Molecular Clouds and Cloud Cores in the Inner Galaxy

N. Z. Scoville, M. S. Yun, D. P. Clemens, D. B. Sanders, W. H. Waller

Astrophysical Journal Supplement Series, V.63, p.821 (1988)

Abstract

CO data from the Massachusetts-Stony Brook Galactic Survey have been analyzed to generate a compilation of clouds and hot cloud cores in the first Galactic quadrant at l=8 to l=90 and b=-1.05 to b=+1.0 degrees. Three lists of CO emission regions are compiled: 1427 emission regions from the general cloud population measured at the 4 K boundary with CO peaks at TR* > 5 K; 255 hot cloud cores measured at the 8 K boundary with peak TR* > 9 K; and 95 clouds associated with 171 radio H II regions.

The clouds associated with H II regions exhibit systematically brighter CO peaks; they are a factor of 2-3 larger and have twice as large a mean velocity dispersions as the general cloud population. Both the H II region clouds and the hot core regions have a Galactic distribution characteristic of a spiral arm population, whereas the colder clouds are much less confined in Galactic azimuthal angle. For giant molecular clouds (GMCs) in the general population, there is no significant difference in the confinement of large or small clouds to the spiral arm locations.

Virial masses are obtained for the large sample of clouds with assigned kinematic distances. The mean H2 density for a GMC of diameter 40 pc (at TR* = 4 K) is 180 cm^-3. For these clouds, a linear relationship is found between the H2 column density (adopting R0 = 8.5 kpc) and the integrated CO emission: N(H2) = 3.6 x 10^20 I(CO) (K km/s) for diameters range 10-100 pc. This relation holds equally well for clouds with and without H II regions. The variation in the Z-dispersion of clouds as a function of cloud mass suggests that more massive GMCs have smaller random velocities, as expected for equipartition. The distribution of H II region locations within GMCs can be approximated by a power law $\rho(r) \propto r^{-1}$, where r is measured from the CO emission centroid. The efficiency for massive star formation estimated from the number and luminosity of the H II regions within individual clouds is found to decrease with increasing cloud mass over the range 2 x 10^5 to 4 x 10^6 solar masses.