Ultracompact H II regions. 2: New high-resolution radio images

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Abstract

Radio continuum observations were made of 59 Infrared Astronomy Satellite (IRAS) sources that have 100 micrometers flux densities greater than or equal to 1000 Jy and far-infrared colors identified with ultracompact (UC) H II regions. Eighty percent were found to have associated compact radio sources. Seventy-five sources were detected at less than or approximately equal to 1 sec resolution at 3.6 and 2 cm wavelengths, for which we provide contour plots and flux density distributions ranging from the radio to the near-infrared. Over half are unresolved and their morphologies undetermined. The remaining sources can be described by only five morphological classes, whose frequency of occurrence is consistent with that of the Wood and Churchwell survey. We calculate physical properties of the nebulae and show that they are consistent with UC photoionized regions. Alternative explanations are explored and found to be unlikely. The correlation of UC H II region positions with proposed spiral arms is examined and found to be well correlated only for the local spiral arm or `spur.' No obvious enhancement of UC H II regions is apparent along the proposed Sagittarius and Scutum arms, probably because of inaccuracies in the kinematic distances. We find the latitude distribution of UC H II regions to lie in the range of absolute value of b_FWHM greater than or equal to 0.5 deg and less than or equal to 0.8 deg. No correlation between size and density of cometary and core-halo UC H II regions is found, consistent with the bow shock interpretation of these morphologies. Spherical and unresolved UC H II regions, however, appear to show a trend toward lower densities with increasing size, as expected for expanding H II regions. The observed ratios of far-infrared to radio flux densities of UC H II regions lie in the range 103 to greater than or approximately equal to 105. By applying the results of model atmospheres, it is shown that this ratio depends on spectral type, ranging from approximately equal to 103 for an O4 star to greater than or approximately equal to 105 for a B3 star. We find that many of the UC H II regions in our sample must be excited by a cluster of stars, and most probably contain significant amounts of dust.

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