Detection of [Ne II] Emission from Young Circumstellar Disks

The Astrophysical Journal (Impact Factor: 5.99). 12/2008; 663(1):383. DOI: 10.1086/518535


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.

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    • "However, models by Meijerink et al. (2008) rather found that there is a linear proportionality between the [NeII] line luminosity and the X-ray luminosity. Pascucci et al (2007) reported a tentative correlation between the [Ne II] and the X-ray luminosities. However, only about 30% of the sources with [Ne II] detections from Lahuis et al. (2007) are identified as X-ray sources (either due to the lack of X-ray emission, sensitivity limited X-ray searches, or object geometry). "
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