Dust extinction in a small molecular cloud

The distribution of matter in a small dark cloud has been investigated observationally by using the extinction of the stellar light that is due to the dust component as a tracer. Here we present the results of a photometric study that has been carried out in the optical spectral region by means of imaging in the B, V, and I bands. The relevant information on the spatial distribution of the dust has been obtained both by counting stars and by measuring their color excesses. While star counts on a reseau grid are easily interpreted as a measurement of the average extinction A(lambda) under the assumption of a locally uniform stellar luminosity function, the color excesses averaged on the same grid are obtained by a comparison of the stellar color distributions among the reddened regions and a reference position. Color excesses have also been derived by comparing the observed stellar colors with the locus of the unreddened stars in the two-color (V-I) versus (B-V) diagram. The extinction map obtained for the globule CB 107 shows that the dust distribution at the boundary of this dark cloud can be measured and compared with models of matter distribution in these objects. By considering a density law such as n proportional to (1+(r/a)(alpha))(-beta), we find that our observations favor models with alpha =2 and beta =1. Indications are also obtained about the spatial variation in this object of the total to selective extinction R-V. In this respect we find that R-V tends to increase toward the innermost regions of CB 107, and this suggests that the mean size of the dust particles also increases. The implications of these observations on the modeling of molecular cloud boundaries are discussed, and some indications on the timescale of the physical processes responsible for their structure are derived. The penetration depth of the interstellar radiation field in these objects is also briefly discussed in the light of its possible consequences on the cloud chemistry. Lastly, the differences that we found in the color excess maps, which were obtained by means of the stellar counts and the cumulative distribution in color, respectively, suggest a clumpy distribution of matter in CB 107.