ISO spectroscopy of compact HII regions in the Galaxy - II. Ionization and elemental abundances

Based on the ISO spectral catalogue of compact H II regions by Peeters et al. (2002), we present a first analysis of the hydrogen recombination and atomic fine-structure lines originated in the ionized gas. The sample consists of 34 H II regions located at galactocentric distances between R-Gal = 0 and 15 kpc. The SWS H I recombination lines between 2 and 8 pin are used to estimate the extinction law at these wavelengths for 14 H II regions. An extinction in the K band between 0 and similar to3 mag has been derived. The fine-structure lines of N, O, Ne, S and At are detected in most of the sources. Most of these elements are observed in two different ionization stages probing a range in ionization potential up to 41 eV. The ISO data, by itself or combined with radio data taken from the literature, is used to derive the elemental abundances relative to hydrogen. The present data thus allow us to describe for each source its elemental abundance. its state of ionization and to constrain the properties of the ionizing star(s). The main results of this study are as follows. The ionization ratios Ar++/Ar+. N++/N+, S+3/S++ and Ne++/Ne+, which measure the degree of ionization and to first order, the hardness of the stellar radiation. seem to increase with R-Gal. These ionization ratios correlate well with each other, implying that the spectral hardening affects equally the full range of ionizing energies. A Galactocentric gradient of N/O (DeltalogN/O = -0.056+/-0.009dex kpc(-1)) is observed in the sense of a decreasing abundance ratio with R-Gal in agreement with previous studies. Abundance gradients for neon and argon are derived of the form Deltalog Ne/H = -0.039 +/- 0.007dex kpc(-1) and Deltalog Ar/H = -0.045 +/- 0.011 dex kpc(-1). These elemental gradients could be enlarged by the existing Galactic T-e gradient. Adopting a T-e gradient of approximately 330 K kpc(-1), the slopes in the Ne/H and Ar/H gradients become -0.06 and -0.07 dex kpc(-1), respectively. Lower limits for the sulphur and oxygen abundances are derived. Nitrogen abundances are derived for 16 sources.