Atomic Diagnostics of X-ray Irradiated Protoplanetary Disks

TL;DR

This paper models atomic line diagnostics in X-ray irradiated protoplanetary disks, predicting observable sulfur and other forbidden line emissions that serve as diagnostics of disk atmospheres.

Contribution

It extends X-ray ionization theory to sulfur and predicts new observable forbidden lines as diagnostics of disk atmospheres, complementing previous neon line studies.

Findings

Sulfur fine-structure line at 25.55μm is below detection limits.

SII forbidden transitions are observable in the upper atmosphere.

Other forbidden lines like OI and CI are potential diagnostics for X-ray irradiated disks.

Abstract

We study atomic line diagnostics of the inner regions of protoplanetary disks with our model of X-ray irradiated disk atmospheres which was previously used to predict observable levels of the NeII and NeIII fine-structure transitions at 12.81 and 15.55mum. We extend the X-ray ionization theory to sulfur and calculate the fraction of sulfur in S, S+, S2+ and sulfur molecules. For the D'Alessio generic T Tauri star disk, we find that the SI fine-structure line at 25.55mum is below the detection level of the Spitzer Infrared Spectrometer (IRS), in large part due to X-ray ionization of atomic S at the top of the atmosphere and to its incorporation into molecules close to the mid-plane. We predict that observable fluxes of the SII 6718/6732AA forbidden transitions are produced in the upper atmosphere at somewhat shallower depths and smaller radii than the neon fine-structure lines. This and other forbidden line transitions, such as the OI 6300/6363AA and the CI 9826/9852AA lines, serve as complementary diagnostics of X-ray irradiated disk atmospheres. We have also analyzed the potential role of the low-excitation fine-structure lines of CI, CII, and OI, which should be observable by SOFIA and Herschel.