Dust and Gas in Astrophysical Systems
Dust is everywhere in astrophysics — literally! It contains most of the heavy elements, forms the building blocks of planets, and dramatically alters our view of everything in the Universe. Yet its dynamics are remarkably poorly-understood. We recently discovered a new super-class of instabilities that manifest in a myriad of different ways, whenever dust moves through gas. These could provide an explanation for a number of decades-old astrophysical mysteries, from the formation of planets to the launching of stellar and galactic winds. For more, see https://arxiv.org/abs/1711.03975 and https://arxiv.org/abs/1801.10166
Turbulence with Dust
Simulation of a turbulent box full of dust grains. Colors show gas vorticity, black points show dust grains. The turbulence efficiently concentrates grains into traps.
"Settling Instability" in Planet Formation
One of the new instabilities discovered is the "settling instability" which occurs as dust settles into a proto-planetary disk before even reaching the midplane. Here see the dust-to-gas ratio (colors, on a log scale) in just the very first few dynamical times as dust begins to settle.
Acoustic Resonant Drag Instability: Non-Linear Case
This is the "acoustic" RDI: dust moving with a constant acceleration through homogenous, neutral gas. Should be simple, but the dust immediately goes unstable and starts to clump up.
Acoustic Resonant Drag Instability: Stronger Case
This case has stronger acceleration than before so forms instabilities even more violently. The dust plumes may be critical for understanding galactic winds.
Acoustic Resonant Drag Instability: Long-Wavelength Case
Now gas is at left (log of density in colors as labeled), with a broad spectrum of grain sizes on right (purple = smallest, yellow = largest). The view is "face on" and we are looking at the very long-wavelength instabilities, which generate large-scale crests and shocks that slam into each other.
MHD RDI: Dust in Magnetized Gas (Example 1)
An example of the RDIs in magnetized gas. The movie shows an xyz projection with gas velocity (colors), dust grains (black points), initial magnetic field in the vertical direction, in a medium with plasma beta of ~1.
MHD RDI: Dust in Magnetized Gas (Example 1): Dust Density
The same simulation as the previous slide, except now showing the dust in a 3D isometric xyz projection. Every point is one of the simulation dust grains, colored by the log of the local density of dust relative to the box-averaged mean.
Magnetic RDIs in a Piece of an HII Region
RDIs in magnetized gas with parameters typical in HII regions, at roughly ~0.1 pc from a bright O star. Box size ~50 au on a side. Dust is drifting mildly super-sonically in a medium with plasma beta of ~10.
Magnetic RDIs in a Piece of an HII Region: Dust Density
The same simulation as the previous slide, now showing the dust in a 3D isometric xyz projection, colored by the log of the local dust density relative to the box-averaged mean.
Magnetic RDIs in a Piece of an HII Region: Larger Box with a Spectrum of Grain Sizes
Like the inner HII region case, but with a larger box (~0.1 pc on a side) and a broad spectrum of grain sizes, a factor of ~100, from grains ~nanometer in size to ~0.1 micron.
Magnetic RDIs in a Piece of an HII Region: How Different Grain Sizes Behave
The same box as the previous, but now showing only the dust grains with color encoding grain size (blue = smallest ~nanometer grains, green = intermediate, red = largest ~0.1 micron grains).
Magnetic RDIs in a Piece of an HII Region: Larger Box with a Spectrum of Grain Sizes (Different Grain Acceleration)
Same box but with grains in the "geometric absorption" limit — acceleration depends on grain size such that terminal velocity becomes independent of grain size. Small grains actually clump more strongly than large grains.
Magnetic RDIs in a Piece of an HII Region: Different Grain Acceleration, Showing Different Grain Size Behavior (palette swap)
Same as the previous, now showing only dust grains with color encoding grain size (blue = smallest, green = intermediate, red = largest).
Magnetic RDIs in a Piece of an HII Region: More Distant, Sub-Sonic Regions
Parameters typical in HII regions but an order-of-magnitude further from the star (~1 pc). The radiation field is weaker and the dust terminal velocity is significantly sub-sonic. Instabilities are not only still present, but lead to more dramatic clumping.
Magnetic RDIs in a Piece of an HII Region: More Distant, Sub-Sonic Regions: Dust Density
The same simulation as the previous slide, now showing the dust in a 3D isometric xyz projection, colored by the log of the local dust density relative to the box-averaged mean.