Near‐Infrared Subarcsecond Observations of Ultracompact H ii Regions

Abstract

We present adaptive optics (AO) assisted J, H, and K/K^{'}images of eight ultracompact H II (UC H II) regions taken with the ALFAand ADONIS AO systems at Calar Alto and La Silla observatories. Theimages show details of the stellar population and the near-IR morphologyof UC H II regions with unprecedented resolution. We have searched forthe ionizing sources of the regions using near-IR photometry. Thespectral type of the ionizing and most luminous stars inferred from ourphotometry has been compared with spectral type estimates from IRAS andpublished radio-continuum measurements. We find that the photometricnear-IR spectral types are earlier than estimates from radio and IRASdata. This difference is alleviated when stellar spherical modelsincluding line blanketing and stellar winds instead of non-LTEplane-parallel models are used to derive the radio- and IRAS-basedspectral types. We also made an attempt to correlate the properties ofthe near-IR ionizing population with MSX data and published CSmeasurements. No correlation was found. We note that in two of theregions (G309.92+0.48 and G61.48+0.09B1), the best candidate to ionizethe region is possibly a supergiant.Based on observations at the ESO 3.6 m telescope on La Silla observatoryunder Program-ID No. 64.I-0606(B) and the 3.5 m telescope on Calar Altoobservatory during the ALFA science verification program.