Detection of [Ne II] emission from young circumstellar disks

Creators: Pascucci, I.; Hollenbach, D.; Najita, J.; Muzerolle, J.; Gorti, U.; Herczeg, G. J.; Hillenbrand, L. A.; Kim, J. S.; Carpenter, J. M.; Meyer, M. R.; Mamajek, E. E.; Bouwman, J.

Abstract

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 MJ) we suggest using this gas line to determine the presence or absence of gas in more evolved circumstellar disks.

Additional Information

© 2007 American Astronomical Society. Print publication: Issue 1 (2007 July 1); received 2006 December 16; accepted for publication 2007 March 22. It is a pleasure to thank all members of the FEPS team for their contributions to the project and to this study. I. P. wishes to thank D.Watson for suggestions in the data reduction of the IRS high-resolution spectra and A. E. Glassgold for helpful discussions on the X-ray model predictions. We thank the referee, Dmitry Semenov, for a very helpful review. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology, under NASA contract 1407. FEPS is pleased to acknowledge support through NASA contracts 1224768, 1224634, and 1224566, administered through JPL.

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