Ionizing the Gas in Protoplanetary Disks by Pebble Collisions

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Ionization of gas in protoplanetary disks is a fundamental process that regulates chemistry and magnetic interactions. The level of ionization can set conditions that decide if the disk is locally turbulent or not. So far, ways of ionization considered are restricted to high temperatures and various high energy radiation sources from stellar X-rays, over cosmic rays to radiation by radioactive decay. A new way of ionizing the disk‘s gas just emerged. With millimeter-sized pebbles being a current standard ingredient in protoplanetary disks and for planet formation, bouncing collisions occur throughout the lifetime of the disk. It was recently shown that collisions charging (tribocharging) is very efficient to promote growth of particle clusters. In addition, it was found - and that is new - that also the gas is ionized during the collision of two pebbles. With an estimated ionization rate of up to 10-15 s-1, the process can be way more efficient than cosmic rays in ionizing disk material and it works better in the dense part of protoplanetary disks. Pebble ionization therefore has a potentially high impact on disk ionization but there are only few quantitative data on the process. With this project, we approach this ionization in laboratory experiments for the first time systematically.