Monte Carlo calculations of the sputtering of grains: Enhanced sputtering of small grains

The sputtering of a small grain embedded in a plasma or hot gas in the interstellar medium (ISM) is calculated using a Monte Carlo binary collision code. This technique is used to describe the transport of the atoms set in motion in the solid by an incident ion or atom and the sputtering from the surface of a grain induced by collisional cascades. The calculations calibrated to laboratory data, describe the enhancement in the yield due to erosion from the edges and back surfaces by the incident particle and the energetic recoils. The angular dependence of the sputtering yield typically used, Y similar to (cos theta)(-1), is shown not to be valid. The grain size effect manifests itself even when the grain radius is only similar to 3 times larger than the mean penetration depth of the incident particle. Sputtering of a grain in a single-fluid shock is used as an example to test the effect of grain size on the erosion rate. The enhancement in the sputtering rate due to grain size exacerbates the problem of small grain destruction, placing more severe constraints on grain growth in the ISM. The results presented are also relevant to the erosion of circumstellar and circumplanetary grains by energetic ions.