High-Temperature Ionization in Protoplanetary Disks

TL;DR

This paper models high-temperature ionization in protoplanetary disks, emphasizing thermionic and ion emission effects, revealing that ionization depends on dust grain work functions and impacts disk dead zones and magnetic coupling.

Contribution

It introduces a novel ionization model incorporating thermionic and ion emission, challenging the traditional Saha equation approach, and applies it to disk physics and planetary atmospheres.

Findings

Ionization sharply increases above 800 K due to thermionic emission.

The inner edge of the dead zone is around 1000 K, influenced by ambipolar diffusion and resistivity.

Short-circuit instability is unlikely to explain chondrule formation.

Abstract

We calculate the abundances of electrons and ions in the hot (> 500 K), dusty parts of protoplanetary disks, treating for the first time the effects of thermionic and ion emission from the dust grains. High-temperature ionization modeling has involved simply assuming that alkali elements such as potassium occur as gas-phase atoms and are collisionally ionized following the Saha equation. We show that the Saha equation often does not hold, because free charges are produced by thermionic and ion emission and destroyed when they stick to grain surfaces. This means the ionization state depends not on the first ionization potential of the alkali atoms, but rather on the grains' work functions. The charged species' abundances typically rise abruptly above about 800 K, with little qualitative dependence on the work function, gas density, or dust-to-gas mass ratio. Applying our results, we find that protoplanetary disks' dead zone, where high diffusivities stifle magnetorotational turbulence, has its inner edge located where the temperature exceeds a threshold value ~1000 K. The threshold is set by ambipolar diffusion except at the highest densities, where it is set by Ohmic resistivity. We find that the disk gas can be diffusively loaded onto the stellar magnetosphere at temperatures below a similar threshold. We investigate whether the "short-circuit" instability of current sheets can operate in disks and find that it cannot, or works only in a narrow range of conditions; it appears not to be the chondrule formation mechanism. We also suggest that thermionic emission is important for determining the rate of Ohmic heating in hot Jupiters.