Extensive Observational Evidence for Massive Star Stellar Wind Variability at Low Metallicities: implications for mass-loss rate determination

TL;DR

This study provides observational evidence that stellar wind variability in massive stars persists at low metallicities, affecting mass-loss rate estimates crucial for understanding stellar and galactic evolution.

Contribution

It extends previous Galactic results to low metallicity environments, quantifying wind structure uncertainty and emphasizing the need for high-cadence UV observations.

Findings

Wind structure is prevalent at low metallicities.

Uncertainty in optical depth measurements is similar to that at Galactic metallicity.

Varying absorption components impact mass-loss models.

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 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% 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.