ISO spectroscopy of (ultra) compact galactic HII regions

In order to study in detail the distribution of galactic element abundances, infrared spectra from 2.5 to 196 mu m were taken towards a sample of similar to 50 compact and ultra compact HII regions using the two spectrographs (LWS and SWS) on board ISO. The sample spans a large range in galactocentric distance (0 to 25 Irpc). We report preliminary results of this work. The SWS-LWS spectra display significant variations from source to source in both spectral shape and content. The fine structure lines of O, N, Ne; Ar, S, C and Si are well detected in most sources with some species (O: N, Ne, Ar) present in different stages of ionisation. The properties of the ionized gas such as the degree of ionization can thus be precisely constrained leading to accurate determinations of the element abundances. First results on the variation of the n(++)/O++ abundance ratio with galactocentric radius are presented. Many of the HII regions show significant dust emission features in the mid-infrared range. Large variations in the relative strengths of the dust bands are found, especially in the 7.7 mu m aid 8.6 mu m bands. These changes appear to be related to the degree of excitation of the sources. For the most obscured sources of the sample, the spectra also display strong absorption features due to ices such as the CO2 4.25 and 15 mu m bands.