# Mid-infrared observations of O-type stars: spectral morphology

## TL;DR

This study presents the first detailed mid-infrared spectral analysis of 16 O-type stars, revealing diverse emission line features and wind formation regions, with implications for classification and future JWST observations.

## Contribution

It provides a comprehensive mid-IR spectral morphology analysis of O-type stars using Spitzer data and NLTE models, highlighting spectral differences and challenges in classification.

## Key findings

- O supergiants show rich emission line spectra with hydrogen and helium features.
- Late O supergiants mainly exhibit hydrogen and HeI lines, early types show hydrogen and HeII.
- O dwarfs have featureless mid-IR spectra, indicating different wind properties.

## Abstract

We present mid-infrared observations for a sample of 16 O-type stars. The data were acquired with the NASA Spitzer Space Telescope, using the IRS instrument at moderate resolution (R $\sim$ 600), covering the range $\sim 10-37$ microns. 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, analyze profile contributions and line-formation regions. The O supergiants present a rich emission line spectra. The most intense features are from hydrogen - $6\alpha$, 7$\alpha$, and $8\alpha$ - which have non-negligible contributions of HeI or HeII lines, depending on the spectral type. The spectrum of early O supergiants is a composite of HI and HeII lines, HeI lines being absent. On the other hand, late O supergiants present features composed mainly by HI and HeI 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.