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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 um in a number of extragalactic H II regionswith the Spitzer Space Telescope. A previous paper presented our data andanalysis for the substantially face-on spiral galaxy M83. Here we report ourresults for the local group spiral galaxy M33. The nebulae selected cover awide range of galactocentric radii (R_G). The observations were made with theInfrared Spectrograph with the short wavelength, high resolution module. Theabove set of five lines is observed cospatially, thus permitting a reliablecomparison of the fluxes. From the measured fluxes, we determine the ionicabundance ratios including Ne++/Ne+, S3+/S++, and S++/Ne+ and find that thereis a correlation of increasingly higher ionization with larger R_G. By samplingthe dominant ionization states of Ne (Ne+, Ne++) and S (S++, S3+) for H IIregions, we can estimate the Ne/H, S/H, and Ne/S ratios. We find from linearleast-squares fits that there is a decrease in metallicity with increasing R_G:d log (Ne/H)/dR_G = -0.058+-0.014 and d log (S/H)/dR_G = -0.052+-0.021 dexkpc-1. There is no apparent variation in the Ne/S ratio with R_G. Unlike ourprevious similar study of M83, where we conjectured that this ratio was anupper 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 higherionization than those in M83. Both Ne and S are primary elements produced inalpha-chain reactions, following C and O burning in stars, making their yieldsdepend very little on the stellar metallicity. Thus, it is expected that Ne/Sremains relatively constant throughout a galaxy. The median (average) Ne/Sratio derived for H II regions in M33 is 16.3 (16.9), just slightly higher than

Spitzer Observations of M33 and the Hot Star, H II Region Connection