2007ApJ...663..383P

We report the detection of [Ne II] emission at 12.81 µm in four out of the six optically thick dust disks observed as part of the FEPS Spitzer Legacy program. In addition, we detect a H I (7-6) emission line at 12.37 µm from the source RX J1852.3-3700. Detections of [Ne II] lines are favored by low mid-infrared excess emission. Both stellar X-rays and extreme ultraviolet (EUV) photons can sufficiently ionize the disk surface to reproduce the observed line fluxes, suggesting that emission from Ne⁺ originates in the hot disk atmosphere. On the other hand, the H I (7-6) line is not associated with the gas in the disk surface, and magnetospheric accretion flows can account for at most ∼30% of the observed flux. We conclude that accretion shock regions and/or the stellar corona could contribute to most of the H I (7-6) emission. Finally, we discuss the observations necessary to identify whether stellar X-rays or EUV photons are the dominant ionization mechanism for Ne atoms. Because the observed [Ne II] emission probes very small amounts of gas in the disk surface (∼10^–6 M_J) we suggest using this gas line to determine the presence or absence of gas in more evolved circumstellar disks.