1996A&A...312..973T

Motivated by the recent finding of AU-scale structure in the 6cm H₂CO absorption line (Marscher et al. 1993), we have performed repeated broad-band CCD imaging observations of two fields extinguished by molecular material (A_V≃1-2mag) in order to search for small 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 7.5AU. Among the sample of 23 measured stars in the two fields studied, no significant fluctuations have been observed that could be attributed to AU-sized dust clumps. We get the following 1σ upper limits on the relative variations of δA_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 results do not support the view that the H₂CO variations detected by Marscher et al. are due to density structure. These variations should then reflect fluctuations of either the relative abundance or excitation of H₂CO molecules. We argue that the molecular component responds differently and much more quickly than dust grains to changes in parameters related to the dynamics, chemistry, and radiation field and that this probably results in the observed differences between the AU-scale spatial structure of the dust and that of H₂CO molecules.