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
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debris disc modelling
Philippe Thébault
Paris Observatory/Stockholm Observatory
Disc modelling: going from there….
…to there…
…and there
Outline
what is a debris disc?
relative timescales:
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
Debris discs
Greaves (2005)
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
coming out.
Imaging of debris discs: visible/near IR
b-Pictoris (1984)
0.5µm
0.5µm
1-2µm
10-20µm
850µm
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
Thermal emission:mid-IR,far-IR,mm
Deriving dust masses:
sub-mm/mm photometry
what we see
what we’d like to know about
Numerical Modelling, why?
SED
SB profile
Numerical Modelling, what for?
AND
OR
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
collisional cascade
~radiation pressure cutoff
unseen parent bodies
size distribution ???
~observational limit
?
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
a5
a4
da
a3
a2
a1
a(1), e(1)
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
collision rates
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
a-3.5 slope
(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
main problem:
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
in short
”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
equilibrium profile
Disc thickness
Future work?