Previous work on protoplanetary dust growth shows halt at centimeter sizes owing to the occurrence of bouncing at velocities of \stackrel { > } { \sim } 0.1 \mathrm { m \leavevmode \nobreak s ^ { -1 } } and fragmentation at velocities \stackrel { > } { \sim } 1 \mathrm { m \leavevmode \nobreak s ^ { -1 } } .
To overcome these barriers , spatial concentration of cm-sized dust pebbles and subsequent gravitational collapse have been proposed .
However , numerical investigations have shown that dust aggregates may undergo fragmentation during the gravitational collapse phase .
This fragmentation in turn changes the size distribution of the solids and thus must be taken into account in order to understand the properties of the planetesimals that form .
To explore the fate of dust pebbles undergoing fragmenting collisions , we conducted laboratory experiments on dust-aggregate collisions with a focus on establishing a collision model for this stage of planetesimal formation .
In our experiments , we analysed collisions of dust aggregates with masses between 1.4 g and 180 g , mass ratios between target and projectile from 125 to 1 at a fixed porosity of 65 % , within the velocity range of 1.5–8.7 \mathrm { m \leavevmode \nobreak s ^ { -1 } } , at low atmospheric pressure of \sim 10 ^ { -3 } mbar and in free-fall conditions .
We derived the mass of the largest fragment , the fragment size/mass distribution , and the efficiency of mass transfer as a function of collision velocity and projectile/target aggregate size .
Moreover , we give recipes for an easy-to-use fragmentation and mass-transfer model for further use in modeling work .
In a companion paper , we utilize the experimental findings and the derived dust-aggregate collision model to investigate the fate of dust pebbles during gravitational collapse .