The Spectroscopic Hertzsprung–Russell Diagram of Hot Massive Stars in the Small Magellanic Cloud

Abstract

Abstract 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 T eff and surface gravities, we find that classical Be stars appear misplaced to low T eff 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 Hertzsprung–Russell diagram. 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 40 M ⊙ at SMC metallicity. We further find a large underabundance of stars above ∼25 M ⊙ near the zero-age main sequence, reminiscent of such earlier findings in the Milky Way and Large Magellanic Cloud.