Computer Science
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009eimw.confe..55r&link_type=abstract
"The Evolving ISM in the Milky Way and Nearby Galaxies, The Fourth Spitzer Science Center Conference, Proceedings of the confere
Computer Science
Ism: Abundances, H Ii Regions, Stars: Atmospheres, Galaxies: Individual (M33)
Scientific paper
We have observed emission lines of [S IV] 10.51, H(7--6) 12.37, [Ne II] 12.81, [Ne III] 15.56, and [S III] 18.71~μm in a number of extragalactic H II regions with the Spitzer Space Telescope. A previous paper presented our data and analysis for the substantially face-on spiral galaxy M83. Here we report our results for the local group spiral galaxy M33. The nebulae selected cover a wide range of galactocentric radii (R_G). The observations were made with the Infrared Spectrograph with the short wavelength, high resolution module. The above set of five lines is observed cospatially, thus permitting a reliable comparison of the fluxes. From the measured fluxes, we determine the ionic abundance ratios including Ne++/Ne^+, S3+/S++, and S++/Ne^+ and find that there is a correlation of increasingly higher ionization with larger R_G. By sampling the dominant ionization states of Ne (Ne^+, Ne++) and S (S++, S3+) for H II regions, we can estimate the Ne/H, S/H, and Ne/S ratios. We find that there is a decrease in metallicity with increasing R_G. There is no apparent variation in the Ne/S ratio with R_G. Unlike our previous similar study of M83, where we conjectured that this ratio was an upper limit, for M33 the derived ratios are likely a robust indication of Ne/S. This occurs because the H II regions have lower metallicity and higher ionization than those in M83. Both Ne and S are primary elements produced in α-chain reactions, following C and O burning in stars, making their yields depend very little on the stellar metallicity. Thus, it is expected that Ne/S remains relatively constant throughout a galaxy. The median (average) Ne/S ratio derived for H II regions in M33 is 16.3 (16.9), just slightly higher than the Orion Nebula value of 14.3. These values are in sharp contrast with the much lower ``canonical", but controversial, solar value of ˜5. A recent nucleosynthesis, galactic chemical evolution model predicts a Ne/S abundance of ˜9. Our observations may also be used to test the predicted ionizing spectral energy distribution of various stellar atmosphere models. We compare the ratio of fractional ionizations , with predictions made from our photoionization models using several of the state-of-the-art stellar atmosphere model grids. The trends of the ionic ratios established from the prior M83 study are remarkably similar, but continued to higher ionization with the present M33 objects.
Browne Dana A.
Brunner Gregory
Colgan Sean J. W.
Csongradi E. J.
Dufour Reginald James
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