Constraining the spatial and thermal structure of the gaseous component of circumstellar disks is crucial for understanding star and planet formation. Models predict that the [Ne II] line at 12.81 μm detected in young stellar objects (YSOs) with Spitzer traces disk gas and its response to high-energy radiation, but such [Ne II] emission may also originate in shocks within powerful outflows. To distinguish between these potential origins for mid-infrared [Ne II] emission and to constrain disk models, we observed 32 YSOs using the high-resolution (R ~ 30,000) mid-infrared spectrograph VISIR at the Very Large Telescope. We detected the 12.81 μm [Ne II] line in 12?objects, tripling the number of detections of this line in YSOs with high spatial and spectral resolution spectrographs. We obtain the following main results. (1) In Class?I objects the [Ne II] emission observed from Spitzer is mainly due to gas at a distance of more than 20-40?AU from the star, where neon is, most likely, ionized by shocks due to protostellar outflows. (2) In transition and pre-transition disks, most of the emission is confined to the inner disk, within 20-40?AU from the central star. (3) Detailed analysis of line profiles indicates that, in transition and pre-transition disks, the line is slightly blueshifted (2-12 km s–1) with respect to the stellar velocity, and the line width is directly correlated with the disk inclination, as expected if the emission is due to a disk wind. (4) Models of EUV/X-ray-irradiated disks reproduce well the observed relation between the line width and the disk inclination, but underestimate the blueshift of the line.
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