ISO spectroscopy of compact HII regions in the Galaxy^*

II. Ionization and elemental abundances

¹ Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands
² SRON, National Institute for Space Reasearch, PO Box 800, 9700 AV Groningen, The Netherlands
³ Institut d'Astrophysique de Marseille, CNRS & Univ. de Provence, BP 8, 13376 Marseille Cedex 12, France
⁴ Institut d'Astrophysique Spatiale, Bât. 121, Université de Paris XI, 91405 Orsay, France
⁵ Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées, 14, Av. E. Belin, 31400 Toulouse, France
⁶ Department of Astronomy, 475 North Charter Street, University of Wisconsin, Madison, WI 53706, USA
⁷ ISO Data Centre, Astrophysics Division, ESA, Villafranca, Spain

Corresponding author: N. L. Martín-Hernández, leticia@astro.rug.nl

Received: 3 August 2001
Accepted: 18 October 2001

Abstract

Based on the ISO spectral catalogue of compact HII regions by [CITE], we present a first analysis of the hydrogen recombination and atomic fine-structure lines originated in the ionized gas. The sample consists of 34HII regions located at galactocentric distances between R_Gal = 0 and 15 kpc. The SWS recombination lines between 2 and 8  are used to estimate the extinction law at these wavelengths for 14 HII 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. A Galactocentric gradient of N/O () 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 and . 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.