Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a...100...42f&link_type=abstract
Astronomy and Astrophysics, Vol.100, P. 42, 1981
Astronomy and Astrophysics
Astrophysics
46
Scientific paper
A detailed discussion of 22 H II regions with small structure in their radio emission is presented. These regions represent the most interesting objects of a λ=6 cm survey of 74 H II regions made with the Westerbork Synthesis Radio Telescope (Felli et al, 1978).
Of the 22 objects discussed, 12 can be classified as compact H II regions with an internal source of excitation. The remaining small diameter radio features are found in more extended H II regions and can be attributed to increases in the electron density at ionization fronts located at the edges of dense molecular clouds. Their ionization can be accounted for by the same early type star which ionizes the diffuse extended H II region in which they are located.
The compact components found in this survey, are usually associated with small bright optical nebulosities, but seldom with diffuse extended low density H II regions. These results imply that, in our sample, the expansion of an H II region plays little role as the trigger of the next generation star formation. Several nebulae in our sample have blister-like configurations and/or bipolar nebulae, in which the ionized gas is flowing away in the two opposite directions perpendicular to a flat, disk-like, molecular cloud. Under assumption that the radio continuum emission of the ionized gas, the infrared emission of the heated dust and the molecular emission are all energized by a single stellar source, the spectral type of the required star is derived independently from the measured flux densities measured in the three wavelength regions for a limited number of nebulae for which sufficient information is available. The values obtained are in good agreement, suggesting that a hidden early-type star can be the main source of energy supply for the radio, infra-red and molecular emission in compact H II region, IR source, molecular hot spot complexes.
Felli Marcello
Harten R. H.
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