16920

Fast-approaching JWST observations of high-redshift galaxies seek to determine the contribution of low-metallicity massive stars to reionization. Yet comma the predicted ionizing fluxes of metal-poor O stars are sensitive to uncertain comma theoretical mass-loss rates and their downstream effects on stellar rotation and evolution. New observational constraints are required to make progress comma but mass-loss measurements from FUV spectra of O stars in nearby galaxies below the metallicity of the SMC (20% Z_sun) remain extremely rare comma particularly for the hottest comma earliest spectral types that dominate the ionizing photon budget. We propose G130M and G160M COS observations of a bright comma early-O star in Sextans A (6% Z_sun) that was identified in NUV imaging obtained in January 2022. This is just the third known extremely metal-poor comma early-O star in any galaxy close enough for detailed study the other two will both be observed at low spectral resolution by ULLYSES. From these FUV spectra comma we willdoublePoint (1) measure the star.s mass-loss rate to test extrapolation of widely used mass-loss recipes to low metallicity (2) use weak metal line detections to determine which metal ions drive winds in the hottest metal-poor O stars and (3) provide crucial constraints not possible at lower resolution to maximize the value of the ULLYSES spectra in Sextans A. Combined with archival and ULLYSES data comma the proposed observations will form the first empirical benchmark for stellar models that predict the ionizing spectra of reionization-era galaxies. The proposed observations are urgently needed to interpret JWST observations of high-redshift galaxies coming this year and can only be done with HST-COS.