A Panchromatic Study of Massive Stars in the Extremely Metal-poor Local Group Dwarf Galaxy Leo A
McQuinn, Kristen B. W.; Williams, Benjamin F.; Choi, Yumi; Gilbert, Karoline M.; Stark, Daniel P.; El-Badry, Kareem; Weisz, Daniel R.; Dalcanton, Julianne J.; Marigo, Paola; Skillman, Evan; McLeod, Anna F.; Senchyna, Peter; Goldman, Steven; Götberg, Ylva; Pastorelli, Giada; Sandford, Nathan R.; Boyer, Martha; Gull, Maude; Ting, Yuan-sen; GuhaThakurta, Puragra
United States, United Kingdom, Italy, Australia
Abstract
We characterize massive stars (M > 8 M ⊙) in the nearby (D ∼ 0.8 Mpc) extremely metal-poor (Z ∼ 5% Z ⊙) galaxy Leo A using Hubble Space Telescope ultraviolet (UV), optical, and near-infrared (NIR) imaging along with Keck/Low-Resolution Imaging Spectrograph and MMT/Binospec optical spectroscopy for 18 main-sequence OB stars. We find that: (a) 12 of our 18 stars show emission lines, despite not being associated with an H II region, suggestive of stellar activity (e.g., mass loss, accretion, binary star interaction), which is consistent with previous predictions of enhanced activity at low metallicity; (b) six are Be stars, which are the first to be spectroscopically studied at such low metallicity-these Be stars have unusual panchromatic SEDs; (c) for stars well fit by the TLUSTY nonlocal thermodynamic equilibrium models, the photometric and spectroscopic values of log(Teff) and log(g) agree to within ∼0.01 dex and ∼0.18 dex, respectively, indicating that near-UV/optical/NIR imaging can be used to reliably characterize massive (M ∼8-30 M ⊙) main-sequence star properties relative to optical spectroscopy; (d) the properties of the most-massive stars in H II regions are consistent with constraints from previous nebular emission line studies; and (e) 13 stars with M > 8M ⊙ are >40 pc from a known star cluster or H II region. Our sample comprises ∼50% of all known massive stars at Z ≲ 10% Z ⊙with derived stellar parameters, high-quality optical spectra, and panchromatic photometry. *This paper includes data gathered with the 10 m Keck Telescopes located at W. M. Keck Observatory, Hawaii and with the 6.5 m Multiple Mirror Telescope (MMT) located at Mt. Hopkins, Arizona.