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Drag and drift – decoupling of the dust and gas phases in astrophysical flows
marzo 14 @ 11:45 - 12:45
Simulating the dynamics of gas and dust in various astrophysical contexts is a topic of intense research. It includes simulations of the turbulent interstellar medium (ISM), active galactic nuclei (AGN), proto-planetary discs, and dusty stellar winds. I will focus on the ISM and dust-driven stellar winds, however, and only briefly mention a few other examples.
Using high-resolution (1024^3) simulations of homogeneous isothermal hydrodynamic turbulence, including a multi-disperse population of 10^7 dust grains imbedded in the gas, one may assess the efficiency of condensation, destruction and coagulation in molecular clouds. Due to hydrodynamic drag, large grains will decouple from a turbulent gas flow, while small grains will tend to follow the motions of the gas. Dust grains of various sizes will also cluster and increase the rate of grain-grain interaction. This may in turn lead to turbulence-driven coagulation and fragmentation.
In a stellar-wind context the decoupling between gas and dust is often called “drift”, which is referring to the fact that the two phases may develop different mean-flow velocities. In a spherically-symmetric setting, a mean-flow model of drift is fairly simple. But there are significant 3D effects, as well as instabilities, which we must take into account. For instance, dust grains of various sizes will cluster and increase the rate of grain-grain interaction. Moreover, due to spatial separation of dust and gas, condensation is likely less effective than in a velocity-coupled case. There are several implications for dust-driven stellar winds which arise from this. One is that the momentum-transfer efficiency (from dust to gas) of large grains may be low as they cluster where the gas is not; another is that the sublimation rate increases as grains are more exposed to the radiation field.