Near-infrared spectroscopy of the ultracompact H II region1 G45.12 + 0.13

We present complete, low resolution IJHK spectroscopy of the ultracompact HIIregion, G45.12+0.13. From the observed HI line strengths, we derive a nearinfrared extinction law that is slightly steeper than the average. Aftercorrection with this extinction law, we find good agreement between theobserved line ratios of HeI, Fe+, Fe++, S+ and S++ and the available atomicdata. Our data show that the density within the core of G45.12+0.13 must be atleast 10^4/cm^3. This is consistent with the known radio structure of the HIIregion and in considerable disagreement with previous work using mid and farinfrared lines. There must also be considerable opacity in the HeI 2 3P-2 3Stransition, and we show how the observed strengths of the other HeI lines areconsistent with this. From modelling the photoionisation structure, we findgood agreement with most of the observed data if the hottest star present hasTeff < 42000K. Consideration of the helium ionisation state places a lowerlimit on this value so that we can also constrain Teff > 38000K. Discrepanciesstill exist between some of the observed and model line ratios, but the mostobvious tend to be the mid-IR observations.Comment: 17 pages Latex source, 5 postscript figures and macros. gzipped tar file. TO appear in Monthly Notices of the Royal Astronimcal Society. Also available by anonymous ftp from ftp://aaoepp.aao.gov.au/local/sll/g45.uu (uuencoded gzipped tar file