Growth in the Dark Cloud L 1251

We have performed optical imaging observations of the dark cloud L1251 at multiple wavelengths, B, V , R, and I, using the 105 cm Schmidt telescope at the Kiso Observatory, Japan. The cloud has a cometary shape with a dense “head” showing star formation activity and a relatively diffuse “tail” without any signs of star formation. We derived extinction maps of AB and AV with a star count method, and also revealed the color excess (EB−V , EV−R, and EV−I) distributions. On the basis of the color excess measurements we derived the distribution of the ratio of total to selective extinction RV over the cloud using an empirical relation between RV and Aλ/AV reported by Cardelli et al. In the tail of the cloud, RV has a uniform value of ∼3.2, close to that often found in the diffuse interstellar medium (∼3.1), while higher values of RV=4−6 are found in the dense head. Since RV is closely related to the size of dust grains, the high RV -values are most likely to represent the growth of dust grains in the dense star-forming head of the cloud. Subject headings: ISM: clouds — dust, extinction — ISM: individual (L1251) — stars: formation