Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a...100...28f&link_type=abstract
Astronomy and Astrophysics, vol. 100, no. 1, July 1981, p. 28-58.
Astronomy and Astrophysics
Astronomy
30
H Ii Regions, Nebulae, Protostars, Radio Sources (Astronomy), Stellar Evolution, Astronomical Catalogs, Astronomical Maps, Cosmic Dust, Electron Density (Concentration), High Resolution
Scientific paper
A statistical study is presented of the properties of a sample of 75 optically visible H II regions from the Sharpless Catalog observed at a frequency of 4.994 GHz by the Westerbork Synthesis Radio Telescope, and the most interesting small-diameter H II components detected in the survey are discussed in detail. The optical and radio properties of the regions are compared, and results are presented of high-and low-resolution observations. The nebulae are divided into five classes according to optical morphology and the presence of obscuration, which classes can be interpreted as representing subsequent evolutionary stages of a single H II region. It is also found that the change of the electron density due to H II region expansion occurs at a higher rate for H II regions ionized by stars of an earlier spectral type, and that the rate of change of the mean electron density for a given type is approximately hyperbolic. Small-diameter, high-density components are found preferentially in small, isolated nebulae with high mean electron densities, while the ratio of the Lyman continuum flux contained in the small-diameter component to that contained in the entire H II region correlates with the mean density of the region. Of the 22 objects discussed individually, 12 can be classified as compact H II regions with an internal source of excitation, and the remaining sources can be attributed to increases in the electron density at ionization fronts located at the edges of dense molecular clouds. From an analysis of radio, infrared and molecular line observations of several sources, it is concluded that a single early-type star embedded in the H II region in the presence of dust can be the energy source of the observed emission.
Felli Marcello
Harten R. H.
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