THE MOLECULAR PROPERTIES OF GALACTIC H II REGIONS

Abstract

We derive the molecular properties for a sample of 301 Galactic H ii regions including 123 ultra compact (UC), 105 compact, and 73 diffuse nebulae. We analyze all sources within the BU-FCRAO Galactic Ring Survey (GRS) of 13 CO emission known to be H ii regions based upon the presence of radio continuum and cm-wavelength radio recombination line emission. Unlike all previous large area coverage 13 CO surveys, the GRS is fully sampled in angle and yet covers ∼ 75 deg 2 of the Inner Galaxy. The angular resolution of the GRS (46 �� ) allows us to associate molecular gas with H ii regions without ambiguity and to investigate the physical properties of this molecular gas. We find clear CO/H ii morphological associations in position and velocity for ∼ 80% of the nebular sample. Compact H ii region molecular gas clouds are on average larger than UC clouds: 2. � 2 compared to 1. � 7. Compact and UC H ii regions have very similar molecular properties, with ∼ 5 K line intensities and ∼ 4k m s −1 line widths. The diffuse H ii region molecular gas has lower line intensities, ∼ 3 K, and smaller line widths, ∼ 3. 5k m s −1 . These latter characteristics are similar to those found for quiescent molecular clouds in the GRS. Our sample nebulae thus show evidence for an evolutionary sequence wherein small, dense molecular gas clumps associated with UC H ii regions grow into older compact nebulae and finally fragment and dissipate into large, diffuse nebulae.