Abstract. 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 µm are used to estimate the extinction law at these wavelengths for 14 H ii regions. An extinction in the K band between 0 and ∼3 mag has been derived. The fine-structure lines of N, O, Ne, S and Ar 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.

Abstract. Infrared spectra between 2.3 and 196 µm were taken towards a sample of 45 compact H ii regions using the two spectrometers (SWS and LWS) on board ISO. The primary goal is to determine the distribution of element abundances in the Galaxy, although there are also many other uses of this database. The spectra contain a wealth of information on the ionized gas and the associated photodissociation regions through the atomic fine-structure lines and on the dust properties via the dust emission bands and the continuum. Significant variations are found from source to source in both spectral shape and content. The sample of H ii regions spans a wide range in galactocentric distance (from 0 to 22 kpc) enabling to investigate the variations of the nebular properties across the Galactic plane. The observations and the data reduction are described in detail in the present paper. The ISO spectral catalogue of compact H ii regions contains the combined SWS-LWS spectra for each of the sources, the fluxes of the atomic fine-structure lines and hydrogen recombination lines, and an inventory of the spectra in terms of molecular lines, dust and ice bands.

Abstract. We present a detailed photoionization model of G29.96-0.02 (hereafter G29.96), one of the brightest Galactic Ultra Compact H ii (UCHII) regions in the Galaxy. This source has been observed extensively at radio and infrared wavelengths. The most recent data include a complete ISO (SWS and LWS) spectrum, which displays a remarkable richness in atomic fine-structure lines. The number of observables is twice as great as the number available in previous studies. In addition, most atomic species are now observed in two ionization stages. The radio and infrared data on G29.96 are best reproduced using a nebular model with two density components: a diffuse (ne ∼ 680 cm −3 ) extended (∼1 pc) component surrounding a compact (∼0.1 pc) dense (ne ∼ 57 000 cm −3 ) core. The properties of the ionizing star were derived using state-of-the-art stellar atmosphere models. CoStar models yield an effective temperature of ∼30 +2 −1 kK whereas more recent non-LTE line blanketed atmospheres with stellar winds indicate somewhat higher values, T eff ∼32-38 kK. This range in T eff is compatible with all observational constraints, including near-infrared photometry and bolometric luminosity. The range 33-36 kK is also compatible with the spectral type O5-O8 determined by Watson & Hanson (1997) when recent downward revisions of the effective temperature scale of O stars are taken into account. The age of the ionizing star of G29.96 is found to be a few 10 6 yr, much older than the expected lifetime of UCHII regions. Accurate gas phase abundances are derived with the most robust results being Ne/S = 7.5 and N/O = 0.43 (1.3 and 3.5 times the solar values, respectively).