Observations of M33 H II Regions: the Ne/S ratio, metallicity, and ionization variations

Details Observations of M33 H II Regions: the Ne/S ratio, metallicity, and ionization variations Observations of M33 H II Regions: the Ne/S ratio, metallicity, and ionization variations

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 /, /, and / vs. / 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.

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