Galactic Chemical Evolution & the Hot Star - H II Region Connection in NGC6822

Details Galactic Chemical Evolution & the Hot Star - H II Region Connection in NGC6822 Galactic Chemical Evolution & the Hot Star - H II Region Connection in NGC6822

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007sptz.prop40910r&link_type=abstract

Spitzer Proposal ID #40910

Computer Science

Scientific paper

H II regions provide fundamental data about heavy element abundances that constrain galactic chemical evolution (GCE) models. We propose to use Spitzer/IRS to follow up on our Spitzer observations of M83 and M33 H II regions. We will measure 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 measuring all the major ionic states of Ne and S in H II regions with an H line, there is a unique opportunity to estimate S3+/S++, Ne++/Ne+, Ne/H, S/H, & Ne/S ratios and test if they vary with galactocentric radius (R_G). The high Ne/S ratios we derived for M83 are likely upper limits due to our estimate of the S abundance not accounting for S+ or dust. The M33 H II regions have much lower metallicity and higher ionization, leading to a truer total S abundance & a Ne/S close to the Orion Nebula value 14. The H II regions in the nearby, dwarf irregular galaxy NGC6822 are both low metallicity (O/H ~3.5x lower than Solar) and high ionization, allowing a reliable derivation of gas-phase Ne/S. The solar Ne abundance is very controversial, with much evidence pointing to a higher Ne value than the current Ne/S~5. NGC6822 observations will test how robust and universal the ratio of ~14 is. Such a finding will place important constraints upon GCE models. Our derivation of ionic abundances from Spitzer data depends on nebular models, which rely on the spectral energy distribution (SED) which comes from stellar atmosphere models. From our M83 and M33 data, we derive / & / vs. / ratios for the various H II regions and compare with theoretical loci, which show a factor >10 spread in y at a given x. The data points best follow the trend of the loci using the supergiant SEDs of Pauldrach, who will compute a set of atmosphere models with metallicity similar to that of NGC6822. Data for a third galaxy with a very different history and gas content will further validate whether or not any SED set preferentially fits the nebular observations.

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