Neon and Oxygen Abundances in M33

Abstract

We present new spectroscopic observations of 13 H II regions in theLocal Group spiral galaxy M33. The regions observed range from 1to 7 kpc in distance from the nucleus. Of the 13 H II regions observed,the [ O III] lambda 4363 line was detected in six regions. Electrontemperatures were thus able to be determined directly from the spectrausing the [ O III] lambda lambda 4959, 5007/lambda 4363 line ratio.Based on these temperature measurements, oxygen and neon abundancesand their radial gradients were calculated. For neon, a gradientof -0.016 +/- 0.017 dex kpc(-1) was computed, which agrees with theNe/H gradient derived previously from ISO spectra. A gradient of-0.012 +/- 0.011 dex kpc(-1) was computed for O/H, much shallowerthan was derived in previous studies. The newly calculated O/H andNe/H gradients are in much better agreement with each other, as expectedfrom predictions of stellar nucleosynthesis. We examine the correlationbetween the WC/WN ratio and metallicity, and find that the new M33abundances do not impact the observed correlation significantly.We also identify two new He II-emitting H II regions in M33, thefirst to be discovered in a spiral galaxy other than the Milky Way.In both cases the nebular He II emission is not associated with Wolf-Rayetstars. Therefore, caution is warranted in interpreting the relationshipbetween nebular He II emission and Wolf-Rayet stars when both areobserved in the integrated spectrum of an H II region.