Detection of AU-Scale Structure in Molecular Clouds

We have detected significant secular changes in the 4.83 GHz H_2_CO absorption line toward the compact extragalactic radio sources NRAO 150 and 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 between Earth and the cloud, the position of the line of sight through each cloud drifts by a transverse distance of ~4 AU per year. The changes in absorption-line profile over a period of 2.05 yr are interpreted in terms of clumps of size <~10 AU in the molecular cloud. The structure observed is qualitatively consistent with that expected if both the supersonic, compressible turbulence hypothesized 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 ~10^14^ and 10^18^ cm is steeper (D >= 1.7) than that found on larger scales from molecular emission line observations (D~1.4). The number of clumps 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 superthermal and probably sonic, while the interclump velocity dispersions are mildly supersonic and possibly Alfvenic.