Observational constraints on the stellar radiation field impinging on transitional disk atmospheres

Judit Szulágyi, Ilaria Pascucci, Péter Ábrahám, Dániel Apai, Jeroen Bouwman, Attila Moór

Research output: Contribution to journalArticlepeer-review

10 Scopus citations


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 μm, [Ne II] at 12.81 μm, and [Ne III] 15.55μm 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 toward 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, then we would expect larger photoevaporation rates and, hence, a reduction of the time available to form planets.

Original languageEnglish (US)
Article number47
JournalAstrophysical Journal
Issue number1
StatePublished - Nov 1 2012


  • circumstellar matter
  • infrared: stars
  • protoplanetary disks
  • stars: pre-main sequence

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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