The spectroscopic Hertzsprung-Russell diagram of hot massive stars in the SMC

TL;DR

This study analyzes 329 hot massive stars in the SMC using spectroscopic data, revealing discrepancies in stellar mass estimates and insights into stellar evolution at low metallicity.

Contribution

It provides the first estimate of the TAMS in the SMC and links the mass-discrepancy problem to nearly gravitationally unbound stellar envelopes.

Findings

Classical Be stars appear misplaced in the sHRD.

Mass estimates from the sHRD are systematically larger for luminous stars.

A large underabundance of stars above 25 M_sun near the ZAMS was found.

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 (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 40$\,$M$_{\odot}$ at SMC metallicity. We further find a large underabundance of stars above $\sim 25\,$M$_{\odot}$ near the ZAMS, reminiscent of such earlier findings in the Milky Way and LMC.