Galactic Chemical Evolution and the Hot Star, H II Region Connection

Details Galactic Chemical Evolution and the Hot Star, H II Region Connection Galactic Chemical Evolution and the Hot Star, H II Region Connection

Sep 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004sptz.prop.3412r&link_type=abstract

Spitzer Proposal ID #3412

Physics

Atomic Physics

Scientific paper

H II regions play a crucial role in the measurement of current interstellar abundances in our Galaxy and others while also serving as laboratories for atomic physics. They provide fundamental data about heavy element abundances that serve to constrain models of galactic chemical evolution.We propose to use Spitzer/IRS to measure the Ne/H and S/H abundances in H II regions in two very different, ~face-on spiral galaxies M83 and M33. By observing face-on galaxies, we will cover a full range of galactocentric radii (R_G) and avoid the extinction problems that plague Milky Way studies of H II regions, particularly in the inner Galaxy. An important advantage compared with prior optical studies is that the IR lines have a weak and similar electron temperature (Te) dependence while optical lines vary exponentially with Te. We plan to observe 5 emission lines: S IV 10.5, Ne II 12.8, Ne III 15.6, S III 18.7, & H7-6 12.4 micron cospatially with IRS/SH. By virtue of being able to measure lines from all the major ionic states of Ne and S in H II regions, together with an H line, in the SAME spectrum, there is a unique opportunity to obtain reliable Ne/H & S/H ratios and determine how they vary with R_G. With our prior efforts to measure the Galactic Ne/O ratio using far-IR lines, we found that the observed Ne++/O++ ratio significantly exceeds model predictions. This has been referred to as the "Ne III problem". The nebular models rely on stellar atmosphere models to provide the ionizing spectral energy distribution (SED) for hot stars. The SED used is the largest source of uncertainty when comparing theory and observations to determine nebular abundances in general (and galactochemical gradients). We will use novel diagnostic tools designed to investigate the Ne III problem via the proposed observations to validate the SED input to the nebular code. Whether or not we solve the Ne III problem, we will produce the most detailed and reliable study by far of abundance variations for S/H and Ne/H in these galaxies.

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