# Radially resolved simulations of collapsing pebble clouds in protoplanetary discs

## TL;DR

This study models the collapse of pebble clouds in protoplanetary discs to understand the internal structure of comet-sized planetesimals, highlighting the influence of gas drag and collision outcomes on porosity and pebble size distribution.

## Contribution

It introduces a model allowing differential contraction of pebble sub-clouds and incorporates gas drag effects, advancing understanding of planetesimal internal structure formation.

## Key findings

- Comets are porous pebble-piles with depth-dependent pebble sizes.
- Surface layers contain a mix of primordial pebbles and fragments.
- Interior regions consist mainly of primordial pebbles with higher porosity.

## Abstract

We study the collapse of pebble clouds with a statistical model to find the internal structure of comet-sized planetesimals. Pebble-pebble collisions occur during the collapse and the outcome of these collisions affect the resulting structure of the planetesimal. We expand our previous models by allowing the individual pebble sub-clouds to contract at different rates and by including the effect of gas drag on the contraction speed and in energy dissipation. Our results yield comets that are porous pebble-piles with particle sizes varying with depth. In the surface layers there is a mixture of primordial pebbles and pebble fragments. The interior, on the other hand, consists only of primordial pebbles with a narrower size distribution, yielding higher porosity there. Our results imply that the gas in the protoplanetary disc plays an important role in determining the radial distribution of pebble sizes and porosity inside planetesimals.