Massive Star Population in the Sextans A Dwarf Galaxy from HST UV Photometry

Abstract We build a catalog of massive (M>8 M⊙) main sequence stars in the metal-poor (∼0.1 Z⊙) dwarf irregular galaxy Sextans A. HST WFC3 UV photometry in the 275 and 336 nm wideband filters is arranged in a Color-Magnitude Diagram (CMD), and overlaid on top of stellar evolutionary tracks from the MIST library. The star properties (mass, age, etc.) are computed with a Finite Element (FE) interpolation of the stellar tracks. The FE method, originally developed to solve structural analysis problems, provides a general framework for interpolating fields inside domains of complex geometry. Besides the interpolated properties, the algorithm computes their gradients with respect to the photometry. These sensitivities yield a direct an efficient estimate of the associated uncertainties. Our catalog contains 655 stars, with the most massive one estimated at 58 ± 11 M⊙. A comparison with a ground-based spectroscopic census of OB stars yields only 8 matches, evidencing the minimal overlap between both datasets. The mass estimates derived from the UV CMD and the spectral classification are in good agreement for the majority of O-type stars found in both datasets. Our catalog provides an extensive list of candidates for followup spectroscopic observation, to improve our understanding of the early evolutionary stages of massive low-metallicity stars.