 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.Methods. We have applied 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-Local Thermodynamic Equilibrium atmospheric code FASTWIND at LMC and SMC metallicities.Results. We have determined physical and wind properties of 97 LMC and 77 SMC massive stars, ranging from O2 to B9 subtypes, representing the majority of the XShootU OB sample (OeBe and candidate SB2 systems are excluded). Results are broadly in agreement with previous optical spectroscopic studies, with evolutionary masses spanning 12–117 M_{⊙ in the LMC and 11–74 M⊙ in the SMC. We have determined a revised Teff-spectral type calibration for Magellanic Cloud stars, identified stars with peculiar radial velocities, and compared 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 Z0.5 for luminous O stars. Studies incorporating ultraviolet mass-loss diagnostics are required for OB stars with weak winds and/or low luminosities.}}