2004ApJS..155..123A

We present adaptive optics (AO) assisted J, H, and K/K' images of eight ultracompact H II (UC H II) regions taken with the ALFA and ADONIS AO systems at Calar Alto and La Silla observatories. The images show details of the stellar population and the near-IR morphology of UC H II regions with unprecedented resolution. We have searched for the ionizing sources of the regions using near-IR photometry. The spectral type of the ionizing and most luminous stars inferred from our photometry has been compared with spectral type estimates from IRAS and published radio-continuum measurements. We find that the photometric near-IR spectral types are earlier than estimates from radio and IRAS data. This difference is alleviated when stellar spherical models including line blanketing and stellar winds instead of non-LTE plane-parallel models are used to derive the radio- and IRAS-based spectral types. We also made an attempt to correlate the properties of the near-IR ionizing population with MSX data and published CS measurements. No correlation was found. We note that in two of the regions (G309.92+0.48 and G61.48+0.09B1), the best candidate to ionize the region is possibly a supergiant.