S. Thoraval et al., A SEARCH FOR EXTINCTION VARIATIONS RELATED TO AU-SCALE STRUCTURE IN MOLECULAR CLOUDS, Astronomy and astrophysics, 312(3), 1996, pp. 973-980
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.