An investigation of grain properties in the Rho Ophiuchi dark cloud

A polarimetric, photometric, and spectroscopic survey of 134 stars in the direction of the Rho Ophiuchi dark cloud, to an absorption depth of Av less than 5-7 mag is presented. The extinction and polarization for 19 stars foreground to the Rho Oph cloud are trivial compared to those observed through the cloud itself. The ratio of total-to-selective extinction, R, determined from the color difference method, is found to increase to R of about 4.1 at Av = 5.0 mag. Grain size, inferred from lambda max, increases with optical depth in a manner that is consistent with a simple model of coagulative grain growth. A high degree of grain alignment under a variety of ISM conditions is indicated. It is suggested that grain growth in Rho Oph is due to coagulation rather than mantle accretion.