Cosmic Dust - Dust Grain Destruction

Dust Grain Destruction

How are the interstellar grains destroyed? There are several ultraviolet processes which lead to grain "explosions" for tiny grains. More prominent for interstellar dust grains is sputtering erosion when energetic atoms or ions pierce the surface of a solid to deposit enough energy to cause secondary ions to be ejected from the resulting internal explosion of fast ions. This causes total mass to be sputtered away with a mean lifetime M/(dM/dt)= 2-4x108 years. Grain-grain collisions also influence the grain size distribution. Stochastic models of such repeating cycles following intermittent reincorporation into molecular clouds to rebuild the overlying shell mass for the grains demonstrate that the refractory grain cores have a much longer mean lifetime against destruction that does the grain mass; viz for cores M/(dM/dt)= 3x109 years, about tenfold longer on average. This survival mechanism explains how the refractory cores can live so long in the interstellar medium. The extreme isotopic compositions of the older cores endows them with a greater potential for cosmic chemical memory. Dust grains incorporated into stars are also formed by complete evaporation, but only a relatively small fraction of the mass of a star-forming cloud actually ends up in stars. Therefore a typical grain core goes through many molecular clouds and has mantles added and later removed many times before the grain core is destroyed.