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 observations of the 4.8 and 8.6 GHz continuum emission towards 11 southern H  regions made with the Australian Telescope Compact Array. The observed objects were selected from the Infrared Space Observatory (ISO) spectral catalogue of compact H  regions ). The morphologies observed for practically all the sources are consistent with them being ionized by a cluster of stars, rather than by a single star. The linear diameters of the regions range from 0.03 pc to 3 pc, the electron densities from 300 to 2.5 × 10 4 cm −3 , and the Lyman continuum photon flux from 10 47 to 10 50 s −1 . We confirm the existence of a relation between the density and size of H  regions which can be fit by a power law shallower than that predicted by the classic Strömgren theory. The radio observations provide, in addition, information about the distribution of the ionized gas within the ISO apertures. As a result of the combined radio and infrared study, estimates of the extinction in the infrared and the metal content of the nebular gas were calculated. In this analysis, we also included several (ultra)compact H  regions previously observed by the Very Large Array. Values for extinction in the K-band between ∼0 and 6 mag are found.The elemental abundances of nitrogen, neon, argon and sulphur were found to decrease with Galactocentric distance. Finally, the degree of ionization of the nebulae is confirmed to be correlated with the metal content.

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).

Abstract.Observations of infrared fine-structure lines provide direct information on the metallicity and ionization structure of H ii regions and indirectly on the hardness of the radiation field ionizing these nebulae. We have analyzed a sample of Galactic and Magellanic Cloud H ii regions observed by the Infrared Space Observatory (ISO) to examine the interplay between stellar content, metallicity and the ionization structure of H ii regions. The observed [S iv] 10.5/[S iii] 18.7 µm and [Ne iii] 15.5/[Ne ii] 12.8 µm line ratios are shown to be highly correlated over more than two orders of magnitude. We have compared the observed line ratios to the results of photoionization models using different stellar energy distributions. The derived characteristics of the ionizing star depend critically on the adopted stellar model as well as the (stellar) metallicity. We have compared the stellar effective temperatures derived from these model studies for a few well-studied H ii regions with published direct spectroscopic determinations of the spectral type of the ionizing stars. This comparison supports our interpretation that stellar and nebular metallicity influences the observed infrared ionic line ratios. We can explain the observed increase in degree of ionization, as traced by the [S iv]/[S iii] and [Ne iii]/[Ne ii] line ratios, by the hardening of the radiation field due to the decrease of metallicity. The implications of our results for the determination of the ages of starbursts in starburst galaxies are assessed.