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A high-resolution search for small-scale structure in Sharpless H II regions at 4.995 GHz. II - General properties of the entire sample. III - Description of selected sources
A statistical study is presented of the properties of a sample of 75optically visible H II regions from the Sharpless Catalog observed at afrequency of 4.994 GHz by the Westerbork Synthesis Radio Telescope, andthe most interesting small-diameter H II components detected in thesurvey are discussed in detail. The optical and radio properties of theregions are compared, and results are presented of high-andlow-resolution observations. The nebulae are divided into five classesaccording to optical morphology and the presence of obscuration, whichclasses can be interpreted as representing subsequent evolutionarystages of a single H II region. It is also found that the change of theelectron density due to H II region expansion occurs at a higher ratefor H II regions ionized by stars of an earlier spectral type, and thatthe rate of change of the mean electron density for a given type isapproximately hyperbolic. Small-diameter, high-density components arefound preferentially in small, isolated nebulae with high mean electrondensities, while the ratio of the Lyman continuum flux contained in thesmall-diameter component to that contained in the entire H II regioncorrelates with the mean density of the region. Of the 22 objectsdiscussed individually, 12 can be classified as compact H II regionswith an internal source of excitation, and the remaining sources can beattributed to increases in the electron density at ionization frontslocated at the edges of dense molecular clouds. From an analysis ofradio, infrared and molecular line observations of several sources, itis concluded that a single early-type star embedded in the H II regionin the presence of dust can be the energy source of the observedemission.
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