IRAS low-resolution spectral observations of H II regions
Abundances of neon and sulfur for 95 H II regions whose spectra were measured by the IRAS Low Resolution Spectrometer (LRS) are determined; and theoretical implications of the neon ionization level and the sulfur-to-neon abundance ratio are discussed. The abundances show a distinct gradient of Ne/H with respect to Galactocentric radius RG, and a smaller gradient for S/H. The neon ionization is high only for sources with both low neon abundance and high luminosity. It is concluded that, due to the fact that the luminosities observed are so high compared to those predicted for the zero-age main sequence star, the most luminous H II regions are probably excited by several stars or a cluster of stars. The observed ionization of neon shows that the effective temperatures of the exciting stars cannot be more than 40,000 K, with an alternative explanation that 04 and 03 stars have Lyman continuum spectra cooler than the 45,000 - 50,000 K effective temperatures that are found for their visible and UV spectra.