Journal
ASTROPHYSICAL JOURNAL
Volume 759, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/759/1/47
Keywords
circumstellar matter; infrared: stars; protoplanetary disks; stars: pre-main sequence
Categories
Funding
- Spitzer Data Analysis grant [1348621]
- NASA/ADP [NNX10AD62G]
- Hungarian Scientific Research Fund [OTKA K101393]
- NASA [NNX10AD62G, 135724] Funding Source: Federal RePORTER
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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 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.
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