Deep-Down Ionization of Protoplanetary Disks

TL;DR

This paper develops a simplified analytical model for deep-down ionization in protoplanetary disks caused by X-rays, crucial for understanding magnetic activity and dead zones.

Contribution

It introduces a closed-form analytical theory for X-ray ionization at depth in protoplanetary disks, including key chemical processes and power-law descriptions of ionization profiles.

Findings

Provides simple power-law formulas for ionization decrease with depth.

Includes key chemical reactions affecting ionization in the disk.

Illustrates non-ideal MHD effects in a T Tauri star disk model.

Abstract

The possible occurrence of dead zones in protoplanetary disks subject to the magneto-rotational instability highlights the importance of disk ionization. We present a closed-form theory for the deep-down ionization by X-rays at depths below the disk surface dominated by far-ultraviolet radiation. Simple analytic solutions are given for the major ion classes, electrons, atomic ions, molecular ions and negatively charged grains. In addition to the formation of molecular ions by X-ray ionization of H2 and their destruction by dissociative recombination, several key processes that operate in this region are included, e.g., charge exchange of molecular ions and neutral atoms and destruction of ions by grains. Over much of the inner disk, the vertical decrease in ionization with depth into the disk is described by simple power laws, which can easily be included in more detailed modeling of magnetized disks. The new ionization theory is used to illustrate the non-ideal MHD effects of Ohmic, Hall and Ambipolar diffusion for a magnetic model of a T Tauri star disk using the appropriate Elsasser numbers.