Cooling, sputtering, and infrared emission from dust grains in fast nonradiative shocks

We model the dynamics, the destruction by sputtering, and the infrared (IR) emission from collisionally heated dust grains in fast (greater than or equal to 400 km s(-1)) astrophysical shocks in order to develop IR diagnostics for the destruction of grains in these environments. The calculations take into account the feedback from sputtering and IR emission on the gas-phase abundances, the cooling, and the ionization and thermal structure of the shock. Sputtering changes the initial grain size distribution, creating a deficiency of small (radius < 50 Angstrom) grains compared to their preshock abundances. The altered grain size distribution depends on shock velocity and the density of the interstellar medium. Dust particles with sizes below approximate to 300 Angstrom are stochastically heated, undergo temperature fluctuations, and radiate an excess of near-infrared emission (lambda less than or equal to 40 mu m) over that expected for grains in thermal equilibrium. This near-infrared excess is a measure of the abundance of small grains and therefore a powerful diagnostic for the amount of destruction the grains were subjected to in the shock. We present here IR spectra from collisionally heated dust for a variety of shocks, and depict the changes in the spectra as a function of postshock column density. Our studies compliment those of Vancura et al. that examined the effects of the release of the sputtered refractory elements on the ultraviolet and X-ray emission. Multiwavelength observations at X-ray, UV, and IR wavelengths are therefore essential in piecing together a comprehensive picture of the physics of grain destruction in fast astrophysical shocks.