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

Abstract

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 104 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 i23P-23S 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 Teff ≤ 42 000 K. Consideration of the helium ionization state places a lower limit on this value, so that we can also constrain Teff≥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.