A SEARCH FOR EXTINCTION VARIATIONS RELATED TO AU-SCALE STRUCTURE IN MOLECULAR CLOUDS

Motivated by the recent finding of AU-scale structure in the 6 cm H2CO absorption line (Marscher et al. 1993), we have performed repeated br oad-band CCD imaging observations of two fields extinguished by molecu lar material (A(V) approximate to 1-2 mag) in order to search for smal l amplitude magnitude variations of stars due to spatial structure in the foreground dust. The time intervals investigated are between a few months and two years which, given the drift in the line of sight due to the earth and sun's motion, allow to probe spatial structure in the foreground extinction at scales smaller than 7.5 AU. Among the sample of 23 measured stars in the two fields studied, no significant fluctu ations have been observed that could be attributed to AU-sized dust cl umps. We get the following 1 sigma upper limits on the relative variat ions of delta A(V)/A(V): 1.5% for 11 stars in the 3C 111 field and 0.8 % for 6 stars in a field towards the high latitude cloud MBM 28. Our r esults do not support the view that the H2CO variations detected by Ma rscher et al. are due to density structure. These variations should th en reflect fluctuations of either the relative abundance or excitation of H2CO molecules. We argue that the molecular component responds dif ferently and much more quickly than dust grains to changes in paramete rs related to the dynamics, chemistry, and radiation field and that th is probably results in the observed differences between the AU-scale s patial structure of the dust and that of H2CO molecules.