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

Abstract

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.

Unveiling Dust Aggregate Structure in Protoplanetary Disks by Millimeter-wave Scattering Polarization

“…By assuming perfect sticking, fluffy dust aggregates (f ≈ 10 −4 ) form in disks and they finally become planetesimals by direct coagulation (Okuzumi et al 2012;Kataoka et al 2013a). These results have also been confirmed by more recent studies (Krijt et al 2015(Krijt et al , 2016Lorek et al 2018). However, as we mentioned in Section 5.1.1, these aggregates are not favored from observations of millimeter-wave scattering polarization.…”

Transport of CO in Protoplanetary Disks: Consequences of Pebble Formation, Settling, and Radial Drift

“…for water ice and dust. For a collection of monomers whose surface properties are a mix between those of water-ice and non-water-ice, the characteristic rolling energy can be interpolated as (Lorek et al 2016(Lorek et al , 2018)…”

Homogeneity of 67P/Churyumov-Gerasimenko as seen by CONSERT: implication on composition and formation

“…-Model A: gravitational collapse of a pebble cloud. Primordial (sub-) micrometer-sized dust and ice grains coagulate into millimeter-to centimeter-sized aggregates (pebbles), which can no longer stick to each other because of the bouncing barrier Lorek et al 2018). These pebbles are locally concentrated by one of many possible processes, for example, by turbulent concentration, sedimentation in a quiescent nebula, or Kelvin-Helmholtz instability (see Johansen et al 2014 for an overview), until the streaming instability is triggered, which further enhances the local concentration of pebbles (Youdin & Goodman 2005).…”