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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 Photophoresis (… which is based on Thermophoresis) What is moving the light mill? • Radiation pressure ? Nope 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 F~ Force maximum at Kn ~ 1 p (Rohatschek 1995) Photophoresis at low pressure (large Kn) force ~ pressure Photophoresis at high pressure (small Kn) force ~ 1/pressure Radiation This is NOT photophoresis! It is not overall pressure difference 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 Tc = = = <1 nc vc nc Tc pc Tw 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) Irradiance Volume FPh = π a Ip Gas Pressure 3 6kthT Temperature 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) and - thermal radiation from the disk Star Particle Photophoresis Drift Residual Gravity Photophoresis Drift Residual Gravity Ring Formation (1) (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) (2) Problem: This mechanism needs light, so it will not work in the dark center of dense protoplanetary disks. (3) 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 (4) 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 (4) 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) (5) Thermal Photophoresis (Disk) Stellar Photophoresis Residual Gravity Gravity Star 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.) (5) 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 Laser Works for all dark dust samples • graphite • vitreous carbon • basalt • iron oxides • silicon carbide • copper Graphite (<10 µm, 150 mbar) 1 cm (Wurm and Krauss, PRL, 2006; Wurm, MNRAS, 2007) 2cm Works for extended light source (halogen lamp) (2 cm x 0.5 cm here) Temperature Profile, Laser from Top t=10s t=1s t=0.1s 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 Irradiance Gas Temperature Erosion Parameter 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.