On the pressure of collisionless particle fluids. The case of solids settling in disks

Details On the pressure of collisionless particle fluids. The case of solids settling in disks On the pressure of collisionless particle fluids. The case of solids settling in disks

2009-05-07

arxiv.org/abs/0905.0959v2

Astronomy and Astrophysics

Astrophysics

Instrumentation and Methods for Astrophysics

Accepted for publication as a research note in Astronomy and Astrophysics. Language edited

Scientific paper

Aims. Collections of dust, grains, and planetesimals are often treated as a pressureless fluid. We study the validity of neglecting the pressure of such a fluid by computing it exactly for the case of particles settling in a disk. Methods. We solve a modified collisionless Boltzmann equation for the particles and compute the corresponding moments of the phase space distribution: density, momentum, and pressure. Results. We find that whenever the Stokes number, defined as the ratio of the gas drag timescale to the orbital timescale, is more than 1/2, the particle fluid cannot be considered as pressureless. While we show it only in the simple case of particles settling in a laminar disk, this property is likely to remain true for most flows, including turbulent flows.

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