Gerhard Wurm

High-velocity dust collisions: forming planetesimals in a fragmentation cascade with final accretion

In laboratory experiments we determine the mass gain and loss in central collisions between centimetre- to decimetre-size SiO2 dust targets and submillimetre- to centimetre-size SiO2 dust projectiles of varying mass, size, shape and at different collision velocities up to ∼56.5 m s −1 . Dust projectiles much larger than 1 mm lead to a small amount of erosion of the target but decimetre targets do not break up. Collisions produce ejecta, which are smaller than the incoming projectile. Projectiles smaller than 1 mm are accreted by a target even at the highest collision velocities. This implies that net accretion of decimetre and larger bodies is possible. Independent of the original size of a considered projectile, after several collisions, all fragments will be of submillimetre size which might then be (re)accreted in the next collision with a larger body. The experimental data suggest that collisional growth through fragmentation and reaccretion is a viable mechanism to form planetesimals.

Photophoresis and the Pile-up of Dust in Young Circumstellar Disks

A rapidly growing number of observations reveal ever more structure in young circumstellar disks that are presumed to be forming planetary systems. Prominent features observed are ring-shaped dust distributions with sharp inner edges around stars like the young, main-sequence star HR 4796A. Models aiming to explain the formation of these dust rings by grain migration incorporate radiation pressure of the central star as one shaping force in radial direction. However, the radiometric effect of photophoresis has been ignored, so far, in this context. This effect is based on a radiation-induced temperature gradient on the surface of a particle and the consequential nonuniform interaction with surrounding gas. The resulting force is able to effectively influence the motion of particles in gaseous environments, but so far photophoresis has been limited to applications in the field of aerosol science. Here we present calculations that underline the relevance of the photophoretic force for the dynamics of particles in gas-rich, optically thin circumstellar disks. Depending on the gas pressure, photophoresis can be stronger than radiation pressure, gas drag, and gravity by orders of magnitude. Then the motion of particles ranging in size from 1 μm to 10 m will be dominated by photophoresis. Since the photophoretic force is a function of the gas density, it provides an efficient mechanism for fast radial migration of particles to a definite distance from the star where the gas density reaches a value at which photophoresis is in equilibrium with all other forces at work. By this effect, material is swept out from the inner region of the disk and piled-up in a more or less confined belt around the star. Thus, the formation of ringlike structures of the dust distribution can most naturally be explained without any further assumptions. Since photophoretic pile-up also works for larger bodies, it might even trigger the formation of Kuiper belts.

The photophoretic sweeping of dust in transient protoplanetary disks

Context: Protoplanetary disks start their lives with a dust free inner region where the temperatures are higher than the sublimation temperature of solids. As the star illuminates the innermost particles, which are immersed in gas at the sublimation edge, these particles are subject to a photophoretic force. Aims: We examine the motion of dust particles at the inner edge of protoplanetary disks due to photophoretic drag. Methods: We give a detailed treatment of the photophoretic force for particles in protoplanetary disks. The force is applied to particles at the inner edge of a protoplanetary disk and the dynamical behavior of the particles is analyzed. Results: We find that, in a laminar disk, photophoretic drag increases the size of the inner hole after accretion onto the central body has become subdued. This region within the hole becomes an optically transparent zone containing gas and large dusty particles (>>10 cm), but devoid of, or strongly depleted in, smaller dust aggregates. Photophoresis can clear the inner disk of dust out to 10 AU in less than 1 Myr. The details of this clearance depend on the size distribution of the dust. Any replenishment of the dust within the cleared region will be continuously and rapidly swept out to the edge. At late times, the edge reaches a stable equilibrium between inward drift and photophoretic outward drift, at a distance of some tens of AU. Eventually, the edge will move inwards again as the disk disperses, shifting the equilibrium position back from about 40 AU to below 30 AU in 1-2 Myr in the disk model. In a turbulent disk, diffusion can delay the clearing of a disk by photophoresis. Smaller and/or age-independent holes of radii of a few AU are also possible outcomes of turbulent diffusion counteracting photophoresis. Conclusions: This outward and then inward moving edge marks a region of high dust concentration. This density enhancement, and the efficient transport of particles from close to the star to large distances away, can explain features of comets such as high measured ratios of crystalline to amorphous silicates, and has a large number of other applications.

On the Importance of Gas Flow through Porous Bodies for the Formation of Planetesimals

Planetesimals and their precursors in protoplanetary disks are very porous. Thus, a gas flow around such bodies will be accompanied by gas flow through them. We calculate how this gas flow will influence the impact of a small body on a body larger than 1 m in size. On the front side of a large body (target) with high porosity there is a boundary layer that is characterized by a gas flow toward the surface. We find that under typical conditions with respect to collisions in protoplanetary disks, fragments of a collision will stay inside this boundary layer. These fragments will return to the target by gas drag. Net growth of the larger body in these secondary collisions will occur. The mechanism works for all sizes up to planetesimal size. This supports the idea that planetesimals (kilometer-sized bodies) build up from collisions of smaller bodies. Subject headings: hydrodynamics — planetary systems: protoplanetary disks — planets and satellites: formation — solar system: formation

Experiments on the photophoretic motion of chondrules and dust aggregates-Indications for the transport of matter in protoplanetary disks

In a set of 16 drop tower experiments the motion of sub-millimeter to millimeter-sized particles under microgravity was observed. Illumination by a halogen lamp induced acceleration of the particles due to photophoresis. Photophoresis on dust-free chondrules, on chondrules, glass spheres and metal spheres covered with SiC dust and on pure SiC dust aggregates was studied. This is the first time that photophoretic motion of millimeter-sized particles has been studied experimentally. The absolute values for the photophoretic force are consistent with theoretical expectations for spherical particles. The strength of the photophoretic force varies for chondrules, dust covered particles and pure dust from low to strong, respectively. The measurements support the idea that photophoresis in the early Solar System can be efficient to transport solid particles outward.

Decimetre dust aggregates in protoplanetary discs

The growth of planetesimals is an essential step in planet formation. Decimetre-size dust agglomerates mark a transition point in this growth process. In laboratory experiments we simulated the formation, evolution, and properties of decimetre-scale dusty bodies in protoplanetary discs. Small sub-mm size dust aggregates consisting of micron-size SiO

An Experimental Study on the Structure of Cosmic Dust Aggregates and Their Alignment by Motion Relative to Gas

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PHOTOPHORETIC SEPARATION OF METALS AND SILICATES: THE FORMATION OF MERCURY-LIKE PLANETS AND METAL DEPLETION IN CHONDRITES

particles randomly interacted with dust targets of varying initial conditions in a continuous sequence of independent collisions. Impact velocities were 7.7 m/s on average and in the range expected for collisions with decimetre bodies in protoplanetary discs. The targets all evolved by forming dust \emph{crusts} with up to several cm thickness and a unique filling factor of 31%

Experiments on centimeter-sized dust aggregates and their implications for planetesimal formation

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Self-sustained levitation of dust aggregate ensembles by temperature-gradient-induced overpressures.

3%. A part of the projectiles sticks directly. In addition, some projectile fragments slowly return to the target by gravity. All initially porous parts of the surface, i.e. built from the slowly returning fragments, are compacted and firmly attached to the underlying dust layers by the subsequent impacts. Growth is possible at impact angles from 0