Cascade Model for Planetesimal Formation by Turbulent Clustering

Thomas Hartlep and Jeffrey N. Cuzzi

The Astrophysical Journal
Volume 892, Number 2, 2020 April 1


“We use a newly developed cascade model of turbulent concentration of particles in protoplanetary nebulae to calculate several properties of interest to the formation of primitive planetesimals and to the meteorite record. The model follows, and corrects, calculations of the primary initial mass function (IMF) of planetesimals by Cuzzi et al., in which an incorrect cascade model was used. Here we use the model of Hartlep et al., which has been validated against several published numerical simulations of particle concentration in turbulence. We find that, for a range of nebula and particle properties, planetesimals may be “born big,” formed as sandpiles with diameters in the range 10–100 km, directly from freely floating particles. The IMFs have a modal nature, with a well-defined peak rather than a power-law size dependence. Predictions for the inner and outer parts of the nebula behave similarly in this regard, and observations of primitive bodies in the inner and outer nebula support such modal IMFs. Also, we present predictions of local particle concentrations on several lengthscales in which particles “commonly” find themselves, which have significance for meteoritical observations of the redox state and isotopic fractionation in regions of chondrule formation. An important difference between these results and those of Cuzzi et al. is that particle growth by sticking must proceed to a radius range of at least one to a few centimeters for the IMF and meteoritical properties to be most plausibly satisfied. That is, as far as the inner nebula goes, the predominant “particles” must be aggregates of chondrules (or chondrule-size precursors) rather than individual chondrules themselves.”