Hossam AlyNov 13, 2023

It's paper day! Together with
@becnealon and @jfg_astro we've obtained what's easily the coolest result in my career so far

Let me introduce you to WInDI, a new dust instability in warped discs
https://arxiv.org/abs/2311.06182
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WInDI: a Warp-Induced Dust Instability in protoplanetary discs

We identify a new dust instability that occurs in warped discs. The instability is caused by the oscillatory gas motions induced by the warp in the bending wave regime. We first demonstrate the instability using a local 1D (vertical) toy model based on the warped shearing box coordinates and investigate the effects of the warp magnitude and dust Stokes number on the growth of the instability. We then run 3D SPH simulations and show that the instability is manifested globally by producing unique dust structures that do not correspond to gas pressure maxima. The 1D and SPH analysis suggest that the instability grows on dynamical timescales and hence is potentially significant for planet formation.

arXiv.org
Show threadHossam Aly
We start with a bit of theory, building on the seminal work of Ogilvie & Latter 2013 who introduced a warped shearing box coordinate frame to study the local motion of gas in the presence of a disc warp. This allowed to uncover 2 oscillation modes in the gas
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Nov 13, 2023 at 5:31amWeb
Show threadHossam AlyNov 13, 2023
A horizontal mode (dubbed 'sloshing' by Dullemond+22) and a vertical 'breathing' mode. These oscillations are due to the fact that the vertical pressure gradient has an oscillating horizontal component in the presence of a warp
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Show threadHossam AlyNov 13, 2023
We add dust to the equations and build a simple 1D (vertical) model to calculate the gas+dust evolution in this warped frame. We immediately find that, while the gas just oscillates, the dust gets compressed significantly
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Show threadHossam AlyNov 13, 2023
This is shown nicely in this schematic (based on values from the calculation). Top panel shows the thickness of gas (blue) and dust (red). While the gas thickness oscillates, the dust gets compressed. This effect increases with the stokes number and warp magnitude, Figs 3, 4
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Show threadHossam AlyNov 13, 2023
Next, we see if this instability also occurs globally. We run 3D global SPH simulations with different Stokes number. WInDI is triggered globally and grows VERY quickly. Here's what it looks like for Stokes=0.1
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Show threadHossam AlyNov 13, 2023
The rings of high dust density seen in these rings are caused by the combination of the sloshing and breathing motions of the gas acting on the dust. Indeed, the breaks in those rings happen at the phase where the sloshing is minimum
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Show threadHossam AlyNov 13, 2023
WInDI happens for all our simulations at different stokes, but the interpretation gets a bit messy for high stokes, because the warp evolves in the gas faster than the dust can follow, so the dust no longer feels the sloshing (breathing) horizontally (vertically)
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Show threadHossam AlyNov 13, 2023
This is shown very nicely in this particle plot in a cut in the disc for the smallest and largest stokes simulations
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Show threadHossam AlyNov 13, 2023
This can have some real consequences for planet formation. SPH isn't very sensitive to instabilities, the fact we see WInDI so robustly here suggests the instability is very strong and doesn't mind a bit of viscosity (unlike most other dust instabilities)
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Show threadHossam AlyNov 13, 2023
And finally, stay tuned for a lot of follow up work in the pipeline :)
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