Observations of selected sources in the 2cm line of H2CO.

One arcminute resolution measurements of the 2(11)-2(12) line of H2CO, at 2 cm, are presented. The observations were made toward nineteen OH and/or H2O maser centers and compact H II regions. In addition, limited maps were made toward eight H II regions; these are shown for 3 sources, W 31, W 43, and W 51, which are extended relative to the telescope beam. Absorption line optical depths were combined with appropriate 6 cm line data to estimate H2 densities. These agree with previous estimates, to a factor of 2-3. The lowest ratio of optical depths at 6 and 2 cm, tan(6)/tan(2) = 0.6, and hence the highest implied density, 10-million/cu cm, were obtained for the molecular cloud toward G 10.6-0.4. For the H2CO maser source in NGC 7538, the lower limit for the ratio of brightness temperatures at 6 cm to 2 cm is greater than 50. A critical discussion of the H2CO excitation model is given.