A Panchromatic Study of Massive Stars in the Extremely Metal-poor Local Group Dwarf Galaxy Leo A

Creators: Gull, Maude; Weisz, Daniel R.; Senchyna, Peter; Sandford, Nathan R.; Choi, Yumi; McLeod, Anna F.; El-Badry, Kareem; Götberg, Ylva; Gilbert, Karoline M.; Boyer, Martha; Dalcanton, Julianne J.; GuhaThakurta, Puragra; Goldman, Steven; Marigo, Paola; McQuinn, Kristen B. W.; Pastorelli, Giada; Stark, Daniel P.; Skillman, Evan; Ting, Yuan-sen; Williams, Benjamin F.

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 (T_(eff)) 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.

Additional Information

© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. M.G. thanks Miriam Garcia, Chris Evans, Sally Oey, Nate Bastian, Morgan Fouseneau, Dietrich Baade, and the referee for helpful discussions, feedback, and/or comments. M.G. acknowledges support of the "Schweizerische Studienstiftung" and UC Berkeley Cranor Fellowship. Support for this work was provided by NASA through grants GO-15275, GO-15921, GO-16162, GO-16717, AR-15056, AR-16120, and HST-HF2-51457.001-A from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Part of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Further, this data was made accessible by the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration. Part of the data presented here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. This work is based on photometric observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute (STScI). STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. This work made extensive use of NASA's Astrophysics Data System Bibliographic Services.

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Resource type: Journal Article
Publisher: American Astronomical Society
Published in: Astrophysical Journal, 941(2), Art. No. 206, ISSN: 0004-637X.