Photoprocesses in protoplanetary disks

TL;DR

This paper reviews how intense UV radiation influences the physical and chemical structure of protoplanetary disks, highlighting differences from traditional PDRs and providing new photorates and cross sections relevant to various astrophysical environments.

Contribution

It introduces new photorates and cross sections for species in protoplanetary disks, considering unique disk parameters and radiation fields, enhancing understanding of disk chemistry and structure.

Findings

UV radiation significantly alters disk chemistry and structure.

New photorates and cross sections are provided for modeling.

Photoprocesses are crucial in diverse astrophysical regions.

Abstract

Circumstellar disks are exposed to intense ultraviolet radiation from the young star. In the inner disks, the UV radiation can be enhanced by more than seven orders of magnitude compared with the average interstellar field, resulting in a physical and chemical structure that resembles that of a dense photon-dominated region (PDR). This intense UV field affects the chemistry, the vertical structure of the disk, and the gas temperature, especially in the surface layers of the disk. The parameters which make disks different from traditional PDRs are discussed, including the shape of the UV radiation field, grain growth, the absence of PAHs, the gas/dust ratio and the presence of inner holes. New photorates for selected species, including simple ions, are presented. Also, a summary of available cross sections at Lyman alpha 1216 A is made. Rates are computed for radiation fields with color temperatures ranging from 4000 to 30,000 K, and can be applied to a wide variety of astrophysical regions including exo-planetary atmospheres. The importance of photoprocesses is illustrated for a number of representative disk models, including disk models with grain growth and settling.