THE NEAR-INFRARED SPECTRUM OF ULTRACOMPACT HII-REGIONS

We present near-infrared spectrophotometry of five southern ultracompa ct (UC) HII regions, namely: IRAS 08546-4254, IRAS 12073-6233, IRAS 15 408-5356, IRAS 17200-3550 and IRAS 17455-2800. The spectra which cover parts of the range 1.25 to 4.1 mu m display the emission lines of ato mic and molecular hydrogen, those of He I, He II and [Fe II]. Near-inf rared imaging results in the Br gamma 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 ar e not the dominant excitation mechanism for the hydrogen emission line in these UC HII regions. From the hydrogen recombination lines we der ive for the present sample a mean visual extinction in the range from 5 to 20 mag. However, the extinction is found to vary significantly ov er the spatial extent (typically in the range 1 to 4 pc) of each of th e five sources. The molecular hydrogen line intensities and ratios are consistent with excitation in the UV photon dominated region at the i nterface of the ionized and molecular gas. Comparison with published m odels shows that the H-2 emission originates in dense gas, i.e. at lea st 10(4) cm(-)3. Except for one source, the derived helium to hydrogen abundance ratios for the present UC HII regions are consistent with t he typical value of 0.112 found in galactic HII regions. In all five u ltracompact HII regions, helium is found to be mainly singly ionized. When available, the HeI 2.058 mu m/Br gamma ratio indicates spectral t ypes for the ionizing star earlier than O9, in agreement with the lumi nosities derived from the IRAS far-infrared flux densities. In two sou rces, we derive a [FeII] 1.644 mu m/Br gamma ratio of similar to 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 sour ces. The fact that a large fraction of Lyman continuum photons is abso rbed by dust within the ionized region is strongly supported by our ob servations. In addition, the spatial distribution of the hot (> 500 K) dust within the ionized region of IRAS 17455-2800 is presented.