ULTRACOMPACT H-II REGIONS .2. NEW HIGH-RESOLUTION RADIO IMAGES

Radio continuum observations were made of 59 IRAS sources that have 10 0 mum 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 sou rces were detected at less than or similar 1'' resolution at 3.6 and 2 cm wavelengths, for which we provide contour plots and flux density d istributions ranging from the radio to the near-infrared. Over half ar e unresolved and their morphologies undetermined. The remaining source s can be described by only five morphological classes, whose frequency of occurrence is consistent with that of the Wood and Churchwell surv ey. 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 reg ion positions with proposed spiral arms is examined and found to be we ll correlated only for the local spiral arm or spur.'' No obvious enha ncement of UC H ii regions is apparent along the proposed Sagittarius and Scutum arms, probably because of inaccuracies in the kinematic dis tances. We find the latitude distribution of UC H II regions to lie in the range 0.5-degrees \b(FWHM)\ less-than-or-equal-to 0.8-degrees. No correlation between size and density of cometary and core-halo UC H I I regions is found, consistent with the bow shock interpretation of th ese morphologies. Spherical and unresolved UC H II regions, however, a ppear to show a trend toward lower densities with increasing size, as expected for expanding H II regions. The observed ratios of far-infrar ed to radio flux densities of UC H II regions lie in the range 10(3) t o greater than or similar to 10(5). By applying the results of model a tmospheres, it is shown that this ratio depends on spectral type, rang ing from approximately 10(3) for an O4 star to greater-than-or-equal-t o 10(5) 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 conta in significant amounts of dust.