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Planet formation bursts at the borders of the dead zone in 2D numerical simulations of circumstellar disks
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
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2009 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 497, no 3, 869-888 p.Article in journal (Refereed) Published
Abstract [en]

Context: As accretion in protoplanetary disks is enabled by turbulent viscosity, the border between active and inactive (dead) zones constitutes a location where there is an abrupt change in the accretion flow. The gas accumulation that ensues triggers the Rossby wave instability, which in turn saturates into anticyclonic vortices. It has been suggested that the trapping of solids within them leads to a burst of planet formation on very short timescales. Aims: We study in the formation and evolution of the vortices in greater detail, focusing on the implications for the dynamics of embedded solid particles and planet formation. Methods: We performed two-dimensional global simulations of the dynamics of gas and solids in a non-magnetized thin protoplanetary disk with the Pencil code. We used multiple particle species of radius 1, 10, 30, and 100 cm. We computed the particles' gravitational interaction by a particle-mesh method, translating the particles' number density into surface density and computing the corresponding self-gravitational potential via fast Fourier transforms. The dead zone is modeled as a region of low viscosity. Adiabatic and locally isothermal equations of state are used. Results: The Rossby wave instability is triggered under a variety of conditions, thus making vortex formation a robust process. Inside the vortices, fast accumulation of solids occurs and the particles collapse into objects of planetary mass on timescales as short as five orbits. Because the drag force is size-dependent, aerodynamical sorting ensues within the vortical motion, and the first bound structures formed are composed primarily of similarly-sized particles. In addition to erosion due to ram pressure, we identify gas tides from the massive vortices as a disrupting agent of formed protoplanetary embryos. We find evidence that the backreaction of the drag force from the particles onto the gas modifies the evolution of the Rossby wave instability, with vortices being launched only at later times if this term is excluded from the momentum equation. Even though the gas is not initially gravitationally unstable, the vortices can grow to Q ≈ 1 in locally isothermal runs, which halts the inverse cascade of energy towards smaller wavenumbers. As a result, vortices in models without self-gravity tend to rapidly merge towards a m = 2 or m =1 mode, while models with self-gravity retain dominant higher order modes (m = 4 or m = 3) for longer times. Non-selfgravitating disks thus show fewer and stronger vortices. We also estimate the collisional velocity history of the particles that compose the most massive embryo by the end of the simulation, finding that the vast majority of them never experienced a collision with another particle at speeds faster than 1 m s-1. This result lends further support to previous studies showing that vortices provide a favorable environment for planet formation.

Place, publisher, year, edition, pages
2009. Vol. 497, no 3, 869-888 p.
Keyword [en]
accretion, accretion disks; hydrodynamics; instabilities; stars: planetary systems: formation; methods: numerical; turbulence
National Category
Physical Sciences
URN: urn:nbn:se:uu:diva-98001DOI: 10.1051/0004-6361/200811265ISI: 000265280500022OAI: oai:DiVA.org:uu-98001DiVA: diva2:173152
Available from: 2009-02-05 Created: 2009-02-05 Last updated: 2011-03-09Bibliographically approved
In thesis
1. Turbulence-Assisted Planetary Growth: Hydrodynamical Simulations of Accretion Disks and Planet Formation
Open this publication in new window or tab >>Turbulence-Assisted Planetary Growth: Hydrodynamical Simulations of Accretion Disks and Planet Formation
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The current paradigm in planet formation theory is developed around a hierarquical growth of solid bodies, from interstellar dust grains to rocky planetary cores. A particularly difficult phase in the process is the growth from meter-size boulders to planetary embryos of the size of our Moon or Mars. Objects of this size are expected to drift extremely rapid in a protoplanetary disk, so that they would generally fall into the central star well before larger bodies can form.

In this thesis, we used numerical simulations to find a physical mechanism that may retain solids in some parts of protoplanetary disks long enough to allow for the formation of planetary embryos. We found that such accumulation can happen at the borders of so-called dead zones. These dead zones would be regions where the coupling to the ambient magnetic field is weaker and the turbulence is less strong, or maybe even absent in some cases. We show by hydrodynamical simulations that material accumulating between the turbulent active and dead regions would be trapped into vortices to effectively form planetary embryos of Moon to Mars mass.

We also show that in disks that already formed a giant planet, solid matter accumulates on the edges of the gap the planet carves, as well as at the stable Lagrangian points. The concentration is strong enough for the solids to clump together and form smaller, rocky planets like Earth. Outside our solar system, some gas giant planets have been detected in the habitable zone of their stars. Their wakes may harbour rocky, Earth-size worlds.

Place, publisher, year, edition, pages
Uppsala: Universitetsbiblioteket, 2009. viii, 102 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 593
accretion, accretion disks, hydrodynamics, instabilities, methods: numerical, solar system: formation, planets and satellites: formation, magnetohydrodynamics (MHD), turbulence, diffusion, stars: planetary systems: formation
National Category
Astronomy, Astrophysics and Cosmology
urn:nbn:se:uu:diva-9537 (URN)978-91-554-7395-2 (ISBN)
Public defence
2009-02-26, Polhemsalem, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 14:00
Available from: 2009-02-05 Created: 2009-02-05Bibliographically approved

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