debris disc modelling. Philippe Thébault Paris Observatory/Stockholm Observatory. Disc modelling: going from there…. …to there…. …and there. Outline. I . Observational data: the need for modelling II . Size and spatial distributions of debris discs III . Collisional avalanches
debris disc modelling
Paris Observatory/Stockholm Observatory
Disc modelling: going from there….
what is a debris disc?
collisions are the dominant process!
Example: β-Pic (Artymowicz, 1997)
protoplanetary discs around YSOs
Young (<107yrs) and massive (~100M) discs, with Mgas/Mdust~100
« protoplanetary » discs
Discoveries of debris discs, the IR trilogy: IRAS/ISO/Spitzer
IRAS (ESA, 1983): all sky survey.
Resolution~0.5’-2’. First IR-excess detection: Vega (“vega-type” stars). ~170 IR-excess detections => ~15% of stars with debris discs
ISO (ESA, 1996): pointed telescope.
Resolution~1.5”-90”. 22 new detections. ~17% of stars with discs. Spectral type dependancy: A:40%, F:9%, G:19%, K:8%.
Spitzer (NASA, 2003): pointed telescope, currently operating.
Resolution~1”-10”. Truckload of new results
Imaging of debris discs: visible/near IR
β-Pictoris: multi wavelength imaging
the debris disc zoo
and some more…
Circumstellar disc observations:
wavelength vs radius probed
Circumstellar discs have been studied at all wavelengths from optical to cm
Different wavelengths probe different locations in the disc; e.g., thermal emission from an optically thin disc, assuming black body grains:
Tdust = 278.3 L*0.25/r0.5
peak = 2898m/T = 10.4r0.5/L*0.25
rprobed = 0.012L*0.5 AU
NIR=0.1AU, MIR=1AU, FIR=30AU, SUB=1000AU (though smaller as not observed at peak)
What do we se? DUST (<1cm)
Scattered light: UV, visible,near-IR
Deriving dust masses:
what we see
what we’d like to know about
Numerical Modelling, why?
Numerical Modelling, what for?
II. Collisional Evolution models
Size Distributions derived from observations are model dependent…
(Li & Greenberg 1998)
what we see
...and restricted to a narrow size range
~radiation pressure cutoff
unseen parent bodies
size distribution ???
doing it the lazy way: the drr3.5dr distribution
Theoretical collisional-equilibrium law dN r-3.5dr
What we don’t see
What we see
many reasons why the dNr3.5dr distribution just doesn’t work
rmaxbecause finite mass
Size Distribution/Evolution:Statistical “Particle in a box” Models
statistical collisional evolution code
High e orbits of grains close to the RPR limit
multi-annulus collisional code (Thebault&Augereau, 2007)
evolution of an extended debris disc
size distribution evolution (Thebault&Augerau, 2007)
(2) overabundance due to the lack of smaller potential impacotrs
(1) Lack of grains< RPR
(3) Depletion due to the overabundance in (2)
(4) Overdensity due to lack of (3), etc…
cutoff size RPR
the ”wavy” size distribution
the ”magical” tcol=(Ωτ)-1formula can also be *very* wrong
why should we care? / Main Conclusions
Be careful when reconstructing ”asteroid belts”
Main Conclusions (2)
III. Collisional avalanches
Grigorieva, Thebault&Artymowicz 2007
Could clumpy or spiral structures be explained by transient violent collisional events?
collisional avalanches: combined statistical and dynamical code
Collisionnal cascade after large planetesimal/cometary breakup
collisional avalanches: face-on spiral structures
collisional avalanches: two-side asymetries
IV. Outer edges of debris discs:
how sharp is sharp?
Outer edges: ”natural” collisional evolution of a narrow ring left to itself
(Thébault & Wu, 2008)
processes at play
the ”universal” a-3.5profile
low mass disc
high mass disc
”extreme” case with SB profile steeper than a-3.5
dynamically ”cold” system: e=2i < 0.01
how likely is it?
high-β grains production is unefficient (low v among parent bodies) while high-β grains destruction is still very efficient (high v among small grains) => Depletion of β>0.1 grains
fitting a ”razor-sharp” edge: HR4796
”natural” outer edge profile
”natural” if highly anisotropic scattering
”natural” only if e<0.01...otherwise, need for ”something” else to act
V. Vertical structure of debris discs (work in progress)
if equipartition: H/a ~ 2<i>~<e> ~ <dV>/VKep
ex: H/a=0.1 => <dV>~ 450m/s
=> Vesc(RBIG) ~ 450m/s => RBIG~500km
a numerical experiment
Q: how do mutual collisions redistribute orbital elements for a population of grains affected by Radiation Pressure?
Thébault & Brahic (1995)
thickening of an initially thin disc