Is Mid-Infrared [Ne II] Emission a Tracer for X-Ray Ionized and Heated Gas in Protoplanetary Disks?


First Author:
Manuel Guedel
Email: guedel AT astro.phys.ethz.ch
Institute of Astronomy, ETH Zurich
Wolfgang-Pauli-Str. 27
Zurich, 8093, Switzerland
Coauthors:
van Boekel, Roy, MPIA Heidelberg, Germany
Lahuis, Fred, Groningen, Netherlands
Najita, Joan, NOAO, Tucson, USA
Henning, Thomas, MPIA Heidelberg, Germany
Glassgld, Alfred, Univ. of Berkeley, USA
van Dishoeck, Ewine, Leiden University, Netherlands
Carr, John, NRL, Washington DC, USA
Briggs, Kevin, ETH Zurich, Switzerland

Abstract
Glassgold et al. (2007) proposed that the forbidden mid-infrared [Ne II] 12.81 micron transition is a tracer of X-ray irradiated and X-ray heated disk gas. [Ne II] observations may detect relatively small amounts of warm gas in the inner, planet-forming disk zone (R=10-20 AU), at the same time confirming the role stellar X-rays play in driving disk dynamics, accretion, photoevaporation, chemistry, and eventually the formation of planets. Theories of [Ne II] line formation in disks predict - for otherwise identical source and disk properties - a linear correlation between the [Ne II] luminosities, L(Ne II), and the X-ray luminosities, LX. However, further parameters may be relevant. We test such predictions using two approaches:
1) We use a sample of 86 classical T Tauri stars with [Ne II] 12.81 micron observations (of which 53 have [Ne II] detections and 54 have X-ray detections) to perform correlation studies, also with other stellar and disk parameters collected from the literature. Although we find a significant correlation between L(Ne II) and LX over 2-3 orders of magnitude, the correlation is dominated by systematic scatter. A tendency is also found for stronger accretors to be more luminous [Ne II] sources. However, stars driving "micro-jets" show systematically enhanced L(Ne II), and a tighter correlation is indeed found between L(Ne II) and the product of LX and L(O I), the latter defining the luminosity in the [O I] 6300A line, often taken to be an indicator for gas in micro-jets.
2) We present a detailed VLT/VISIR case study of [Ne II] emission in and around the T Tau triple. This system shows an extremely high L(Ne II) = 5E30 erg/s. The emission is concentrated on the embedded T Tau S binary, but widely dispersed components are found along structures that have previously been identified as outflow features. A high-velocity component (v = 126 km/s, blueshifted) is identified at the position of T Tau N. Both results suggest that the [Ne II] 12.81 micron flux is prominently formed in outflows and jets, probably as a result of irradiation with stellar X-rays. The line may therefore be an interesting tracer for X-ray irradiated jets/outflows. These findings do not exclude additional [Ne II] contributions from disk surfaces, photoevaporative flows, or accretion flows close to the star.



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