arXiv subjects

Gerhard Wurm

Publications and source records attributed to Gerhard Wurm.

At least 73 records · Page 4Linked to original sources

arXiv2013

Ice aggregate contacts at the nm-scale

Aggregation of ice particles is a fundamental process in the interstellar medium as well as in planet formation. Dedicated to study the contact physics of nm-ice particles we developed a thermal gradient force microscope. This allows us and was used to measure pull-off forces with a resolution on the nN-scale and to measure rolling. Furthermore, based on a free probe, it also allows us to study twisting torques for the first time. The experiments show that torques required to twist are significantly larger than that macroscopic models scaled down to the nm-size range would predict. This implies that (ice) aggregates in astrophysical settings with small constituents are more robust against restructuring than previously thought. They likely grow as fractal aggregates to larger size before they restructure and during later compact growth they likely retain a higher porosity during further evolution towards planetesimals.

astro-ph.EP↗

arXiv2013

Radiative forces on macroscopic porous bodies in protoplanetary disks: laboratory experiments

In optically thin parts of protoplanetary disks photophoresis is a significant force not just for dust grains, but also for macroscopic bodies. The absolute strength on the supposedly highly porous objects is not known in detail as yet. We set up a low pressure torsion balance and studied photophoretic forces. We investigated the dependence on plate dimensions and on ambient pressure and considered the influence of channels through the plates. As samples for full (no channel) plates we used tissue with 2mm thickness and circular shape with diameters of 10mm, 30mm and 50mm. The influence of channels was probed on rectangular-shaped circuit boards of 35mm x 35mm area and 1.5mm thickness. The number of channels was 169 and 352. At low pressure, the absolute photophoretic force is proportional to the cross section of the plates. At high pressure, gas flow through the channels enhances the photophoretic force. The pressure dependence of the radiative force can (formally) be calculated by photophoresis on particles with a characteristic length. We derived two characteristic length scales l depending on the plate radius r_1, the channel radius r_2, and the thickness of the plate which equals the length of the channel d as l=r^{0.35} x d^{0.65}. The highest force is found at a pressure p_max = 15 x l^{-1}Pa mm. In total, the photophoretic force on a plate with channels can be well described by a superposition of the two components: photophoresis due to the overall size and cross section of the plate and photophoresis due to the channels, both with their characteristic pressure dependencies. We applied these results to the transport of large solids in protoplanetary disks and found that the influence of porosity on the photophoretic force can reverse the inward drift of large solids, for instance meter-sized bodies, and push them outward within the optically thin parts of the disk.

astro-ph.EP↗

arXiv2013

Accretion through the inner edges of protoplanetary disks by a giant solid state pump

At the inner edge of a protoplanetary disk solids are illuminated by stellar light. This illumination heats the solids and creates temperature gradients along their surfaces. Interactions with ambient gas molecules lead to a radial net gas flow. Every illuminated solid particle within the edge is an individual small gas pump transporting gas inward. In total the inner edge can provide local mass flow rates as high as $\dot{M} = 10^{-5} M_{\odot}$ yr$^{-1}$.

astro-ph.EP↗

arXiv2013

Photophoretic separation of metals and silicates: the formation of Mercury like planets and metal depletion in chondrites

Mercury's high uncompressed mass density suggests that the planet is largely composed of iron, either bound within metal (mainly Fe-Ni), or iron sulfide. Recent results from the MESSENGER mission to Mercury imply a low temperature history of the planet which questions the standard formation models of impact mantle stripping or evaporation to explain the high metal content. Like Mercury, the two smallest extrasolar rocky planets with mass and size determination, CoRoT-7b and Kepler-10b, were found to be of high density. As they orbit close to their host stars this indicates that iron rich inner planets might not be a nuisance of the solar system but be part of a general scheme of planet formation. From undifferentiated chondrites it is also known that the metal to silicate ratio is highly variable which must be ascribed to pre-planetary fractionation processes. Due to this fractionation most chondritic parent bodies - most of them originated in the asteroid belt - are depleted in iron relative to average solar system abundances. The astrophysical processes leading to metal silicate fractionation in the solar nebula are essentially unknown. Here, we consider photophoretic forces. As these forces particularly act on irradiated solids, they might play a significant role for the composition of planetesimals forming at the inner edge of protoplanetary discs. Photophoresis can separate high thermal conductivity materials (iron) from lower thermal conductivity solids (silicate). We suggest that the silicates are preferentially pushed into the optical thick disk. Subsequent planetesimal formation at the edge moving outwards leads to metal rich planetesimals close to the star and metal depleted planetesimals further out in the nebula.

astro-ph.EP↗

arXiv2013

From Planetesimals to Dust: Low Gravity Experiments on Recycling Solids at the Inner Edge of Protoplanetary Disks

Transporting solids of different sizes is an essential process in the evolution of protoplanetary disks and planet formation. Large solids are supposed to drift inward; high-temperature minerals found in comets are assumed to have been transported outward. From low-gravity experiments on parabolic flights we studied the light-induced erosion of dusty bodies caused by a solid-state greenhouse effect and photophoresis within a dust bed's upper layers. The gravity levels studied were 0.16g, 0.38g, 1g, and 1.7g. The light flux during the experiments was 12 +/- 2 kW/m^2 and the ambient pressure was 6 +/- 0.9 mbar. Light-induced erosion is strongly gravity dependent, which is in agreement with a developed model. In particular for small dusty bodies ((sub)-planetesimals), efficient erosion is possible at the optically thin inner edges of protoplanetary disks. Light-induced erosion prevents significant parts of a larger body from moving too close to the host star and be being subsequently accreted. The small dust produced continues to be subject to photophoresis and is partially transported upward and outward over the surface of the disk; the resulting small dust particles observed over the disk's lifetime. The fraction of eroded dust participates in subsequent cycles of growth during planetesimal formation. Another fraction of dust might be collected by a body of planetary size if this body is already present close to the disk edge. Either way, light induced erosion is an efficient recycling process in protoplanetary disks.

astro-ph.EP↗

arXiv2012

Thermal and Photophoretic Properties of Dust Mantled Chondrules and Sorting in the Solar Nebula

Many chondrules are found to be surrounded by a fine grained rim. Supposedly, these rims were acquired from dust accreted to the chondrules in a protoplanetary disk. In numerical simulations we study the heat transfer in illuminated bare and dust mantled chondrules. The calculations consider the chondrule size, the dust mantle size, and the thermal conductivities of both components as parameters. We calculate the photophoretic force and compare the numerical results to analytical approximations. We give an expression to quantify the photophoretic force on a spherical particle in the free molecular regime to better than 2%. We describe the influence of a dust mantle on the photophoretic strength by an effective thermal conductivity of the core-mantle particle. The effective thermal conductivity significantly depends on the size ratio between mantle and chondrule but not on the absolute sizes. It also strongly depends on the thermal conductivity of the mantle with minor influence of the thermal conductivity of the chondrule. The size ratio between rim and chondrule in meteorites is found to vary systematically with overall size by other authors. Based on this, our calculations show that a photophoretic size sorting can occur for dust mantled chondrules in optically thin disks or at the evolving inner edge of the solar nebula.

astro-ph.EP↗

arXiv2012

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

The first macroscopic bodies in protoplanetary disks are dust aggregates. We report on a number of experimental studies with dust aggregates formed from micron-size quartz grains. We confirm in laboratory collision experiments an earlier finding that producing macroscopic bodies by the random impact of sub-mm aggregates results in a well-defined upper-filling factor of 0.31 \pm 0.01. Compared to earlier experiments, we increase the projectile mass by about a factor of 100. The collision experiments also show that a highly porous dust-aggregate can retain its highly porous core if collisions get more energetic and a denser shell forms on top of the porous core. We measure the mechanical properties of cm-sized dust samples of different filling factors between 0.34 and 0.50. The tensile strength measured by a Brazilian test, varies between 1 kPa and 6 kPa. The sound speed is determined by a runtime measurement to range between 80 m/s and 140 m/s while Young's modulus is derived from the sound speed and varies between 7MPa and 25MPa. The samples were also subjected to quasi-static omni- and uni-directional compression todetermine their compression strengths and flow functions. Applied to planet formation, our experiments provide basic data for future simulations, explain the specific collisional outcomes observed in earlier experiments, and in general support a scenario where collisional growth of planetesimals is possible.

astro-ph.EP↗

arXiv2012

Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains

Collisions of mm-size dust aggregates play a crucial role in the early phases of planet formation. We developed a laboratory setup to observe collisions of dust aggregates levitating at mbar pressures and elevated temperatures of 800 K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller than 25 \mum in size. We find that for all impact energies in the studied range sticking occurs at a probability of 32.1 \pm 2.5% on average. In general, the sticking probability decreases with increasing impact parameter. The sticking probability increases with energy density (impact energy per contact area). We also observe collisions of aggregates that were formed by a previous sticking of two larger aggregates. Partners of these aggregates can be detached by a second collision with a probability of on average 19.8 \pm 4.0%. The measured accretion efficiencies are remarkably high compared to other experimental results. We attribute this to the rel. large dust grains used in our experiments, which make aggregates more susceptible to restructuring and energy dissipation. Collisional hardening by compaction might not occur as the aggregates are already very compact with only 54 \pm 1% porosity. The disassembly of previously grown aggregates in collisions might stall further aggregate growth. However, owing to the levitation technique and the limited data statistics, no conclusive statement about this aspect can yet be given. We find that the detachment efficiency decreases with increasing velocities and accretion dominates in the higher velocity range. For high accretion efficiencies, our experiments suggest that continued growth in the mm-range with larger constituent grains would be a viable way to produce larger aggregates, which might in turn form the seeds to proceed to growing planetesimals.

astro-ph.EP↗

arXiv2011

Ice Particles Trapped by Temperature Gradients at mbar Pressure

In laboratory experiments we observe that ice particles (\leq100 μm) entrained in a low pressure atmosphere (~1 mbar) get trapped by temperature gradients between three reservoirs at different tempertature. Confining elements are a peltier element at 250 K (bottom), a liquid nitrogen reservoir at 77 K (top) and the surrounding vacuum chamber at 293 K. Particle levitation and trapping is modeled by an interplay of thermophoresis, photophoresis and gravity. A number of ice particles are trapped simultaneously in close spatial distance to each other at least up to minutes and are accessible for further experiments.

physics.ins-det↗

arXiv2011

A Mechanism to Produce the Small Dust Observed in Protoplanetary Disks

Small (sub)-micron dust is present over the entire lifetime of protoplanetary disks. As aggregation readily depletes small particles, one explanation might be that dust is continuously generated by larger bodies in the midplane and transported to the surface of the disks. In general, in a first step of this scenario, the larger bodies have to be destroyed again and different mechanisms exist with the potential to accomplish this. Possible destructive mechanisms are fragmentation in collisions, erosion by gas drag or light induced erosion. In laboratory experiments we find that the latter, light induced erosion by Knudsen compression and photophoresis, can provide small particles. It might be a preferred candidate as the dust is released into a low particle density region. The working principle of this mechanism prevents or decreases the likelihood for instant re-accretion or re-growth of large dense aggregates. Provided that there is a particle lift, e.g. turbulence, these particles might readily reach the surface of the disk.

astro-ph.EP↗

arXiv2011

Self-sustained Levitation of Dust Aggregate Ensembles by Temperature Gradient Induced Overpressures

In laboratory experiments we observe dust aggregates from 100 μm to 1 cm in size composed of micrometer sized grains levitating over a hot surface. Depending on the dust sample aggregates start to levitate at a temperature of 400 K. Levitation of dust aggregates is restricted to a pressure range between 1--40 mbar. The levitating is caused by a Knudsen compressor effect. Based on thermal transpiration through the dust aggregates the pressure increases between surface and aggregates. Dust aggregates are typically balanced 100 μm over the surface. On a slightly concave surface individual aggregates are trapped at the center. Ensembles of aggregates are confined in a 2D plane. Aggregates are subject to systematic and random translational and rotational motion. The levitated aggregates are well suited to study photophoretic or thermophoretic forces on dust aggregates or the mutual interaction between dust aggregates.

cond-mat.other↗

arXiv2011

The implications of particle rotation on the effect of photophoresis

In a laboratory experiment, water-ice aggregates are trapped in a vacuum chamber at a pressure of 2 mbar due to photophoresis and thermophoresis. The particles are located between a Peltier element at the bottom at 250 K and a reservoir of liquid nitrogen at the top at 77 K. Particle sizes vary between 20 micrometres and a few hundred micrometres. It is found that 95% of all the particles rotate about their vertical axis. A qualitative model is developed which explains why particles should mainly align to and rotate around the vertical. The results imply that rotation does not decrease the vertical strength of photophoretically driven motion in, e.g., protoplanetary discs.

astro-ph.EP↗

arXiv2011

Porosities of Protoplanetary Dust Agglomerates from Collision Experiments

Aggregation of dust through sticking collisions is the first step of planet formation. Basic physical properties of the evolving dust aggregates strongly depend on the porosity of the aggregates, e.g. mechanical strength, thermal conductivity, gas-grain coupling time. Also the outcome of further collisions depends on the porosity of the colliding aggregates. In laboratory experiments we study the growth of large aggregates of $\sim$ 3 mm to 3 cm through continuous impacts of small dust agglomerates of 100 $μ$m size, consisting of $μ$m grains at different impact velocities. The experiments show that agglomerates grow by direct sticking as well as gravitational reaccretion. The latter can be regarded as suitable analog to reaccretion of fragments by gas drag in protoplanetary disks. Experiments were carried out in the velocity range between 1.5 m/s and 7 m/s. With increasing impact velocities the volume filling factor of the resulting agglomerates increases from $ϕ= 0.2$ for 1.5 m/s to $ϕ= 0.32$ for 7 m/s. These values are independent of the target size. Extrapolation of the measured velocity dependence of the volume filling factor implies that higher collision velocities will not lead to more compact aggregates. Therefore, $ϕ= 0.32$ marks a degree of compaction suitable to describe structures forming at $\rm v > 6\, m/s$. At small collision velocities below 1 m/s highly porous structures with $ϕ\approx 0.10$ will form. For intermediate collision velocities porosities vary. Depending on the disk model and resulting relative velocities, objects in protoplanetary disks up to dm-size might evolve from highly porous ($ϕ\approx 0.10$) to compact ($ϕ= 0.32$) with a more complex intermediate size range of varying porosity.

astro-ph.EP↗

arXiv2011

Impact Angle Influence in High Velocity Dust Collisions during Planetesimal Formation

We have examined the influence of impact angle in collisions between small dust aggregates and larger dust targets through laboratory experiments. Targets consisted of \mum-sized quartz dust and had a porosity of about 67%; the projectiles, between 1 and 5 mm in diameter, were slightly more compact (64% porosity). The collision velocity was centered at 20 m/s and impact angles range from 0° to 45°. At a given impact angle, the target gained mass for projectiles smaller than a threshold size, which decreases with increasing angle from about 3 mm to 1 mm. The fact that growth is possible up to the largest angles studied supports the idea of planetesimal formation by sweep-up of small dust aggregates.

astro-ph.EP↗

arXiv2011

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$_2$ 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% $\pm$3%. A part of the projectiles sticks directly. In addition, some projectile fragments slowly return to the target by gravity. All initially porous parts of the surface, i.e. built from the slowly returning fragments, are compacted and firmly attached to the underlying dust layers by the subsequent impacts. Growth is possible at impact angles from 0$^{\circ}$ (central collision) to 70$^{\circ}$. No growth occurs at steeper dust surfaces. We measured the velocity, angle, and size distribution of collision fragments. The average restitution coefficient is 3.8% or 0.29 m/s ejection velocity. Ejecta sizes are comparable to the projectile sizes. The high filling factor is close to the most compact configuration of dust aggregates by local compression ($\sim 33$%). This implies that the history of the surface formation and target growth is completely erased. In view of this, the filling factor of 31% seems to be a universal value in the collision experiments of all self-consistently evolving targets at the given impact velocities. We suggest that decimetre and probably larger bodies can simply be characterised by one single filling factor.

astro-ph.EP↗

arXiv2011

Breaking the Ice: Planetesimal Formation at the Snowline

Recently Saito & Sirono (2011) proposed that large ice aggregates which drift in- wards in protoplanetary disks break up during sublimation, ejecting embedded silicate particles. This would lead to a concentration of small solid particles close to the snow- line. In view of this model we carried out laboratory experiments where we observed freely levitating ice aggregates sublimating. We find that frequent break up is indeed very common. Scaled to a 10 cm aggregate about 2x10^4 small silicate aggregates might result. This supports the idea that sublimation of drifting ice aggregates might locally increase the density of small dust (silicate) particles which might more easily be swept up by larger dust aggregates or trigger gravitational instability. Either way this might locally boost the formation of planetesimals at the snowline.

astro-ph.EP↗

arXiv2011

Low-velocity collisions of centimeter-sized dust aggregates

Collisions between centimeter- to decimeter-sized dusty bodies are important to understand the mechanisms leading to the formation of planetesimals. We thus performed laboratory experiments to study the collisional behavior of dust aggregates in this size range at velocities below and around the fragmentation threshold. We developed two independent experimental setups with the same goal to study the effects of bouncing, fragmentation, and mass transfer in free particle-particle collisions. The first setup is an evacuated drop tower with a free-fall height of 1.5 m, providing us with 0.56 s of microgravity time so that we observed collisions with velocities between 8 mm/s and 2 m/s. The second setup is designed to study the effect of partial fragmentation (when only one of the two aggregates is destroyed) and mass transfer in more detail. It allows for the measurement of the accretion efficiency as the samples are safely recovered after the encounter. Our results are that for very low velocities we found bouncing as could be expected while the fragmentation velocity of 20 cm/s was significantly lower than expected. We present the critical energy for disruptive collisions Q*, which showed up to be at least two orders of magnitude lower than previous experiments in the literature. In the wide range between bouncing and disruptive collisions, only one of the samples fragmented in the encounter while the other gained mass. The accretion efficiency in the order of a few percent of the particle's mass is depending on the impact velocity and the sample porosity. Our results will have consequences for dust evolution models in protoplanetary disks as well as for the strength of large, porous planetesimal bodies.

astro-ph.EP↗

arXiv2011

Dust Ejection from Planetary Bodies by Temperature Gradients: Laboratory Experiments

Laboratory experiments show that dusty bodies in a gaseous environment eject dust particles if they are illuminated. We find that even more intense dust eruptions occur when the light source is turned off. We attribute this to a compression of gas by thermal creep in response to the changing temperature gradients in the top dust layers. The effect is studied at a light flux of 13 kW/(m*m) and 1 mbar ambient pressure. The effect is applicable to protoplanetary disks and Mars. In the inner part of protoplanetary disks, planetesimals can be eroded especially at the terminator of a rotating body. This leads to the production of dust which can then be transported towards the disk edges or the outer disk regions. The generated dust might constitute a significant fraction of the warm dust observed in extrasolar protoplanetary disks. We estimate erosion rates of about 1 kg/s for 100 m parent bodies. The dust might also contribute to subsequent planetary growth in different locations or on existing protoplanets which are large enough not to be susceptible to particle loss by light induced ejection. Due to the ejections, planetesimals and smaller bodies will be accelerated or decelerated and drift outward or inward, respectively. The effect might also explain the entrainment of dust in dust devils on Mars, especially at high altitudes where gas drag alone might not be sufficient.

astro-ph.EP↗