<div style="text-align: center;">Harvard Applied Mechanics Colloquium</div><div style="text-align: center;"><br></div><div style="text-align: center;">Collisional Cosmogony</div><div style="text-align: center;"><br></div><div style="text-align: center;">
John Wettlaufer</div><div style="text-align: center;">Yale University</div><div style="text-align: center;"><br></div><div style="text-align: center;">Wednesday, October 13, 4 pm</div><div style="text-align: center;">Pierce Hall Room 209</div>
<br>Abstract<br>The formation of a solar system such as ours is believed to have <br>followed a multi-stage process around a protostar and its associated <br>accretion disk. Whipple first noted that<br>planetesimal growth by particle agglomeration is strongly influenced <br>
by gas drag, and Cuzzi and colleagues have shown that when midplane <br>particle mass densities approach or exceed those of the gas, solid- <br>solid interactions dominate the drag effect. The size dependence of <br>the drag creates a ``bottleneck'' at the meter scale with such bodies <br>
rapidly spiraling into the central star, whereas much smaller or <br>larger particles do not. Independent of whether the origin of the <br>drag is angular momentum exchange with gas or solids in the disk, <br>successful planetary accretion requires rapid planetesimal rapid <br>
growth to km scales. A commonly accepted picture is that for <br>collisional velocities Vc above a certain threshold value, Vth ~ <br>0.1-10 cm/s, particle agglomeration is not possible; elastic rebound <br>overcomes attractive surface and intermolecular forces. However, if <br>
perfect sticking is assumed for all ranges of interparticle collision <br>speeds the bottleneck can be overcome by rapid planetesimal growth. <br>While previous work has dealt with the influences of collisional <br>pressures and the possibility of particle fracture or penetration, the <br>
basic role of the phase behavior of matter--phase diagrams, amorphs <br>and polymorphs--has been neglected. I discuss that novel aspects of <br>surface phase transitions provide a physical basis for efficient <br>sticking through collisional melting/amphorphization/polymorphization <br>
and subsequent fusion/annealing to extend the collisional velocity <br>range of primary accretion (1-100 m/s), which encompasses both typical <br>turbulent RMS speeds and the velocity differences between boulder <br>sized and small grains (1-50 m/s). Therefore, as inspiraling meter <br>
sized bodies collide with smaller particles in this high velocity <br>collisional fusion regime they grow sufficiently rapidly to km scale <br>and settle into stable Keplerian orbits in ~ 10^5 years before stellar <br>
wind clears the disk of source material. The basic theory applies to <br>low and high melting temperature materials and thus to the inner and <br>outer regions of a nebula.<br><br><a href="https://www.seas.harvard.edu/news-events/calendars/applied_mechanics_colloquia">https://www.seas.harvard.edu/news-events/calendars/applied_mechanics_colloquia</a><br>