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Photophoresis and Planet Formation
Dr. Gerhard Wurm
Institut für Planetologie, Univ. Münster
[email protected]
The magic word for the next hour is
(… which is based on Thermophoresis)
What is moving the light mill?
• Radiation pressure ?
What is moving the light mill?
Something called photophoresis
… but what is this really?
Pressure dependence of photophoresis in experiments
• Light mill does not rotate at 1 bar
• It starts rotating at lower pressure
~10 mbar at given light flux
photophoretic force > friction in needle bearing
• It increases in speed to ~10-2 mbar
• It rotates at constant speed to at least ~10-5 mbar
v = const
tf F = const
tf ~1/p
Force maximum at Kn ~ 1
(Rohatschek 1995)
Photophoresis at low pressure (large Kn)
force ~ pressure
Photophoresis at high pressure (small Kn)
force ~ 1/pressure
This is NOT photophoresis!
It is not overall pressure
This IS photophoresis!
Photophoresis at high pressure
Within the gas, on the large scale there has to be
no mass flow and
pressure is the same everywhere
(v: „diffusion velocity“, many molecule collisions)
Photophoresis at high pressure
nwvw nw Tw pw Tc
nc vc nc Tc
pc Tw
p = nk BT
pw = pc
nwvw < nc vc
Thermal creep:
net mass flow
from cold to warm side
(v: mean thermal velocity)
Photophoresis at high pressure
Some names:
• Thermophoresis: a particle moving in a temperature gradient
• Temperature gradient can be
• within the gas
• within the particle
• between gas and particle (both at different but one temperature)
• Photophoresis: Thermophoresis induced by illumination
Photophoresis (Approximation for LOW pressure)
FPh =
π a Ip
Gas Pressure
Thermal Conductivity
How strong can photophoresis be?
Solar flux
Rohatschek 85
... with solar flux it ...
• can be stronger than Earth‘s gravity (I will show you some experiment soon)
• can be million times stronger than radiation pressure on dust
• can act on at least 10 cm bodies efficiently
Does planet formation care about photophoresis?
The effect is supposed to be used technically
on space station experiment (ICAPS)
to manipulate particle clouds studying aspects
of planet formation.
(see also work by Blum, von Borstel, Steinbach)
... but does it occur
in the early phases of planet formation itself?
What conditions do we need?
- a radiation source (sun light is fine for a light mill)
- some gas (e.g. 10-2 mbar in a light mill)
What conditions do we find in protoplanetary disks?
- some gas (e.g. 10-2 mbar in minimum mass nebula at 1AU)
- star light (inner disk edge, further out in later stages)
- thermal radiation from the disk
Residual Gravity
Residual Gravity
Ring Formation
(Krauss & Wurm, 2005,
Herrmann & Krivov, 2008 (sol. lum.)
Courtesy NASA/JPL-Caltech / T. Pyle
Concentrate and sort chondrules (Wurm & Krauss, 2006)
(complementarity matrix/chondrules can be kept,
if acting at an inner disk edge)
This mechanism needs light, so it will not work in the dark
center of dense protoplanetary disks.
CAIs (calcium aluminum rich inclusions)
• Drifting towards the sun
• they are last to evaporate.
• With other dust gone, the disk is transparent and
• CAIs are pushed back into the disk by photophoresis.
They survive
These disk „holes“ (at least some) still have
significant amounts of gas!
(e.g. TW Hya (Calvet et al. 2002, Rettig et al. 2004))
Courtesy NASA/JPL-Caltech / T. Pyle
Transport of matter to comets
Clearing the disk
Courtesy NASA/JPL-Caltech / T. Pyle
(Krauss & Wurm, ApJ, 2005, Petit et al., LPSC, 2006,
Krauss et al., A&A, 2007, Herrmann & Krivov, A&A, 2008)
But photophoresis not only works with visible radiation
Infrared works just as well (if not better)
Thermal Photophoresis (Disk)
Stellar Photophoresis
Dense Disk
(Wurm & Haack, in prep.)
Known to work in Earth‘s thermal field for stratospheric particles (Beresnev et a. 2003)
Resulting values for
visible photospheric surface / pressure scale height for dust
in the inner 10 AU
H/h = 1 .. 4
(Chiang 2001)
Does photophoresis create the surface of the disk?
(Wurm& Haack, in prep.)
Let‘s assume a dusty body gets out of the shadow
but is too big for photophoresis to move it, e.g.
meter bodies drifting inwards, emerging from the edge.
Close to the star (say 0.1 AU) the irradiance is very high.
(100 solar constants or > 100 kW/m2)
Does the light have any effect?
O.k. the object heats up a bit but otherwise?
Greenhouse Effect and Thermophoresis in Dust Layers
Works for all dark dust
• graphite
• vitreous carbon
• basalt
• iron oxides
• silicon carbide
• copper
(<10 µm, 150 mbar)
1 cm
(Wurm and Krauss, PRL, 2006; Wurm, MNRAS, 2007)
Works for extended light
source (halogen lamp)
(2 cm x 0.5 cm here)
Temperature Profile, Laser from Top
Wurm & Krauss, PRL, 2006
Wurm, MNRAS, 2007
(Solid State) Greenhouse Effect
Thermophoresis on Dust Particles
Wurm & Krauss, PRL, 2006
Wurm, MNRAS, 2007
• Universal
process which disassembles
a dusty body to sub-mm units.
• Requirements are gas, light, and loosly bound dust
Gas Pressure
Gas Temperature
Threshold Value (Continuous erosion, Basalt, 0..100µm)
Is the erosion parameter > 1 under any kind of (protoplanetary) disk condition?
(Wurm, MNRAS, 2007)
(Wurm, MNRAS, 2007)
Courtesy NASA/JPL-Caltech / T. Pyle
In the inner disk …
• Dusty bodies up to planetesimal size drifting (i.e. meter size) or moving
otherwise into the erosion zone lose dust and get detstroyed.
• However, this dust is not lost for the disk but can be recycled.
• This dust can be pushed back into the disk (dust edge) by photophoresis
and radiation pressure or can be sent over the disk.
• This dust might also be collected by exisiting planets and might explain
high mass, gas poor planets (Sato et al. 2005) which supposedly
form in gas starved regions.