Shock Processing of Large Grains in the Interstellar Medium

There is a growing body of evidence for the existence of large (>0.25 μm) dust grains in the interstellar medium (ISM). Large presolar grains have been found in meteors and have been directly detected flowing into the solar system from the ISM by the Ulysses and Galileo spacecraft. While extending the grain size distribution to grains of this size presents problems in accounting for elemental abundances (and they have thus been left out of standard grain models, such as that of Mathis and coworkers) their presence may have important consequences for the evolution of dust in the ISM. We present the results of calculations of the processing of large grains in shocks, including all known destruction mechanisms. We break from earlier techniques in that we explicitly follow the trajectories of the grains rather than assuming tight coupling with the gas. As the grains traverse the shock they are subject to magnetic and drag forces and different environments for charging, as well as thermal and nonthermal sputtering, vaporization, and shattering. We find markedly different behaviors for different combinations of shock speed and grain size. The fate of the grains can be described as coupled, reflected, or escaped, with the degree of grain destruction dependent on the type of trajectory followed. Grains reflected into the preshock gas are accelerated to high speeds before being destroyed, possibly creating the seeds for cosmic-ray acceleration. Grains that escape, on the other hand, suffer little destruction and may act as a reservoir of material that is decoupled from the gas.