Fast Accretion of Small Planetesimals by Protoplanetary Cores

By R. Rafikov. 2003

141 citations recorded by the source corpus.

We explore the dynamics of small planetesimals coexisting with massive protoplanetary cores in a gaseous nebula. Gas drag strongly affects the motion of small bodies, leading to the decay of their eccentricities and inclinations, which are excited by the gravity of protoplanetary cores. Drag acting on larger (≳1 km), high-velocity planetesimals causes a mere reduction of their average random velocity. By contrast, drag qualitatively changes the dynamics of smaller (≲0.1–1 km), low-velocity objects: (1) small planetesimals sediment toward the midplane of the nebula, forming a vertically thin subdisk; (2) their random velocities rapidly decay between successive passages of the cores, and as a result, encounters with cores typically occur at the minimum relative velocity allowed by the shear in the disk. This leads to a drastic increase in the accretion rate of small planetesimals by the protoplanetary cores, allowing cores to grow faster than expected in the simple oligarchic picture, provided that the population of small planetesimals contains more than roughly 1% of the solid mass in the nebula. Fragmentation of larger planetesimals (≳1 km) in energetic collisions triggered by the gravitational scattering by cores can easily channel this amount of material into small bodies on reasonable timescales (<1 Myr in the outer solar system), providing a means for the rapid growth (within several million years at 30 AU) of rather massive protoplanetary cores. Effects of inelastic collisions between planetesimals and presence of multiple protoplanetary cores are discussed.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/9f5978da40ff3a24b4a2d0c75c5d7d96022c7757

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In Astronomy and Astrophysics

Citations in the corpus

Atmospheres of Protoplanetary Cores: Critical Mass for Nucleated Instability (2004, methodology): “This regime corresponds to the boundary between the shear- and dispersion-dominated dynamical regimes (Stewart & Ida 2000) and may occur during oligarchic growth of protoplanetary embryos by accretion of large planetesimals (Kokubo & Ida 1998; Rafikov 2004b).”
Planet Formation around Stars of Various Masses: The Snow Line and the Frequency of Giant Planets (2007, methodology): “Our choice of 105 yr for the Jovian core is relatively short, and is based on likely fragmentation (e.g. Kenyon & Bromley 2004a) and the rapid accretion of small ∼100m planetesimals in the shear dominated regime (e.g. Rafikov 2004; Chambers 2006).”
Separating gas-giant and ice-giant planets by halting pebble accretion (2014, background): “Collisional fragments of planetesimals (0.1−1 km) have a reduced scale height and can be accreted more rapidly by a factor of 1/ √ ψ (Rafikov 2004).”
DIRECT IMAGING OF A COLD JOVIAN EXOPLANET IN ORBIT AROUND THE SUN-LIKE STAR GJ 504 (2013, background): “…that may enable the cores of giant planets to grow in the shorter timescale: effective damping of small planetesimal’s eccentricity due to gas drag (Rafikov 2004) or efficient capture of planetesimals due to atmospheric drag (Inaba & Ikoma 2003) may accelerate the accumulations of planetesimals…”
On the Location of the Snow Line in a Protoplanetary Disk (2006, background): “The most widely accepted theory for the formation of gas giants is the ’core accretion’ model (e.g., Pollack et al. 1996; Rafikov 2004), which requires a core of 5− 15 M⊕ (Guillot 2005).”
Rapid growth of gas-giant cores by pebble accretion (2012, background): “Accretion of planetesimal fragments from a thin midplane, as discussed in A32, page 7 of 13 Rafikov (2004) is more efficient, with the accretion rate being proportional to √ αṀH.”
Forming the cores of giant planets from the radial pebble flux in protoplanetary discs (2014, background): “Destruction of large planetesimals into smaller fragments can speed up core growth (Rafikov 2004; Ormel & Klahr 2010).”
A COMBINED SUBARU/VLT/MMT 1–5 μm STUDY OF PLANETS ORBITING HR 8799: IMPLICATIONS FOR ATMOSPHERIC PROPERTIES, MASSES, AND FORMATION (2011, background): “Their formalism implicitly assumes that planetesimals have an isotropic velocity dispersion (va ∼ vz), which is valid as long as the scale height of planetesimals accreted by cores (vz/Ω) is larger than the core’s impact parameter, Rcore √ (1 + θ) (Rafikov 2004), where θ is the Safranov number.”
Can Giant Planets Form by Direct Gravitational Instability? (2004, background): “Rafikov (2004) have previously presented arguments against in situ formation of “hot Jupiters” via the core instability.”
The effect of gas drag on the growth of protoplanets. Analytical expressions for the accretion of small bodies in laminar disks (2010, background): “…gravity of the central star) is derived by a number of studies to be( dM dt ) gf ≈ 11α1/2p RhvhΣ (1) (e.g., Ida & Nakazawa 1989; Greenberg et al. 1991; Rafikov 2004; Weidenschilling 2005; Ormel et al. 2010b; the numerical constant is adopted from Inaba et al. 2001), where Σ is the density in…”
Variations on Debris Disks: Icy Planet Formation at 30-150 AU for 1-3 M☉ Main-Sequence Stars (2008, methodology): “For typical conditions in planetesimal disks, Rafikov (2004) derived 1 1/6 to 1/5.”
Dynamics of pebbles in the vicinity of a growing planetary embryo: hydro-dynamical simulations (2012, background): “It has been argued that planet migration (Alibert et al. 2004) or the radial drift of sub-km planetesimals due to gas drag (Rafikov 2004) break the isolation of the cores from the disk of solids but, again, N-body simulations show that the relative drift of planetesimals and cores simply collects…”