| Abstract
| - 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 theSpitzer 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 (RG). 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 largerRG. 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 from linear least-squares fits that there is a decrease in metallicity with increasingRG: d log (Ne/H)/dRG=−0.058 ± 0.014 and d log (S/H)/dRG=−0.052 ± 0.021 dex kpc−1. There is no apparent variation in the Ne/S ratio withRG. 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 the Ne/S ratio 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. The same methodology is applied toSpitzer observations recently published for three massive H ii regions: NGC 3603 (Milky Way), 30 Dor (LMC) and N 66 (SMC) as well as for a group of blue compact dwarf galaxies. We find median Ne/S values of 14.6, 11.4, 10.1, and 14.0, respectively. All of 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 an 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 〈Ne++〉/〈S++〉, 〈Ne++〉/〈S3+〉, and 〈Ne++〉/〈Ne+〉 versus 〈S3+〉/〈S++〉 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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