Harvesting the decay energy of $^{26}$Al to drive lightning discharge in protoplanetary discs

TL;DR

This paper proposes that decay energy from $^{26}$Al can generate lightning in protoplanetary discs by charging pebble structures through positron emission, potentially explaining chondrule formation.

Contribution

It introduces a novel mechanism where $^{26}$Al decay leads to large-scale charging and lightning in protoplanetary discs, linking radioactive decay to early planetary processes.

Findings

Positron emission from $^{26}$Al causes significant charging of pebble structures.

Lightning discharge driven by radioactive decay can produce flash-heating of dust.

The energy from $^{26}$Al decay is sufficient to melt solids in the disc.

Abstract

Chondrules in primitive meteorites likely formed by recrystallisation of dust aggregates that were flash-heated to nearly complete melting. Chondrules may represent the building blocks of rocky planetesimals and protoplanets in the inner regions of protoplanetary discs, but the source of ubiquitous thermal processing of their dust aggregate precursors remains elusive. Here we demonstrate that escape of positrons released in the decay of the short-lived radionuclide $^{26}$Al leads to a large-scale charging of dense pebble structures, resulting in neutralisation by lightning discharge and flash-heating of dust and pebbles. This charging mechanism is similar to a nuclear battery where a radioactive source charges a capacitor. We show that the nuclear battery effect operates in circumplanetesimal pebble discs. The extremely high pebble densities in such discs are consistent with conditions during chondrule heating inferred from the high abundance of sodium within chondrules. The sedimented mid-plane layer of the protoplanetary disc may also be prone to charging by the emission of positrons, if the mass density of small dust there is at least an order of magnitude above the gas density. Our results imply that the decay energy of $^{26}$Al can be harvested to drive intense lightning activity in protoplanetary discs. The total energy stored in positron emission is comparable to the energy needed to melt all solids in the protoplanetary disc. The efficiency of transferring the positron energy to the electric field nevertheless depends on the relatively unknown distribution and scale-dependence of pebble density gradients in circumplanetesimal pebble discs and in the protoplanetary disc mid-plane layer.