Inside-out Planet Formation. IV. Pebble Evolution and Planet Formation Timescales

Xiao Hu (胡晓)1,2, Jonathan C. Tan1,3, Zhaohuan Zhu (朱照寰)2, Sourav Chatterjee4, Tilman Birnstiel5, Andrew N. Youdin6, and Subhanjoy Mohanty7

Astrophysical Journal 857, 20 Link to Article [DOI: 10.3847/1538-4357/aaad08]
1Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
2Department of Physics and Astronomy, University of Nevada, Las Vegas, 4505 South Maryland Parkway, Las Vegas, NV 89154, USA
3Department of Physics, University of Florida, Gainesville, FL 32611, USA
4Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
5University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, D-81679 Munich, Germany
6Department of Astronomy and Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
7Imperial College London, 1010 Blackett Lab, Prince Consort Rd., London SW7 2AZ, UK

Systems with tightly packed inner planets (STIPs) are very common. Chatterjee & Tan proposed Inside-out Planet Formation (IOPF), an in situ formation theory, to explain these planets. IOPF involves sequential planet formation from pebble-rich rings that are fed from the outer disk and trapped at the pressure maximum associated with the dead zone inner boundary (DZIB). Planet masses are set by their ability to open a gap and cause the DZIB to retreat outwards. We present models for the disk density and temperature structures that are relevant to the conditions of IOPF. For a wide range of DZIB conditions, we evaluate the gap-opening masses of planets in these disks that are expected to lead to the truncation of pebble accretion onto the forming planet. We then consider the evolution of dust and pebbles in the disk, estimating that pebbles typically grow to sizes of a few centimeters during their radial drift from several tens of astronomical units to the inner, lesssim1 au scale disk. A large fraction of the accretion flux of solids is expected to be in such pebbles. This allows us to estimate the timescales for individual planet formation and the entire planetary system formation in the IOPF scenario. We find that to produce realistic STIPs within reasonable timescales similar to disk lifetimes requires disk accretion rates of ~10−9 M yr−1 and relatively low viscosity conditions in the DZIB region, i.e., a Shakura–Sunyaev parameter of α ~ 10−4.

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