The pebble accretion model for planet formation: understanding thermal and chemical histories of astrophysical pebbles

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In the theory of planet formation, one of the biggest unknowns is how small pebbles, that is, particles of roughly millimeter to centimeter size, coagulate to form the initial planetary building blocks (planetesimals). A new model called "pebble accretion" might be able to solve this problem. In the model, the gas in the protoplanetary disk slows the pebbles around a planetesimal by frictional drag. The slowed pebbles gently accrete onto the planetesimal instead of hitting at high speed and fragmenting. However, there many unknowns in the model, and nobody knows how well it will work. This project will refine the pebble accretion model by studying the chemical changes that occur in pebbles as they move through the gas in the accretion disk, and by accounting for the thermodynamics of the planetesimal. Success of the proposed work will give a better understanding of how planetary systems form, including our own Solar System. The team will compare their results to Solar System bodies, as well as observations of planetary accretion disks in other solar systems (where planets are currently forming) by the Atacama Large Millimeter/Sub-Millimeter Array (ALMA) facility.This program focuses on numerical hydrodynamics simulations of the pebble accretion process, initially with two separate efforts that are later combined. One effort will incorporate the chemical evolution of pebbles as they move through the gas in the accretion disk. The other will incorporate realistic thermodynamics for the planetesimal and surrounding proto-atmosphere, as well as the effects of atmospheric drag on the infalling pebbles. The team will separately produce global numerical simulations of pebble accretion which track the history and chemical evolution of all solids accreted by the planet. Finally, the team will combine both approaches into one self-consistent model and use the results to develop synthetic observations that they can directly compare to ALMA observations of accretion disks and sub-disk structures.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.