AAS 202nd Meeting, May 2003
Session 24 When Do Planets Form?
Topical Oral, Tuesday, May 27, 2003, 8:30-10:00am and 10:45am-12:30pm, 204

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[24.05] The Epoch of Planetesimal Formation

A.N. Youdin (University of California, Berkeley), F.H. Shu (National Tsing Hua University)

The Epoch of Planetesimal Formation

Our research addresses the hypothesis that kilometer sized planetesimals form directly by the gravitational fragmentation of a layer of small (\la cm) midplane solids. The advantage of this mechanism is that one need not rely on uncertain particle sticking efficiencies. It is particularly advantageous to bypass the difficulties of pairwise collisional growth in the meter size range. However small solids are tightly coupled to the gas motions. Any midplane turbulence must therefore be characterized by very low \alpha values to allow efficient settling to the midplane. Thus, for the gravitational instability mechanism, planetesimals should form relatively late in the lifetime of a YSO, e.g. near the boundary between Class II (classical) and Class III (weak-lined) T-Tauri stars. Additionally our model argues for a gradual buildup of the solid/gas ratio of protoplanetary disks which greatly aids particle settling to the midplane. Several mechanisms (bipolar outflows, radial migration, photoionization, and layered accretion) can contribute to this enrichment on relevant 105 to 106 year timescales.

The late formation of planetesimals has implications for the mass of planetary systems and for the metallicity enrichment of gas giant planets and their host stars. The talk will also present recent results on the non-linear development of the two phase (gas and solid) gravitational instability. The focus will be on the timescale of this process and the implications for subsequent phases of planet formation.

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Bulletin of the American Astronomical Society, 35 #3
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