Segregation of dust and abundance inhomogeneities in globular clusters

Abstract

Segregation of dust in globular cluster protoclouds is considered as a possible mechanism for producing the observed heavy element abundance inhomogeneities within globular clusters. Segregation of dust towards the protocloud center produces a radial gradient in the dust-to-gas mass ratio within the cloud. Stars that form from material with different dust-to-gas mass ratio then have different heavy element abundances. A radial abundance gradient is thus naturally produced. If the globular cluster has a long relaxation time, the radial gradient may survive up to the present time, otherwise only star to star variations in abundance are observed. For any significant abundance variations to be produced it is required that the dust segregation time-scale be of the order of or less than the protocloud lifetime. The segregation time-scales are estimated to be between 10 to the 7th and 10 to the 8th yr, so that primordial abundance inhomogeneities can be established by dust segregation processes provided the globular cluster protoclouds had lifetimes of about 10 to the 8th yr, similar to the mean lifetime of the large interstellar clouds.