FORMALDEHYDE OBSERVATIONS OF MOLECULAR CLOUDS (INTERSTELLAR)

The 1(,11) (--->) 1(,10) and 2(,12) (--->) 2(,11) K-doublet transitions H(,2)CO have been observed in absorption towards 26 strong continuum radio sources in the first galactic quadrant. Ninety-seven discrete absorption components have been detected in the 6 cm 1(,11) (--->) 1(,10) line. Of these, 69 have corresponding detections in the 2 cm 2(,12) (--->) 2(,11) transition. We have analyzed the data using a 16-level collisional excitation model and the collisional rate coefficients of Green et al. (1978), employing a spherically symmetric Large Velocity Gradient approximation to treat the radiative transfer. We obtain median H(,2) densities of 9 x 10('4) cm('-3) for clouds near HII regions and 7 x 10('4) cm('-3) for clouds well removed from HII regions. Estimates of the ortho- H(,2)CO column density based on our observations, when combined with conservative estimates of the ortho-H(,2)CO abundance, suggest that the H(,2)CO traces a significant fraction (20-80%) of the total H(,2) column density seen in CO. No evidence is found in our data sample for the existence of a significant population of molecular clouds comprised solely of low density (i. e. n(H(,2))(, )< 10('4) cm('-3)) gas.;Finally, we present high spatial resolution observations of the 2 cm H(,2)CO emission in the (rho) Oph B cloud core. These observations indicate that the core contains 1-4 M(,o) of dense (n(H(,2)) (DBLTURN) 4-5 x 10('6) cm('-3)) molecular gas within a region 0.05 x 0.02 pc in linear extent. Systematic variations in the position of the peak line intensity with velocity suggest that the core may be comprised of at least two small rotating fragments. These fragments contain masses typical of low-mass stars and have densities and linear sizes expected of protostars.;Maps of the 6 cm and 2 cm H(,2)CO absorption in the dense molecular core TMC-1 are also presented. The spatial extent and overall morphology of the H(,2)CO absorption agree with maps made in other molecular species. However, a homogeneous LVG model analysis of the data yields H(,2) density estimates an order of magnitude lower than those obtained from other species. We believe that the anomalously low density which we obtain points toward the likelihood of inhomogeneities (e. g. core-halo structure) within TMC-1.