Previous
abstract 
Next abstract
We have detected significant changes in the line profile of $H_2CO$ absorption toward two compact extragalactic radio sources using the Very Large Array at a wavelength of 6 cm. The background sources, NRAO 150 and 3C 111, have cores of angular sizes less than 0.5 milliarcseconds. The absorption occurs in galactic molecular clouds that happen to lie along the line of sight to the background source.
Because of parallax, solar motion and proper motion, the position of the cloud penetrated by the line of sight to the background source drifts at a rate of a few AU yr$^{-1}$. The changing line profile therefore reveals the presence of AU-scale clumps of high density (~10$^7 cm^{-3}$), far out of pressure equilibrium with the bulk of the cloud. These clumps may correspond to the same compressible, supersonic turbulence thought to cause clumping on larger scales, with density vs. size distribution following standard scaling laws for molecular clouds.
This research is supported in part by NSF grant AST-9116525. The Very Large Array is operated by the National Radio Astronomy Observatory which is operated by Associated Universities, Inc. under contract with the U.S. National Science Foundation.
