The spectroscopic Hertzsprung-Russell diagram of hot massive stars in the SMC
N. Castrp, M. S. Oey, L. Fossati and N. Langer
 University of Michigan,  Space Research Institute, Austrian Academy of Sciences and  Argelander-Institut für Astronomie, Universität Bonn
We present a comprehensive stellar atmosphere analysis of 329 O- and B-type stars in the Small Magellanic Cloud (SMC) from the RIOTS4 survey. Using spectroscopically derived effective temperature (Teff) and surface gravities, we find that classical Be stars appear misplaced to low Teff and high luminosity in the spectroscopic Hertzsprung-Russell diagram (sHRD). Together with the most luminous stars in our sample, the stellar masses derived from the sHRD for these objects are systematically larger than those obtained from the conventional HRD. This suggests that the well-known, spectroscopic mass-discrepancy problem may be linked to the fact that both groups of stars have outer envelopes that are nearly gravitationally unbound. The non-emission-line stars in our sample mainly appear on the main-sequence, allowing a first estimate of the terminal-age main-sequence (TAMS) in the SMC, which matches the predicted TAMS between 12 and 40M⊙ at SMC metallicity. We further find a large underabundance of stars above ∼25M⊙ near the ZAMS, reminiscent of such earlier findings in the Milky Way and LMC.
Reference: arXiv 1810.04682
Status: Manuscript has been accepted
Comments: 22 pages, 12 figures, 1 table; accepted for publication in ApJ