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Unformatted text preview: arXiv:0807.1114v2 [astro-ph] 11 Jul 2008 Physics and Astrophysics of Planetary Systems, Les Houches 2008 Editors : will be set by the publisher EAS Publications Series, Vol. ?, 2008 FROM GRAINS TO PLANETESIMALS Andrew N. Youdin 1 Abstract. This pedagogical review covers an unsolved problem in the theory of protoplanetary disks: the growth of dust grains into plan- etesimals, solids at least a kilometer in size. I summarize timescale constraints imposed on planetesimal formation by circumstellar disk observations, analysis of meteorites, and aerodynamic radial migra- tion. The infall of lessorsimilar meter-sized solids in a hundred years is the most stringent constraint. I review proposed mechanisms for plan- etesimal formation. Collisional coagulation models are informed by laboratory studies of microgravity collisions. The gravitational collapse (or Safronov-Goldreich-Ward) hypothesis involves detailed study of the interaction between solid particles and turbulent gas. I cover the ba- sics of aerodynamic drag in protoplanetary disks, including radial drift and vertical sedimentation. I describe various mechanisms for particle concentration in gas disks – including turbulent pressure maxima, drag instabilities and long-lived anticylonic vortices. I derive a general result for the minimum size for a vortex to trap particles in a sub-Keplerian disk. Recent numerical simulations demonstrate that particle clumping in turbulent protoplanetary disks can trigger gravitational collapse. I discuss several outstanding issues in the field. 1 Introduction This chapter serves as a pedagogical introduction to planetesimal formation and related processes in protoplanetary disks. I defer to Dominik et al. (2007) for a comprehensive review of research in this rapidly progressing field. Section 2 serves as an extended introduction by discussing timescale constraints from disk obser- vations, meteoritics, and dynamics. The purposeful overlap with other chapters in this volume aims to reinforce the overlap between sub-fields. Section 3 describes formation mechanisms: collisional coagulation ( § 3.1) and gravitational collapse Thanks to Theirry Montmerle and everyone at the Laboratoire d’Astrophysique de l’Observatoire de Grenoble for a thoroughly enoyable and rewarding school. 1 C.I.T.A., University of Toronto, 60 St. George St., Toronto ON M5S 3H8 CANADA c circlecopyrt EDP Sciences 2008 DOI: (will be inserted later) 2 Physics and Astrophysics of Planetary Systems, Les Houches 2008 ( § 3.2). Section 4 covers the dynamics of gas drag in protoplanetary disks, in- cluding the basic form of drag forces ( § 4.1), vertical settling and radial migration ( § 4.2), and particle trapping mechanisms ( § 4.3)....
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