An outflow origin of the [Ne II] emission in the T Tauri triplet^*

¹ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany e-mail: boekel@mpia.de
² Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
⁵ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d'Astrophysique, Bât. 709, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France

Received: 28 November 2008
Accepted: 22 January 2009

Abstract

Context. The 12.81 μm [Ne ii] line has recently gained interest as a potential tracer of gas in the tenuous surface layers of circumstellar disks and in outflow-related shocks. Evidence has been found for a proportionality between [Ne ii] emission and X-ray luminosity, supporting the hypothesis that X-rays are responsible for the required ionization and heating of the gas. Alternatively, ionization and heating by EUV photons and in J-type (dissociative) shocks has been proposed.

Aims. The T Tau multiple system harbors three stars with circumstellar disks, at least one strong X-ray source (T Tau N), and regions of shocked gas in the immediate vicinity. ISO and Spitzer spectra revealed remarkably strong [Ne ii] emission, but because of insufficient spatial and spectral resolution those observations could neither pinpoint where in the system the [Ne ii] emission arises, nor identify the emission mechanism. We aim to clarify this by observing the system with enough resolution to spatially separate the various components and spectrally resolve the line emission.

Methods. We performed high-resolution (R = 30 000) spectroscopy of the T Tau triplet at ~04 spatial resolution with VISIR at the VLT early February 2008. We spatially separated T Tau N from the southern close binary T Tau S, as well as the structures of shocked gas surrounding the stars. The individual southern components Sa and Sb remained spatially unresolved in our observations.

Results. The dominant component of [Ne ii] emission is centered on T Tau S and has a spatial extent of FWHM ~ 11 in a Gaussian fit. We detect spatially extended red-shifted emission NW of the system and fainter blue-shifted emission to the SE, which we associate with the N-S outflow from T Tau S. Only a small fraction of the [Ne ii] emission appears directly related to the X-ray bright northern component. Shocks may account for a substantial and possibly dominant fraction of the observed [Ne ii] emission. We estimate the total [Ne ii] flux to be 23±610^-16 Wm^-2, in good agreement with the values measured by ISO in late 1997 and Spitzer in early 2004.

Conclusions. Our observations show that outflows rather than the disk surface may dominate the observed [Ne ii] emission in stars with strong outflow activity. We propose [Ne ii] emission in jets as a major factor causing the observed large scatter in the L_X vs.  relation. We argue that T Tau S is the driving source of the T Tau “NW-blob”.