Toward Planetesimals: Dense Chondrule Clumps in the Protoplanetary Nebula

Abstract

We outline a scenario which traces a direct path from freely-floating nebula particles to the first 10-100km-sized bodies in the terrestrial planet region, producing planetesimals which have properties matching those of primitive meteorite parent bodies. We call this primary accretion. The scenario draws on elements of previous work, and introduces a new critical threshold for planetesimal formation. We presume the nebula to be weakly turbulent, which leads to dense concentrations of aerodynamically size-sorted particles having properties like those observed in chondrites. The fractional volume of the nebula occupied by these dense zones or clumps obeys a probability distribution as a function of their density, and the densest concentrations have particle mass density 100 times that of the gas. However, even these densest clumps are prevented by gas pressure from undergoing gravitational instability in the traditional sense (on a dynamical timescale). While in this state of arrested development, they are susceptible to disruption by the ram pressure of the differentially orbiting nebula gas. However, self-gravity can preserve sufficiently large and dense clumps from ram pressure disruption, allowing their entrained particles to sediment gently but inexorably towards their centers, producing 10-100 km "sandpile" planetesimals. Localized radial pressure fluctuations in the nebula, and interactions between differentially moving dense clumps, will also play a role that must be allowed for in future studies. The scenario is readily extended from meteorite parent bodies to primary accretion throughout the solar system.

Planet formation bursts at the borders of the dead zone in 2D numerical simulations of circumstellar disks

“…The clumps are held together by self-gravity if the gravitational Weber number (in analogy with the surface tension case) is less than a critical value, close to 1. The gravitational Weber number is defined as the ratio of the drag to self-gravitational accelerations Cuzzi et al (2008) further point that in numerical models, artificial viscosity can largely exaggerate the disrupting effect of the ram pressure. This happens because, as the clumps are small, they are deeply within the viscous range of the grid, whereas in the real solar nebula the dissipation happens at much smaller scales.…”

“…In Sect. 4 we analyze the formation and evolution of the protoplanetary embryos, focusing on stability against erosion (Paraskov et al 2006;Cuzzi et al 2008) and tides from the gas, which we identify as an important disrupting agent. In Sect.…”

Hiding in the Shadows. Ii. Collisional Dust as Exoplanet Markers

“…The well known centimeter and meter barriers (Weidenschilling 1977;Morbidelli et al 2008) along with the fragility of 1 km planetesimals (Leinhardt et al 2009;Nelson & Gressel 2010) will perturb this power law, especially in the case of the centimeter barrier, which can result in mass "piling up" in smaller sizes. Also, if planetesimals are not constructed via dust coagulation, then pathways for the formation of gravitational instability, such as turbulent concentration and streaming instability (e.g., Johansen et al 2006;Cuzzi et al 2008;Bai & Stone 2010;Gressel et al 2011), can circumvent the growth of intermediate-sized objects completely such that Equation (7) no longer holds, even as an approximation.…”

The Emerging Paradigm of Pebble Accretion

“…How planetesimals themselves form is a different topic. Recent popular models hypothesize that planetesimals could form from a population of pebble-sized particles (Youdin and Goodman 2005;Johansen et al 2007;Cuzzi et al 2008). Planetesimal formation and pebble accretion can therefore operate sequentially.…”