Vertical settling of pebbles in turbulent circumbinary discs and the in situ formation of circumbinary planets

Abstract

The inner-most regions of circumbinary discs are unstable to a parametric instability whose non-linear evolution is hydrodynamical turbulence. This results in significant particle stirring, impacting on planetary growth processes such as the streaming instability or pebble accretion. In this paper, we present the results of three-dimensional, inviscid global hydrodynamical simulations of circumbinary discs with embedded particles of 1 cm size. Hydrodynamical turbulence develops in the disc, and we examine the effect of the particle back-reaction on vertical dust. We find that higher solid-to-gas ratios lead to smaller gas vertical velocity fluctuations, and therefore to smaller dust scale heights. For a metallicity Z = 0.1, the dust scale height near the edge of the tidally-truncated cavity is $\sim 80{{\ \rm per\ cent}}$ of the gas scale height, such that growing a Ceres-mass object to a 10 M⊕ core via pebble accretion would take longer than the disc lifetime. Collision velocities for small particles are also higher than the critical velocity for fragmentation, which precludes grain growth and the possibility of forming a massive planetesimal seed for pebble accretion. At larger distances from the binary, turbulence is weak enough to enable not only efficient pebble accretion but also grain growth to sizes required to trigger the streaming instability. In these regions, an in-situ formation scenario of circumbinary planets involving the streaming instability to form a massive planetesimal followed by pebble accretion on to this core is viable. In that case, planetary migration has to be invoked to explain the presence of circumbinary planets at their observed locations.

How cooling influences circumbinary discs

“…All of the mentioned studies so far have used vertically integrated, locally isothermal models, where the cooling timescale is negligible compared to the orbital period of the gas, ignoring changes to the thermal profile of the disc. Kley et al (2019) introduced a radiatively cooled model for the circumbinary disc with migrating planets and Pierens et al (2020Pierens et al ( , 2021) used a three-dimensional (3D) model with βcooling including dust to study the impact of the turbulent disc on in situ planet growth close to the binary.…”

Hydrodynamical turbulence in eccentric circumbinary discs and its impact on the in situ formation of circumbinary planets

“…Growing a massive planetesimal seed in-situ through pebble accretion also appears to be very challenging. The inner disc is prone to a parametric instability that generates hydrodynamical turbulence (Papaloizou 2005;Barker & Ogilvie 2014), the main impact of which is to stir up the pebble layer, resulting in inefficient pebble accretion (Pierens et al 2020(Pierens et al , 2021.…”

Global N-body simulations of circumbinary planet formation around Kepler-16 and -34 analogues I: Exploring the pebble accretion scenario

“…Ways to overcome the problems with in situ formation have been explored, including having extremely massive protoplanetary discs (Marzari & Scholl 2000;Martin et al 2013;Meschiari 2014;Rafikov & Silsbee 2015), or if the fragments are reaccreted and form second or later generations of planetesimals (Paardekooper & Leinhardt 2010). More recently, it has been shown that in situ pebble accretion scenarios also suffer from difficulties because a parametric instability can generate hydrodynamical turbulence that stirs up pebbles, rendering pebble accretion onto planetary embryos inefficient (Pierens et al 2020(Pierens et al , 2021.…”