MAGNITUDE AND COLOR DISTRIBUTION OF BACKGROUND STARS AS A PROBE OF SMALL-SCALE STRUCTURE IN THE SPATIAL-DISTRIBUTION OF INTERSTELLAR DUST

In order to determine the amplitude of small scale fluctuations of the extinction induced by molecular material, we have reconsidered the st ar count method in the context of fragmented clouds. We show that by s tudying the full magnitude distribution it is possible not only to est imate extinctions in a more: reliable way than by using the classical procedure, but also to derive constraints on cloud structure at very s mall scales (down to few mu arcsec). This technique has been applied t o a low latitude cloud located at a distance of about 200 pc in the IC 5146 complex. Analyses of CCD B, V, R, i photometric data on more tha n one thousand stars indicate that the cloud extinction is close to un iform over the 45 arcmin(2) field. To better constrain the fluctuation s of the extinction, the distribution of the colors of background star s is considered. Analysis of the V - i data implies that variations ar e present but at a level no larger than sigma(A(v))/A(v) approximate t o 25%. The accurate 42 '' (corresponding to 0.01 pc) resolution map of the V - i color excess obtained indicates that most of these variatio ns occur at scales larger than approximate to 1 arcmin. We therefore c onclude that the contribution of small scales to extinction fluctuatio ns is quite small and that, at these scales, molecular material behave s as if it were homogeneous regarding the transfer of continuum UV rad iation. A comparison of our extinction data with a high resolution map of the same field in the J = 1-0 and J = 2-1 CO lines shows a complet e absence of correlation between Av and I(CO) at these small scales, w hich implies an upper limit for the H-2-I(CO) conversion factor (defin ed as delta N(H-2)/delta I(CO)) of about 10(19) H-2 molecules cm(-2)/K km s(-1). We discuss the implications of this lack of correlation on the structure of molecular clouds and on the origin of the large spati al variability displayed by CO emission.