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

We present complete, low-resolution IJHK spectroscopy of the ultracompact H II region, G45.12 + 0.13. From the observed H I line strengths, we derive a near-infrared extinction law that is slightly steeper than the average. After correction with this extinction law, we find good agreement between the observed line ratios of He I, Fe+, Fe2+, S+ and S++ and the available atomic data. Our data show that the density within the core of G45.12 + 0.13 must be at least 10(4) cm(-3). This is consistent with the known radio structure of the H II region and in considerable disagreement with previous work using mid- and far-infrared lines. There must also be considerable opacity in the He I 2(3)P-2(3)S transition, and we show how the observed strengths of the other He I lines are consistent with this. From modelling the photoionization structure, we find good agreement with most of the observed data if the hottest star present has T-eff less than or equal to 42 000 K. Consideration of the helium ionization state places a lower limit on this value, so that we can also constrain T-eff greater than or equal to 38 000 K. Discrepancies still exist between some of the observed and model line ratios, but the most obvious tend to be the mid-infrared observations.