Total to Selective Extinction in the Dark Globule CB 107

Abstract

We discuss optical (B, V, and I) and near-infrared (J, H, and Ks) imaging observations of the small dark cloud CB 107. The cloud is projected against a rich stellar background, making it possible to detect ~3900 stars in all six photometric bands. After an accurate choice of a reference region, we obtained the color excess Eλ-V maps and derived the distribution of the total to selective extinction RV across the cloud by adopting three different techniques. The RV map obtained closely follows the optical shape of CB 107 with the value RV 3.1, typical of the diffuse interstellar medium, corresponding to the optical limit of the globule. Larger values up to RV ~ 5 are found in the innermost regions, indicating an increase in the dust grain mean size. On the basis of our results, the three methods adopted have been found to be almost equivalent. We also estimated the RV values along the lines of sight of individual stars that, considered in a given spatial box, have been used to obtain both average value and dispersion, as a function of the position. A scatter plot of these two quantities shows that the dispersion σRV increases with RV similarly to the correlation between extinction dispersion σAV and AV already known in the literature. Finally, we discuss the RV versus AV scatter plot on the basis of a simple model that helps us to give a plausible physical interpretation of this correlation.