Spitzer Space Telescope Observations of Sulfur and Neon Abundances in Extragalactic H II Regions

Presented by Reginald Dufour, Rice University

I will discuss the results of recent observations of the infrared spectra of H II regions in the spiral galaxies M33 & M83 and the irregular galaxy NGC 6822 made with the IRS on the Spitzer Space Telescope. These observations were the first measurements of the infrared emission lines of Ne+ and S+3 in the HII regions - enabling improved determinations of the abundances of the primary nucleosynthesis elements neon and sulphur in the ISM of three quite different systems. We analyzed our results, which also include measurements of Ne+2 and S+2, using photoionization models of H II regions with a variety of input stellar spectral energy distributions (SEDs) and discovered that the supergiant atmospheres of Pauldrach et al. track the ionization variations best. The resulting averaged Ne/S ratios determined for the ISM of the three galaxies were higher than the (controversial) solar value, as well as values predicted by stellar nucleosynthesis and galactic chemical evolution models. I then compare our results to other studies of Ne and S in Galactic and extragalactic H II regions and conclude with a discussion of potential future advancements in H II region abundance determination problems afforded by the upcoming infrared astronomy missions SOFIA and JWST.