First JWST/NIRSpec Spectroscopy of O Stars in the Small Magellanic Cloud

Abstract Determining how much mass is removed by stellar winds is crucial to understanding massive star evolution and feedback. However, traditional spectroscopic diagnostics in the UV and optical are not sensitive enough to characterize weak stellar winds of OB stars in low-metallicity environments. A new tool to access weak stellar winds is provided by spectroscopy in the infrared (IR). Stellar atmosphere models indicate that the hydrogen Br α line at λ 4.05 μ m is a useful mass-loss rate indicator, particularly at low metallicity. The unprecedented capabilities of the NIRSpec spectrograph on board the James Webb Space Telescope allow us to measure this line in spectra of massive stars in other galaxies. In this work, we present the first NIRSpec spectra of O-type stars in the Small Magellanic Cloud (SMC), which has a metallicity of only 20% solar. Our sample consists of 13 stars with spectral types ranging from O2 to O9.5, including supergiants, giants, and dwarfs. The stars belong to NGC 346, the most massive young cluster in the SMC. We describe the observing strategy and data reduction, highlighting the treatment of the nebular background emission. The spectra cover the 2.8–5.1 μ m wavelength range, and we detect the Br α line in emission in each of our sample stars. Using a combination of spectral and photometric data ranging from the UV to the IR, we improve the measurements of stellar luminosity and reddening. A first qualitative comparison of the observed Br α line with stellar atmosphere models shows its potential as a wind diagnostic for weak-winded stars.