From: arXiv.org e-Print archive
Posted: Friday, January 23, 2015
Daniel Carrera, Anders Johansen, Melvyn B. Davies
(Submitted on 21 Jan 2015)
The size distribution of asteroids and Kuiper belt objects in the solar system is difficult to reconcile with a bottom-up formation scenario due to the observed scarcity of objects smaller than ∼100 km in size. Instead, planetesimals appear to form top-down, with large 100-1000 km bodies forming from the rapid gravitational collapse of dense clumps of small solid particles. In this paper we investigate the conditions under which solid particles can form dense clumps in a protoplanetary disk.
We use a hydrodynamic code to model the interaction between solid particles and the gas inside a shearing box inside the disk, considering particle sizes from sub-millimeter-sized chondrules to meter-sized rocks. We find that particles down to millimeter sizes can form dense particle clouds through the run-away convergence of radial drift known as the streaming instability. We make a map of the range of conditions (strength of turbulence, particle mass-loading, disk mass, and distance to the star) which are prone to producing dense particle clumps.
Finally, we estimate the distribution of collision speeds between mm-sized particles. We calculate the rate of sticking collisions and obtain a robust upper limit on the particle growth timescale of ~105 years. This means that mm-sized chondrule aggregates can grow on a timescale much smaller than the disk accretion timescale (~106−107 years). Our results suggest a pathway from the mm-sized grains found in primitive meteorites to fully formed asteroids.
We speculate that asteroids may form from a positive feedback loop, where coagulation produces chondrule aggregates, that experience particle clumping, while in turn particle clumping reduces collision speeds and enhances coagulation. Future simulations should model coagulation and the streaming instability together to explore this feedback loop further.
Comments:18 pages. Submitted to A&A. Comments welcome
Subjects:Earth and Planetary Astrophysics (astro-ph.EP)
Cite as:arXiv:1501.05314 [astro-ph.EP] (or arXiv:1501.05314v1 [astro-ph.EP] for this version)
From: Daniel Carrera
[v1] Wed, 21 Jan 2015 21:00:15 GMT (6395kb,D)
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