DETECTION OF AU-SCALE STRUCTURE IN MOLECULAR CLOUDS

We have detected significant secular changes in the 4.83 GHz H2CO abso rption line toward the compact extragalactic radio sources NRAO 150 an d 3C 111 with the Very Large Array. In each case the absorption occurs within a molecular cloud in our Galaxy against the submilliarcsecond core of the background radio source. Because of relative motion betwee n Earth and the cloud, the position of the line of sight through each cloud drifts by a transverse distance of approximately 4 AU per year. The changes in absorption-line profile over a period of 2.05 yr are in terpreted in terms of clumps of size less than or similar 10 AU in the molecular cloud. The structure observed is qualitatively consistent w ith that expected if both the supersonic, compressible turbulence hypo thesized to explain superthermal line widths and the clumping observed on subparsec scales extend to AU scales. Quantitative agreement with such a model is attained if (1) the intraclump velocity dispersion is independent of clump size L, (2) the density versus size scaling law n is-proportional-to L-1 is approximately valid down to AU size scales, and (3) the clump size distribution law dN/dL is-proportional-to L-(D +1) between approximately 10(14) and 10(18) cm is steeper (D greater-t han-or-equal-to 1.7) than that found on larger scales from molecular e mission line observations (D almost-equal-to 1.4). The number of clump s along the line of sight is in the range 20-50 toward NRAO 150 and 30 -100 toward 3C 111. The intraclump velocity dispersions are mildly sup erthermal and probably sonic, while the interclump velocity dispersion s are mildly supersonic and possibly Alfvenic.