Blowing in the wind

Context. Comet formation by gravitational instability requires aggregates that trigger the streaming instability and cluster in pebble-clouds. These aggregates form as mixtures of dust and ice from (sub-)micrometre-sized dust and ice grains via coagulation in the solar nebula. Aim. We investigate the growth of aggregates from (sub-)micrometre-sized dust and ice monomer grains. We are interested in the properties of these aggregates: whether they might trigger the streaming instability, how they compare to pebbles found on comets, and what the implications are for comet formation in collapsing pebble-clouds. Methods. We used Monte Carlo simulations to study the growth of aggregates through coagulation locally in the comet-forming region at 30 au. We used a collision model that can accommodate sticking, bouncing, fragmentation, and porosity of dust- and ice-mixed aggregates. We compared our results to measurements of pebbles on comet 67P/Churyumov-Gerasimenko. Results. We find that aggregate growth becomes limited by radial drift towards the Sun for 1 μm sized monomers and by bouncing collisions for 0.1 μm sized monomers before the aggregates reach a Stokes number that would trigger the streaming instability (Stmin). We argue that in a bouncing-dominated system, aggregates can reach Stmin through compression in bouncing collisions if compression is faster than radial drift. In the comet-forming region (~30 au), aggregates with Stmin have volume-filling factors of ~10−2 and radii of a few millimetres. These sizes are comparable to the sizes of pebbles found on comet 67P/Churyumov-Gerasimenko. The porosity of the aggregates formed in the solar nebula would imply that comets formed in pebble-clouds with masses equivalent to planetesimals of the order of 100 km in diameter.

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“…Turbulent stirring is caused by the turbulent motion of the gas. According to Cuzzi & Hogan (2003); Ormel & Cuzzi (2007), we have…”

Section: A2 Drag Forcementioning

confidence: 99%

Local growth of dust- and ice-mixed aggregates as cometary building blocks in the solar nebula

Lorek

Lacerda

Blum

2018

A&A

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“…Chondrule precursors with a radius of 300 μm, as we have assumed in our model, have almost the exact size to be optimally concentrated. Particle concentrations C ∼ 10 are easily achieved (Cuzzi & Hogan 2003).…”

Section: Thermal Histories Of Chondrules In Planetary Embryo Bow Shocksmentioning

confidence: 99%

“…Of course, only small fractions of the volume of the nebula see concentrations this high, but these regions are where most chondrule precursors reside. The length scale over which a concentration C is achieved roughly scales as (10 6 km)/C (Cuzzi & Hogan 2003), so even regions with C = 100 are >10 4 km in extent and thus are wider than the planetary embryo bow shock itself. We are justified in assuming a uniform value of C when computing chondrule thermal histories.…”

Section: Thermal Histories Of Chondrules In Planetary Embryo Bow Shocksmentioning

confidence: 99%

Chondrule Formation in Bow Shocks Around Eccentric Planetary Embryos

Morris

Boley

Desch

et al. 2012

ApJ

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Recent isotopic studies of Martian meteorites by Dauphas & Pourmand have established that large (∼3000 km radius) planetary embryos existed in the solar nebula at the same time that chondrules-millimeter-sized igneous inclusions found in meteorites-were forming. We model the formation of chondrules by passage through bow shocks around such a planetary embryo on an eccentric orbit. We numerically model the hydrodynamics of the flow and find that such large bodies retain an atmosphere with Kelvin-Helmholtz instabilities allowing mixing of this atmosphere with the gas and particles flowing past the embryo. We calculate the trajectories of chondrules flowing past the body and find that they are not accreted by the protoplanet, but may instead flow through volatiles outgassed from the planet's magma ocean. In contrast, chondrules are accreted onto smaller planetesimals. We calculate the thermal histories of chondrules passing through the bow shock. We find that peak temperatures and cooling rates are consistent with the formation of the dominant, porphyritic texture of most chondrules, assuming a modest enhancement above the likely solar nebula average value of chondrule densities (by a factor of 10), attributable to settling of chondrule precursors to the midplane of the disk or turbulent concentration. We calculate the rate at which a planetary embryo's eccentricity is damped and conclude that a single planetary embryo scattered into an eccentric orbit can, over ∼10 5 years, produce ∼10 24 g of chondrules. In principle, a small number (1-10) of eccentric planetary embryos can melt the observed mass of chondrules in a manner consistent with all known constraints.

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“…In addition, these lags also cause particles to acquire relative velocities among themselves. While the general problem of calculating these relative velocities has received considerable attention in the basic fluid dynamics community (see Cuzzi & Hogan 2003, for references; henceforth CH03), the formalism most frequently used in the astrophysics community was developed by Völk et al (1980) and Markiewicz et al (1991, henceforth MMV). In these works the final results are given in terms of integrals that were not solved analytically.…”

Section: Introduction and Outlinementioning

confidence: 99%

Closed-form expressions for particle relative velocities induced by turbulence

Ormel¹,

Cuzzi²

2007

A&A

428

505

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In this note we present complete, closed-form expressions for random relative velocities between colliding particles of arbitrary size in nebula turbulence. These results are exact for very small particles (those with stopping times much shorter than the large eddy overturn time) and are also surprisingly accurate in complete generality (that is, also apply for particles with stopping times comparable to, or much longer than, the large eddy overturn time). We note that some previous studies may have adopted previous simple expressions, which we find to be in error regarding the size dependence in the large particle regime.

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Blowing in the wind

Cited by 98 publications

References 17 publications

Local growth of dust- and ice-mixed aggregates as cometary building blocks in the solar nebula

Local growth of dust- and ice-mixed aggregates as cometary building blocks in the solar nebula

Chondrule Formation in Bow Shocks Around Eccentric Planetary Embryos

Closed-form expressions for particle relative velocities induced by turbulence

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