VLA1 OBSERVATIONS OF THE 6 CENTIMETER AND 2 CENTIMETER LINES OF H2CO IN THE DIRECTION OF W49

Formaldehyde absorption at 6 cm and 2 cm toward the radio continuum source W49 has been mapped with the Very Large Array with an angular resolution of 2″ and a velocity resolution of 0.5 km s-1. Gaussian components have been fitted to the H2CO profiles obtained in the direction of the embedded ultracompact H II regions (UC H II). H2CO absorption occurs over the velocity range -2 km s-1 to +20 km s-1. Large differences in opacity are found toward the different compact H II components of W49 North. The strongest absorption with τ ∼ 1 occurs for both transitions of H2CO toward the westernmost UC H II region A. Broad absorption (ΔV ∼ 13 km s-1) with low opacity (τ ≤ 0.2) is seen toward the shell-like H II region G. The opacity is low (τ ∼ 0.1) toward the H II region W49 South which is located at the southeastern edge of the molecular cloud. The density of molecular hydrogen, n(H2), and the column density of formaldehyde molecules, N(H2CO), have been determined for each velocity component. The H2 densities are high throughout the region; values range from 6 × 104 to 5 × 105 cm-3. Most of the H2CO column densities lie in the range 1014-5 × 1014 cm-2, with extreme values of 1013 cm-2 obtained for W49 South and 4 × 1015 cm-2 obtained for component A of W49 North. Although the mean abundance of H2CO relative to H2 is between 10-9 and 2 × 10-9, it may well be an order of magnitude higher in the direction of UC H II region A. The main features of the H2CO observations toward W49 North can be explained if the absorption occurs within the dense core of the molecular cloud and arises close to the UC H II regions. The structure of the molecular core is determined from a comparison of the H2CO absorption, OH maser emission, and bipolar CO outflow. The results are discussed in the context of the model for W49 North of a collapsing cloudrotating disk proposed by Welch et al. in 1987.