Grain Processing in Interstellar Shocks

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Session 14 -- Interstellar Dust and Extinction
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[14.05] Grain Processing in Interstellar Shocks

K.J.Borkowski (UMD), E.Dwek (NASA/GSFC)

Radiative shocks propagating through the diffuse interstellar medium (ISM) are the primary sites where the destruction of interstellar dust grains occurs. Dust grains entering a shock front are efficiently accelerated to high velocities in the compressive section of the radiative shock through the betatron process. Binary collisions between fast moving grains are then effective in vaporizing and fragmenting a significant fraction of the grain population.

We present a detailed study of the fragmentation and vaporization of dust grains in radiative shocks propagating through the diffuse ISM. For that purpose we have formulated and solved numerically the integro-differential equations governing the evolution of the grain size distribution. Our treatment of grain fragmentation and vaporization is based on our analysis of physical processes involved in grain-grain collisions. We take into account both partial and complete fragmentation and vaporization, as well as sputtering of dust grains induced by their fast motion with respect to the ambient gas.

Grain fragmentation was found to be the primary factor in controlling the evolution of the grain size distribution. This process is responsible for the virtual absence of grains with radii larger than 0.05--0.1~$\mu$m and for the increase in the population of grains with smaller sizes after their passage through our standard 100~km s$^{-1}$ shock with the preshock density and magnetic field of 0.25~cm$^{-3}$ and 1~$\mu$G, respectively. The grain mass returned to gas through kinetic sputtering and vaporization in grain-grain collisions is equal to 20--30\%\ of the total grain mass, with only a moderate dependence on the details of the fragmentation process. We will discuss how these results depend on the shock velocity and the preshock magnetic field.

This research is funded by NASA RTOP \#188-44-53-05.

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