Ionizing protoplanetary discs in pebble collisions

Abstract

ABSTRACT We introduce collisions of solids as a new and efficient ionization mechanism for gas in protoplanetary discs, which especially operates in the dense midplane of protoplanetary discs. This idea is sparked by laboratory experiments where we found that charge, which is exchanged by grains in mutual collision (tribocharging), is not tied to their surfaces alone. As kind of collateral effect, charges also become entrained into the gas phase, i.e. collisions ionize the protoplanetary disc. Therefore, solids are not only sinks of charges in discs but also sources. A first estimate shows that ionization rates in the midplane at 1 AU in the range of $10^{-19}\!-\!10^{-15}\,{\rm s}^{-1}$ seem feasible depending on the assumption of rather calm or highly turbulent conditions with radial particle pile up.