Other
Scientific paper
Jun 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005sptz.prop20057r&link_type=abstract
Spitzer Proposal ID #20057
Other
Scientific paper
H II regions play a crucial role in the measurement of current interstellar abundances 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 the nearby spiral galaxy M33. By observing a substantially face-on galaxy with a significant abundance gradient, we will cover a full range of galactocentric radii (R_G) & abundances, and avoid the extinction problems that plague optical/near IR Milky Way studies, 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. Our prior measurements of the Galactic Ne/O ratio using far-IR lines, showed 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. These observations will pave the way for similar Spitzer studies of other galaxies to yield the most detailed, reliable data by far of S/H and Ne/H abundance variations.
Colgan Sean
Dufour Reginald
Erickson Edwin
Haas Michael
Pauldrach Adi
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