2017MNRAS.470.2710M

We present mid-infrared (mid-IR) observations for a sample of 16 O-type stars. The data were acquired with the NASA Spitzer Space Telescope, using the IRSinstrument at moderate resolution (R ∼ 600), covering the range of ∼10-37 µm. Our sample includes early, mid and late O supergiants and dwarfs. We explore for the first time their mid-IR spectral morphology in a quantitative way. We use NLTE expanding atmosphere models to help with line identifications, analyseprofile contributions and line-formation regions. The O supergiants present a rich emission line spectra. The most intense features are from hydrogen - 6 α, 7 α and 8 α - that have non-negligible contributions of He I or He II lines, depending on the spectral type. The spectrum of early O supergiants is a composite of H I and He II lines, He Ilines being absent. On the other hand, late O supergiants present features composed mainly by H I and He I lines. All emission lines are formed throughout the stellar wind. We found that O dwarfs exhibit a featureless mid-IR spectrum. Two stars of our sample exhibit very similar mid-IR features, despite having a very different optical spectral classification. The analysis of O-type stars based on mid-IR spectra alone to infer spectral classes or to estimate physical parameters may thus be prone to substantial errors. Our results may therefore inform spectroscopic observations of massive stars located in heavily obscured regions and help establish an initial framework for observations of massive stars using the Mid-Infrared Instrument on the James Webb Space Telescope.