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Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..792t&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Other
Scientific paper
Introduction Although it is clear that dust particles embedded in a protoplanetary disk have grown to planetesimals, kmsize bodies, the growth mechanism is still an unsolved problem. Theoretical and experimental studies showed that the first growth phase from dust to cm-size aggregates happens on short time scales and is well understood so far [1][2]. With increasing aggregate sizes dust agglomerates become more compact and collision velocities are getting larger. In a laminar Solar Nebula with some turbulence added these velocities reach values of ~60 m/s for m-size objects[3]. In this parameter range collisions lead to compaction and fragmentation of the aggregates. For fractal aggregates fragmentation has already been observed for collision velocities of ~ 1 m/s [1] whereas more compact bodies just bounce of each other up to several m/s [4][5]. Nevertheless growth has been observed in collision experiments with compact mm-size projectiles and larger targets of the same size at velocities between 13 m/s and 25 m/s [5]. The experiments described in this work were performed to determine under which conditions growth is possible with velocities up to ~60 m/s. Results We present laboratory experiments in which we determine the mass gain and loss in central collisions between mm to cm-size SiO2 dust projectiles and cm to dm size targets of the same material. Experiments were performed with different projectile and target masses and covered a velocity range from 25 m/s up to 53 m/s. In this work we show that under certain conditions net growth in this parameter range is possible. Collisions in this parameter range can be described by a three step model. (1) In the first step the projectile always fragments into smaller pieces and some ten percent of the projectile mass stick to the target material and leads to a mass gain of the target in form of a cone sticking to the target surface. (2) In the second step some target material also is ejected and there is some near field damage around the direct impact site. This near field damage can range from a thin circular trench around the sticking projectile material to a crater which also includes the impact site as material loss. Figure 1 gives an example for a collision with near field damage leading to mass loss. The balance between these two processes determines the mass balance of a collision. (3) When the impact energies are too high a large area of the target surface can be eroded away. This far field damage depends on the target size and thus is not important for realistic conditions. Due to the size dependance of the balance between near and far field damage small projectiles can lead to a net growth even with high velocities. Conclusions Based on experimental results we present a growth model which can be a solution to the question how planetesimal growth is possible once dust aggregates are compact and velocities tend to be destructive. Projectiles of a few mm in size hit a larger aggregate with a size of ~30 cm and leave a crater in the target surface which leads to a slight mass loss. In this colision the projectiles fragment into smaller pieces whereas the target mass hardly changes. In processes of this kind a large amount of sub-mm particles is formed which can lead to a mass gain in a following collisions. Although fragmentation plays a significant role in the high velocity range planetesimal growth via mutual collisions is possible. References [1] Blum, J. and Wurm, G. (2008) ARAA, in press. [2] Dominik, C. and Tielens, A. G. G. M. (1997) ApJ,480, 647-673. [3] Weidenschilling, S. J. and Cuzzi, J. N. (1993) in Levy, E. H. and Lunine, J. I., Protostars and Planets III, 1031-1060. [4] Blum, J. and Münch, (1993) Icarus, 106, 98-116. [5] Wurm, G. et al. (2005) Icarus, 178, 253-263.
Teiser Jens
Wurm Gerhard
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