Size-dependent charging of dust particles in protoplanetary disks

Can turbulence cause charge separation and lightning?

¹ Austrian Academy of Science, Space Research Institute, Schmiedlstrasse 6, 8042 Graz, Austria
e-mail: Thorsten.Balduin@oeaw.ac.at
² TU Graz, Faculty of Mathematics, Physics and Geodesy, Petersgasse 16, 8010 Graz, Austria
³ Centre for ExoLife Sciences (CELS), Niels Bohr Institute, Østervoldgade 5, 1350 Copenhagen, Denmark
⁴ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany

Received: 16 March 2023
Accepted: 1 August 2023

Abstract

Context. Lightning can have a profound impact on the chemistry of planetary atmospheres. In a similar manner, as protoplanetary disks are the foundation of planet formation, the emergence of lightning in protoplanetary disks can substantially alter their chemistry.

Aims. We aim to study under which conditions lightning could emerge within protoplanetary disks.

Methods. We employed the PRODIMO code to make 2D thermo-chemical models of protoplanetary disks. We included a new way of how the code handles dust grains, which allows the consideration of dust grains of different sizes. We investigated the chemical composition, dust charging behavior, and charge balance of these models to determine which regions could be most sufficient for lightning.

Results. We identify six regions within the disks where the charge balance is dominated by different radiation processes and find that the emergence of lightning is most probable in the lower and warmer regions of the midplane. This is due to the low electron abundance (n_е/n_〈H〉 < 10⁻¹⁵) in these regions and dust grains being the most abundant negative charge carriers (n_Z/n_〈H〉 > 10⁻¹³). We find that NH4⁺ is the most abundant positive charge carrier in those regions at the same abundances as the dust grains. We developed a method of inducing electric fields via turbulence within this mix of dust grains and NH₄⁺. The electric fields generated with this mechanism are however several orders of magnitude weaker than required to overcome the critical electric field.