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Debris Discs: Exploring Dust Dynamics and Spatial Structures

This crash course provides an overview of debris discs, their properties, and the techniques used to study them. It covers the composition, structure, and evolution of debris discs, as well as numerical simulations to explain observed spatial structures. The course also explores the potential presence of planets in debris discs.

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Debris Discs: Exploring Dust Dynamics and Spatial Structures

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  1. « Debris » discs A crash course in numerical methods Philippe Thébault Paris Observatory/Stockholm Observatory

  2. « debris disc », what are they? • Definition (Lagrange et al., 2000) • Around Main Sequence Stars • Ldust/L*<<1 • M(dust+gas)<0.01M* • Mgas<<10Mdust (dust dynamics not controled by gas) • Grain Lifetime << Age of the Star • Which means • Evolved System  ProtoPlanetary discs • Planet formation already over • Collisionally eroding system (what we see is NOT primordial stuff)

  3. What do we see?DUST !(<1cm) • Total flux = photometry • One wavelength shows disk is there • Two wavelengths determines dust temperature • Model fitting with multiple wavelengths (Spectral Energy Distribution) • Composition = spectroscopy • Can be used like multiple photometry • Also detects gas and compositional features • Structure = imaging • Give radial structure directly and detects asymmetries • But rare as high resolution and stellar suppression required Scattered light: UV, visible,near-IR Thermal emission:mid-IR,far-IR,mm

  4. « protoplanetary » discs Debris discs Is there a M(t) evolution?

  5. 0.5µm 0.5µm 1-2µm 10-20µm 850µm b-Pictoris: the crowned queen of debris discs

  6. « debris discs » around MSS

  7. Numerical Simulations, why? • What are discs made of? • Size Distribution • Total mass • “hidden” bigger parent bodies (>1cm) OR AND • What is going on? • Explain the Observed Spatial Structures • Presence of Planets?

  8. Numerical simulations are about making the right approximations

  9. collisional cascade ~radiation pressure cutoff unseen parent bodies size distribution ??? ~observational limit Size Distribution/Evolution:the basic problem

  10. Size Distributions derived from observations are model dependent (Li & Greenberg 1998)

  11. ? Theoretical collisional-equilibrium law dN R-3.5dR What we don’t see What we see

  12. Size Distribution/Evolution:Statistical “Particle in a box” Models • Principle • Dust grains distributed in Size Bins (and possibly spatial/velocity bins) • “Collision” rates between all size-bins • Each bini-binj interaction produces a distribution of binl<max(i,j) fragments • Approximations/Simplifications • No (or poor) dynamical Evolution • No (or poor) spatial resolution

  13. How to do it • « Particle in a box » Principle • Collision Outcome prescription (lab.experiments)

  14. a5 a4 da a3 a2 a1 a(1), e(1) High e orbits of grains close to the RPR limit 103yrs 104yrs 105yrs 106yrs 107yrs EXAMPLE: Collisional dust production in debris discs Thébault & Augereau (2006) • Numerical model • Extended Disc: 0-120AU • Size range: 1μm – 50km • Cratering, fragmentation, etc... strong departure from the ”equilibrium” distribution in dN R-3.5dR

  15. Dynamical Evolution ModelsGOAL: explain the observed structures Warps Spirals Offsets Brightness asymmetries Rings & Clumps

  16. Structures could be caused by: • Companion Star perturbation • Encounter with a passing star • Embedded planet(s): torques, resonances,… • Isolated violent event: Cometary breakup? • Complex dust/gas interactions • …

  17. Dynamical Evolution:Deterministic “N-Body” models • Principle • Dust grains represented by “test particles” • Forces: FG-Star(s), FG-planet(s),FRad-Pres, FGas-Drag,… • Step-by-Step Equation of motion integatror:Runge-Kutta, Swift,… • Approximations/Simplifications • Nnumerical~104-105 <<<< Nreal • No Size Distribution

  18. Integrator Example:Runge-Kutta Equation of motion

  19. Wyatt (2005) Augereau & Papaloizou (2004) simulations Passing star 0.2MJup planet at 250AU An Example: HD141569 350AU observations

  20. Another one: Fomalhaut 133AU observations simulations 15AU

  21. warps: A way to get a warp, is to introduce a planet into the disk on an orbit inclined to the disc midplane This causes disc near planet to become aligned with the planet, but that far away keeping the initial symmetry plane HST image of  Pic Model Augereau et al. (2001) Heap et al. (2000)

  22. Indirect detection of planets through debris disc structures (?) Theory tells us: debris discs and (proto)-planets should co-exist…

  23. Wyatt (2006)

  24. An attempt at coupling both approach:“Super-Particles”(Grigorieva, Artymowicz&Thebault,2006) Collisionnal cascade after planetesimal/cometary breakup

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