恒星辐射场撞击过渡盘大气的观测约束
Observational Constraints on the Stellar Radiation Field Impinging on Transitional Disk Atmospheres
摘要：
在前主序星光谱中测量的中红外原子线和离子线比率是撞击盘表面的辐射场硬度的敏感指标。我们提出了一个低分辨率的斯皮策IRS搜索[Ar II]在6.98 $μ$ m，[Ne II]在12.81 $μ$ m，和[Ne III] 15.55 $μ$ m线在56个过渡盘。这些物体的特点是近红外线减少，但远红外线过量发射很强，是将恒星辐射场限制在圆盘上的理想目标，因为它们的光谱不受喷流/外流的冲击发射或分子发射谱线的污染。在证明我们可以检测到[Ne II]线，并恢复其流量从低分辨率光谱，在这里，我们报告的[Ar II]线朝向原行星盘的第一个检测。我们没有在任何来源中检测到[Ne III]发射。我们的[Ne II]/[Ne III]线通量比结合文献数据表明，软EUV或X射线光谱产生这些气体线。此外，[Ar II]/[Ne II]线通量比指向软X射线和/或软EUV恒星光谱作为[Ar II]和[Ne II]发射层的盘的电离源。如果软X射线成分在EUV中占主导地位，那么我们预计会有更大的光蒸发率，因此形成行星的时间会减少。
英文摘要：
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 $μ$m, [Ne II] at 12.81 $μ$m, and [Ne III] 15.55 $μ$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.
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