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Aggregation of dust through sticking collisions is the first step of planetformation. Basic physical properties of the evolving dust aggregates stronglydepend on the porosity of the aggregates, e.g. mechanical strength, thermalconductivity, gas-grain coupling time. Also the outcome of further collisionsdepends on the porosity of the colliding aggregates. In laboratory experimentswe study the growth of large aggregates of $\sim$ 3 mm to 3 cm throughcontinuous impacts of small dust agglomerates of 100 $\mu$m size, consisting of$\mu$m grains at different impact velocities. The experiments show thatagglomerates grow by direct sticking as well as gravitational reaccretion. Thelatter can be regarded as suitable analog to reaccretion of fragments by gasdrag in protoplanetary disks. Experiments were carried out in the velocityrange between 1.5 m/s and 7 m/s. With increasing impact velocities the volumefilling factor of the resulting agglomerates increases from $\phi = 0.2$ for1.5 m/s to $\phi = 0.32$ for 7 m/s. These values are independent of the targetsize. Extrapolation of the measured velocity dependence of the volume fillingfactor implies that higher collision velocities will not lead to more compactaggregates. Therefore, $\phi = 0.32$ marks a degree of compaction suitable todescribe structures forming at $\rm v > 6\, m/s$. At small collision velocitiesbelow 1 m/s highly porous structures with $\phi \approx 0.10$ will form. Forintermediate collision velocities porosities vary. Depending on the disk modeland resulting relative velocities, objects in protoplanetary disks up todm-size might evolve from highly porous ($\phi \approx 0.10$) to compact ($\phi= 0.32$) with a more complex intermediate size range of varying porosity.

Porosities of Protoplanetary Dust Agglomerates from Collision Experiments