IRAS low-resolution spectral observations of HII regions

Abundances of neon and sulfur are determined for 95 H II regions whose spectra were measured by the IRAS Low Resolution Spectrometer (LRS). Fifty-one H II regions with spectra in the IRAS LRS Atlas were used, plus an additional 44 H II regions whose spectra were taken from the IRAS LRS Data Base. Because of the large size of the sample, a number of correlations can be studied. The abundances show gradients with respect to Galactocentric radius R-G and number of ionizing photons s(-1), N-Lyc: log Ne/H = 3.636 - 0.086(+/- 0.013)R-G - 0.149(+/- 0.047) log N-Lyc, and log S/H = 1.795 - 0.051(+/- 0.013)R-G - 0.138(+/- 0.047) log N-Lyc. The Ne/H gradient is steeper than the S/H gradient: log S/H = - 1.87 + 0.82(+/- 0.09) log Ne/H, and the S/Ne ratio is a function of the Ne/H ratio S/Ne = - 0.305 - 0.095(+/- 0.017) log Ne/H. The neon ionization is high only for sources with both low neon abundance and high luminosity. Theoretical implications of the neon ionization level and the sulfur-to-neon abundance ratio are discussed. The total IRAS far-infrared luminosities and N-Lyc of the sources are compared to those predicted for the zero-age main sequence. The H II regions all lie close to the main-sequence line, showing that all or almost all the stellar flux must be absorbed by the IR emitting dust. Many of the H II regions are more luminous than even an O3 star; they are probably excited by several stars or even a cluster of stars. The ionization of neon (Ne++/Ne+), as compared to that of model H II regions, shows that the effective temperatures of the exciting stars cannot be more than 40,000 K. An alternative explanation is that O4 and O3 stars have Lyman continuum spectra cooler than the 45,000-50,000 K effective temperatures that are found for their visible and UV spectra.