OBSERVATIONAL PROBES OF THE SMALL-SCALE STRUCTURE OF MOLECULAR CLOUDS

We probe the small-scale structure of several Galactic molecular clouds that occult extragalactic radio sources by examining time and spatial variations of their absorption line profiles. Using the Very Large Array over a 3.4 yr period, we detect significant time variability of 6 cm formaldehyde absorption toward the compact sources 3C 111, NRAO 150, and with marginal significance, BL Lac. This reveals the presence of structure in the dense gas on scales of order 10 AU. If the variability is due solely to density structure, the volume densities of the clumps implied are greater than or similar to 10(6) cm(-3). The radio galaxy 3C 111 has extended structure on a scale of 1.'4-3.'4 (0.14-0.34 pc). We find substantial spatial variations in the molecular (H2CO, OH) absorption-line profiles at this scale toward this source. The variations in the molecular lines are greater than those found in the atomic neutral hydrogen. These studies demonstrate that molecular clouds contain significant structure in density and/or excitation from subparsec to AU scales.