Stellar Winds of O-type Stars Traced by High- ionization Fine-structure Emission Lines with JWST/MIRI

We investigate the presence of high-ionization fine-structure emission lines across a range of 22 OB-type stars observed with JWST-MIRI as part of calibration programs and the Webb Investigation of Silicates, Carbons, and Ices and the Measuring Extinction and Abundances of Dust programs. Mid-infrared (MIR) wind emission is detected in four late O dwarfs (O8 V–O9 V), one early O dwarf (O5 V), and there are tentative detections in an additional three stars (O8.5 II, O8.5 IV, and O9 I). We measure the wind speeds and make estimates of lower limits on the mass-loss rates of five O-type stars from broad, flat-topped emission in the fine-structure line of [Ne v] 14.3 μm. We find terminal wind speeds that are generally in agreement with empirical trends, but note that in some cases the wind speeds are surprisingly low. We highlight two main takeaways from this sample, which combine to establish an exciting new window into the winds of massive stars. First, a new diagnostic ability is gained from lines formed at much higher ionization, larger spatial extent, and longer wavelengths than typical wind diagnostics. Second, there is a frequent incidence of MIR emission in O-type stars, even in the “weak-wind” regime where wind emission is often not detected in the UV and optical.