SULFUR AND NEON IN EXTRAGALACTIC H II REGIONS

we will use iso to observe ir emission lines of s iii, s iv, ne ii, ne iii, and brackettalpha in h ii regions in two spiral galaxies and three irregular galaxies. the data will be used to determine the variation of the abundances of sulfur and neon over the metallicity range 0.1 to 2.0 times solar, and to compare the variation of these elements with that of oxygen. because of the weak temperature dependence of the ir lines, these measurements provide critical measurements of abundances in metalrich h ii regions, where electron temperatures are poorly known. comparison of the ir lines of s iii and ne iii with their optical counterparts will provide a measure of electron temperatures in the h ii regions and of the amplitude of temperature inhomogeneities in the nebulae, with important consequences for elemental abundances inferred from h ii region spectra. there is considerable debate whether sulfur abundance gradients in spiral galaxies are shallower than oxygen abundance gradients. if sulfur gradients are indeed shallower than for oxygen, it would imply either that the massive star imf varies with metallicity or that the yield of s from massive stars varies with metallicity. we propose to measure the s iii 18.7 and 33.6 micron lines, plus the s iv 10.5 micron line, to determine sulfur abundances in h ii regions, providing an independent test of the trend for sulfur. neon is generally considered to vary in lockstep with oxygen. however, a recent study of h ii galaxies found ne++/o++ increasing with metal licity. this result was attributed to an incorrect ionization correction for ne++, but could be suspect because of possible bias in the analysis. we propose to measure ne ii 12.8 microns and ne iii 15.6 microns in our h ii region sample to determine independently neon abundances and to test the validity of commonly used ionization correction schemes for neon. truncated!, Please see actual [truncated!, Please see actual data for full text]