  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 (E_{B-V}, E_{V-R}, and
E_{V-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 ~3.2,
close to that often found in the diffuse interstellar medium (~3.1), 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.

Astrophysics