High-resolution maps of 6 centimeter formaldehyde: clumping in molecular clouds

VLA maps of W31, W33, W49, and W51 in the 110--111 line of ortho-H2CO are presented. The maps were made with angular resolutions of 10''--15'' and a velocity resolution of 1.8 km s-1. The total velocity coverage was 45 km s-1. In all of the regions measured, the ortho-H2CO distribution breaks up into a number of condensations, or clumps. For W33 and W49, all of the single-dish line flux density is present in our VLA maps; for the other two, about 70% is measured. Averaged over all of the sources, the median clump size is 0.5 pc with a range from <0.2 pc (one-half of the beam size) to >1.4 pc. The median apparent optical depth is 0.4 with a range from <0.1 to 1.1. The median line width (FWHP) is 5.0 km s-1, with a range from <1 km s-1 to 12 km s-1. The median space densities of ortho-H2CO are calculated on a source-by-source basis. Combining these with H2 densities, we find that the (ortho-H2CO/H2) ratios are about an order of magnitude larger than the prediction of Wootten and colleagues published in 1978. Based on line width, the clumps can be placed in two categories: those having FWHP widths < or =1 km s-1 and those having widths larger than 3 km s-1. The first group could follow the relation given by Larson in 1981 and might have masses which agree with virial estimates. For the second group, comparisons of our data with the analysis of Larson show that (1) our clumps have approx.10% of the sizes predicted from the line widths, and (2) the masses of most of these clumps toward W31 and W51 are 20%--60% of the virial mass estimates