Crystalline Silicates as a Probe of Disk Formation History

Abstract

We present a new perspective on the crystallinity of dust in protoplanetary disks. The dominant crystallization by thermal annealing happens in the very early phases of disk formation and evolution. Both the disk properties and the level of crystallinity are thereby directly linked to the properties of the molecular cloud core from which the star+disk system was formed. We show that under the assumption of single-star formation, rapidly rotating clouds produce disks that after the main infall phase (i.e., in the optically revealed class II phase) are rather massive and have a high accretion rate but low crystallinity. Slowly rotating clouds, on the other hand, produce less massive disks with lower accretion rates but high levels of crystallinity. Cloud fragmentation and the formation of multiple stars complicates the problem and necessitates further study. The underlying physics of the model is insufficiently understood to provide the precise relationship between crystallinity, disk mass, and accretion rate. But the fact that with standard input physics the model produces disks that, in comparison to observations, appear to have either too high levels of crystallinity or too high disk masses demonstrates that the comparison of these models to observations can place strong constraints on the disk physics. The question to ask is not why some sources are so crystalline, but why some other sources have such a low level of crystallinity.