Large dust grains in the dark cloud B5

Abstract

Polarization measurements of stars in the region of the dark cloud B5 indicate that the average dust grain size in the cloud is much larger than the mean interstellar grain size. Within the cloud, the average grain size is found to increase with decreasing distance from the cloud core where IRAS detected a compact infrared source IRS 1. Dust grains with an average size as large as about twice the mean size of the interstellar grains could be present in the inner regions of the cloud. It is suggested that dust grains in B5 grew in size by the accretion of condensable heavy elements from the gas on a time-scale of ≡106yr, and gravitational segregation of the larger grains towards the cloud core on a longer time-scale (≡3×107yr) caused the observed radial variation in grain size and the lower than expected extinction in the region outside the core.