Extensive observational evidence for massive star stellar wind variability at low metallicities: implications for mass-loss rate determination

ABSTRACT Mass-loss from massive stars is fundamental to stellar and galactic evolution and enrichment of the interstellar medium. Reliable determination of mass-loss rate is dependent upon unravelling details of massive star outflows, including optical depth structure of the stellar wind. That parameter introduces significant uncertainty due to the nearly ubiquitous presence of large-scale optically thick wind structure. We utilize suitable available ultraviolet spectra of 20 Large and Small Magellanic Cloud (LMC, SMC) OB stars to extend existing Galactic results quantifying uncertainty inherent in individual observations to lower metallicity environments. This is achieved by measuring standard deviations of mean optical depths of multiple observations of suitable wind-formed absorption profiles as a proportion of their mean optical depths. We confirm earlier findings that wind structure is prevalent at low metallicities and demonstrate that quantifying the consequent uncertainty is to some extent possible, despite the near-complete absence of time series ultraviolet (UV) spectroscopic observations in those environments. We find that the uncertainty inherent in any single observation of stellar wind optical depth at low metallicity is of similar magnitude to that already identified at Galactic metallicity (up to 45 per cent for cooler OB stars). We further demonstrate how the effect of varying narrow absorption components in wind-formed UV spectral profiles is unlikely to be properly accounted for in existing mass-loss models. We present further evidence of a binary companion to the SMC O-type giant star AzV 75. The importance of obtaining high-cadence multi-epoch, or genuine time series, UV spectroscopic observations at low metallicities is highlighted.