[Baps] Wettlaufer talk on planet formation

[Sarah Stewart-Mukhopadhyay]

Harvard Applied Mechanics Colloquium

Collisional Cosmogony

John Wettlaufer
Yale University

Wednesday, October 13, 4 pm
Pierce Hall Room 209

Abstract
The formation of a solar system such as ours is believed to have
followed a multi-stage process around a protostar and its associated
accretion disk.  Whipple first noted that
planetesimal growth by particle agglomeration is strongly influenced
by gas drag, and Cuzzi and colleagues have shown that when midplane
particle mass densities approach or exceed those of the gas, solid-
solid interactions dominate the drag effect.  The size dependence of
the drag creates a ``bottleneck'' at  the meter scale with such bodies
rapidly spiraling into the central star, whereas much smaller or
larger particles do not.    Independent of whether the origin of the
drag is angular momentum exchange with gas or solids in the disk,
successful planetary accretion requires rapid planetesimal rapid
growth to km scales.  A commonly accepted picture is that for
collisional velocities Vc above a certain threshold value, Vth ~
0.1-10 cm/s, particle agglomeration is not possible; elastic rebound
overcomes attractive surface and intermolecular forces. However, if
perfect sticking is assumed for all ranges of interparticle collision
speeds the bottleneck can be overcome by rapid planetesimal growth.
While previous work has dealt with the influences of collisional
pressures and the possibility of particle fracture or penetration, the
basic role of the phase behavior of matter--phase diagrams, amorphs
and polymorphs--has been neglected.  I discuss that novel aspects of
surface phase transitions provide a physical basis for efficient
sticking through collisional melting/amphorphization/polymorphization
and subsequent fusion/annealing to extend the collisional velocity
range of primary accretion (1-100 m/s), which encompasses both typical
turbulent RMS speeds and the velocity differences between boulder
sized and small grains (1-50 m/s).  Therefore, as inspiraling meter
sized bodies collide with smaller particles in this high velocity
collisional fusion regime they grow sufficiently rapidly to km scale
and settle into stable Keplerian orbits in ~ 10^5 years before stellar
wind clears the disk of source material. The basic theory applies to
low and high melting temperature materials and thus to the inner and
outer regions of a nebula.

https://www.seas.harvard.edu/news-events/calendars/applied_mechanics_colloquia
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