Observational Constraints on the Stellar Radiation Field Impinging on Transitional Disk Atmospheres

TL;DR

This study uses Spitzer IRS spectra to detect atomic and ionic lines in transitional disks, constraining the stellar radiation field impacting disk atmospheres and implications for planet formation.

Contribution

First detection of [Ar II] lines in protoplanetary disks and analysis indicating a soft X-ray/EUV stellar spectrum influences disk ionization.

Findings

Detected [Ar II] lines in protoplanetary disks.

No [Ne III] emission detected in the sample.

Line ratios suggest a soft X-ray/EUV radiation field.

Abstract

Mid-infrared atomic and ionic line ratios measured in spectra of pre-main sequence stars are sensitive indicators of the hardness of the radiation field impinging on the disk surface. We present a low-resolution Spitzer IRS search for [Ar II] at 6.98 $\mu$m, [Ne II] at 12.81 $\mu$m, and [Ne III] 15.55 $\mu$m lines in 56 transitional disks. These objects, characterized by reduced near-infrared but strong far-infrared excess emission, are ideal targets to set constraints on the stellar radiation field onto the disk because their spectra are not contaminated by shock emission from jets/outflows or by molecular emission lines. After demonstrating that we can detect [Ne II] lines and recover their fluxes from the low-resolution spectra, here we report the first detections of [Ar II] lines towards protoplanetary disks. We did not detect [Ne III] emission in any of our sources. Our [Ne II]/[Ne III] line flux ratios combined with literature data suggest that a soft-EUV or X-ray spectrum produces these gas lines. Furthermore, the [Ar II]/[Ne II] line flux ratios point to a soft X-ray and/or soft-EUV stellar spectrum as the ionization source of the [Ar II] and [Ne II] emitting layer of the disk. If the soft X-ray component dominates over the EUV than we would expect larger photoevaporation rates hence a reduction of the time available to form planets.