Grain growth in the dark cloud L1251

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 A(B) and A(V) 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 R-V over the cloud using an empirical relation between R-V and A(lambda)/A(V) reported by Cardelli et al. In the tail of the cloud, R-V has a uniform value of similar to3.2, close to that often found in the diffuse interstellar medium (similar to3.1), while higher values of R-V = 4-6 are found in the dense head. Since R-V is closely related to the size of dust grains, the high R-V-values are most likely to represent the growth of dust grains in the dense star-forming head of the cloud.