Molecular clouds associated with compact H II regions. I - General properties

Details Molecular clouds associated with compact H II regions. I - General properties Molecular clouds associated with compact H II regions. I - General properties

Jun 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981apj...246..761h&link_type=abstract

Astrophysical Journal, Part 1, vol. 246, June 15, 1981, p. 761-787. Research supported by the Miller Institute for Basic Resear

Astronomy and Astrophysics

Astrophysics

72

H Ii Regions, Interstellar Gas, Molecular Clouds, Spectrum Analysis, Stellar Evolution, Stellar Mass, Abundance, Ammonia, Carbon Monoxide, Cesium, Emission Spectra, Nebulae, Stellar Spectra

Scientific paper

Very compact H II regions with typical sizes less than 0.1 pc, excited by stars with spectral types B2 to O7, are found associated with warm and dense molecular clouds. Typical values of n(H2) > l04 cm-3, Tk = 20-40 K, and M = 102 - 104 Msun are deduced from molecular line studies of NH3, CS, and CO. These regions are suggested to be the less evolved versions of a molecular cloud-H II region complex such as in Orion. A clumpy structure in the NH3 emission cannot be ruled out. If this is important, n(H2) > 105 cm-3, and the presence of small hot cores with size scales ∼0.1 pc (Tex/50 K)-1/2 is implied. The deduced masses, the fraction of the total H2 column density residing within the condensations9 and the ability of dust to heat the gas are all influenced by the presence of clumpy structures within the molecular clouds. A rough correlation is found between the maximum stellar mass in a region and the mass of the associated molecular cloud condensation. The association of stars of spectral types later than B0 with cloud mass less than 102 Msun appears to imply that the cloud was already fairly dense [n(H2) > l03 cm-3] prior to the collapse and fragmentation process as described by Larson.

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