1996A&A...306..593A

We present near-infrared spectrophotometry of five southern ultracompact (UC) HII regions, namely: IRAS 08546-4254, IRAS 12073-6233, IRAS 15408-5356, IRAS 17200-3550 and IRAS 17455-2800. The spectra which cover parts of the range 1.25 to 4.1 µm display the emission lines of atomic and molecular hydrogen, those of He I, He II and [Fe II]. Near-infrared imaging results in the Brγ line have also been obtained for IRAS 12073-6233 and IRAS 17455-2800. The atomic hydrogen line ratios are found to be consistent with the recombination line theory using a normal interstellar extinction law. This result implies that shocks are not the dominant excitation mechanism for the hydrogen emission line in these UC HII regions. From the hydrogen recombination lines we derive for the present sample a mean visual extinction in the range from 5 to 20mag. However, the extinction is found to vary significantly over the spatial extent (typically in the range 1 to 4pc) of each of the five sources. The molecular hydrogen line intensities and ratios are consistent with excitation in the UV photon dominated region at the interface of the ionized and molecular gas. Comparison with published models shows that the H₂ emission originates in dense gas, i.e. at least 10⁴cm^–3. Except for one source, the derived helium to hydrogen abundance ratios for the present UC HII regions are consistent with the typical value of 0.112 found in galactic HII regions. In all five ultracompact HII regions, helium is found to be mainly singly ionized. When available, the HeI 2.058µm/Brγ ratio indicates spectral types for the ionizing star earlier than O9, in agreement with the luminosities derived from the IRAS far-infrared flux densities. In two sources, we derive a [FeII] 1.644µm/Brγ ratio of ∼0.07 similar to what is usually measured in other HII regions. This result implies again that shocks are not the dominant processes in these sources. The fact that a large fraction of Lyman continuum photons is absorbed by dust within the ionized region is strongly supported by our observations. In addition, the spatial distribution of the hot (>500K) dust within the ionized region of IRAS 17455-2800 is presented.