DETECTION OF [Ne ii] EMISSION FROM YOUNG CIRCUMSTELLAR DISKS

We report the detection of [Ne ii] emission at 12.81 m 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 (7Y6) emission line at 12.37 m 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 fromNe originates in the hot disk atmosphere. On the other hand, the H i (7Y6) line is not associated with the gas in the disk surface, andmagnetospheric 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 (7Y6) emission. Finally, we discuss the observations necessary to identify whether stellar X-rays or EUV photons are the dominant ionizationmechanism forNe atoms. Because the observed [Ne ii] emission probes very small amounts of gas in the disk surface ( 10 6 MJ ) we suggest using this gas line to determine the presence or absence of gas inmore evolved circumstellar disks. Subject headingg s: circumstellar matter — infrared: stars — line: identification — planetary systems: protoplanetary disks — stars: individual (HD 143006, PDS 66, [PZ99] J161411.