Size and Temperature Distribution of Planetesimals

Abstract:

During the earlier stages of accretion, the mass distribution of planetesimals evolved through a discontinuous process of collisions (merging of planetesimals). In between collisions, the internal temperature of the planetesimals varied continuously by radioactive heat release, mostly the decay of 36Al, and radiation at their surface. The collisions were also accompanied by a partial release of the kinetics energy of the impactor in the target planetesimal. Although the distribution of planetesimal sizes have been previously discussed, this is not the case of their temperatures. In this presentation we will present a statistical model able to capture the distribution of planetesimal temperatures and masses as a function of time. Even in simulations where the accretion is rapid enough so that in the temperature-mass distribution, large and molten planetesimals (with radii larger than 50 km formed in less than 500 kyrs) are present, the average planetesimals remain often small and cold (with radii smaller than 5 km and temperature of only a few 100 K). The swarm of planetesimals contains indeed a large number of small objets that cannot retain the heat produced by radioactivity. This points to a necessary very fast accretion (50 km planetesimals formed in less than 100 kyrs) to melt the majority of planetesimals.