PARTICLE CLUMPING AND PLANETESIMAL FORMATION DEPEND STRONGLY ON METALLICITY

By Anders Johansen, A. Youdin, M. Mac Low. 2009

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We present three-dimensional numerical simulations of particle clumping and planetesimal formation in protoplanetary disks with varying amounts of solid material. As centimeter-size pebbles settle to the mid-plane, turbulence develops through vertical shearing and streaming instabilities. We find that when the pebble-to-gas column density ratio is 0.01, corresponding roughly to solar metallicity, clumping is weak, so the pebble density rarely exceeds the gas density. Doubling the column density ratio leads to a dramatic increase in clumping, with characteristic particle densities more than 10 times the gas density and maximum densities reaching several thousand times the gas density. This is consistent with unstratified simulations of the streaming instability that show strong clumping in particle-dominated flows. The clumps readily contract gravitationally into interacting planetesimals on the order of 100 km in radius. Our results suggest that the correlation between host star metallicity and exoplanets may reflect the early stages of planet formation. We further speculate that initially low-metallicity disks can be particle enriched during the gas dispersal phase, leading to a late burst of planetesimal formation.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/f018ce7c20ac10f4d46e209b10293564303c8ed0

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Citations in the corpus

Concentrating small particles in protoplanetary disks through the streaming instability (2016, methodology): “The former includes long-lived local pressure maxima (Whipple 1972; Johansen et al. 2009a; Bai & Stone 2014) or vortices (Barge & Sommeria 1995; Bracco et al. 1999; Klahr & Hubbard 2014; Lyra 2014), in which solids passively follow the underlying flow of gas by friction.”
FORMATION OF KUIPER BELT BINARIES BY GRAVITATIONAL COLLAPSE (2010, methodology): “We do not attempt to extract our initial data from the Johansen et al. (2007, 2009) simulations because they have low resolution (<10 grid cells) across the densest clumps.”
Forming Planetesimals in Solar and Extrasolar Nebulae (2009, background): “Simulations of MRI turbulence, both global (Fromang & Nelson 2005) and in large shearing boxes (Johansen et al. 2009a), show that inverse cascades produce pressure perturbations that survive many tens of orbits and have in-plane sizes larger than hg.”
Rapid growth of gas-giant cores by pebble accretion (2012, background): “On the other hand, one can circumvent this so-called meter barrier with turbulence induced by the magnetorotational instability (MRI, Balbus & Hawley 1991), which excites local pressure bumps, ideal regions for dust particle trapping and growth (Whipple 1972; Johansen et al. 2009a).”
Building Terrestrial Planets (2012, background): “ Gy ago; Bouvier et al. 2007; Burkhardt et al. 2008). Sufficient clumping of small particles to form planetesimals is possible only if the solid/gas density ratio is larger than some threshold value (Johansen et al. 2009). This ratio in principle increases with time due to the progressive removal of”
THE MASS AND SIZE DISTRIBUTION OF PLANETESIMALS FORMED BY THE STREAMING INSTABILITY. I. THE ROLE OF SELF-GRAVITY (2015, methodology): “Three-dimensional calculations that include the mutual gravitational forces between particles have been used to study how planetesimals form (Johansen et al. 2007, 2009b, 2011, 2012, 2015).”
Adding particle collisions to the formation of asteroids and Kuiper belt objects via streaming instabilities (2011, background): “…clumping of particles (Youdin & Goodman 2005; Johansen & Youdin 2007; Bai & Stone 2010b), seeding a gravitational collapse into bound clumps (Johansen et al. 2009) and further to Appendices are available in electronic form at http://www.aanda.org solid planetesimals (Nesvorný et al. 2010).”
THE EXOPLANET CENSUS: A GENERAL METHOD APPLIED TO KEPLER (2011, background): “This trend has been a powerful guide to identifying the mechanisms responsible for planetesimal formation (Youdin & Shu 2002; Youdin & Goodman 2005; Johansen et al. 2009), see Chiang & Youdin (2010) for a review.”
How to form planetesimals from mm-sized chondrules and chondrule aggregates (2015, background): “But even inside an MRI-free “dead zone” (Gammie 1996), the Kelvin-Helmholtz instability can be very effective at producing turbulence that blocks particle sedimentation to the midplane before they can reach the Roche density (Weidenschilling 1980; Youdin & Shu 2002; Lee et al. 2010).”
The effect of gas drag on the growth of protoplanets. Analytical expressions for the accretion of small bodies in laminar disks (2010, background): “…less well to the gas and relative velocities increase, culminating in the so called metersize barrier, which, at our current level of understanding, can best be overcome by the combined efforts of turbulent concentration and gravitational collapse (Johansen et al. 2007, 2009; Cuzzi et al. 2010).”
Planetesimal formation near the snowline: in or out? (2017, background): “The enhancement of the ice surface density near the snowline is relatively modest, but enrichment factors of a few can still be important for triggering streaming instability (Johansen et al. 2009).”
Forming the cores of giant planets from the radial pebble flux in protoplanetary discs (2014, background): “One can therefore imagine a scenario where during the disc dissipation phase the metallicity is elevated by gas removal and pebbles are efficiently converted into planetesimals by the streaming instability (Johansen et al. 2009; Bai & Stone 2010).”
Close-in planetesimal formation by pile-up of drifting pebbles (2016, methodology): “This pile-up is required by the planetesimal formation models which include the streaming instability (Johansen et al. 2009; Bai & Stone 2010a).”
WIND-DRIVEN ACCRETION IN PROTOPLANETARY DISKS. I. SUPPRESSION OF THE MAGNETOROTATIONAL INSTABILITY AND LAUNCHING OF THE MAGNETOCENTRIFUGAL WIND (2013, background): “Current models for planetesimal formation such as the streaming instability (Johansen et al. 2009; Bai & Stone 2010a, 2010b) and gravitational instability (Youdin 2011) generally favor weak turbulence and small radial pressure gradient.”