Volume 543, July 2012
|Number of page(s)||20|
|Section||Interstellar and circumstellar matter|
|Published online||22 June 2012|
On the origin of [Ne II] emission in young stars: mid-infrared and optical observations with the Very Large Telescope⋆,⋆⋆
1 ISDC Data Centre for Astrophysics, Université de Genève, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
2 Observatoire Astronomique de l’Université de Genève, Chemin de Maillettes 51, 1290 Sauverny, Switzerland
3 University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
4 Spitzer Science Center, California Institute of Technology, 220-6 1200 East California Boulevard, Pasadena, CA 91125, USA
5 Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309-0389, USA
6 Universitäts-Sternwarte München, Scheinerstrasse 1, 81679 München, Germany
7 Cluster of Excellence Origin and Structure of the Universe, Boltzmannstrasse 2, 85748 Garching, Germany
Received: 25 October 2011
Accepted: 9 May 2012
Context. The [Ne II] line 12.81 μm was proposed to be a good tracer of gas in the environments of proto-planetary disks; its origin is explained by different mechanisms: jets in outflows, photo-evaporative disk winds driven by stellar X-rays/EUV or by the X-ray irradiated proto-planetary disk atmosphere. Previous Spitzer studies gave hints toward the neon emitting mechanism by exploring correlations between the line luminosity and properties of the star-disk system. These studies concluded that the origin of the emission is likely related to accretion and outflows, with some influence from X-rays.
Aims. We provide direct constraints on the origin of the [Ne II] emission using high-spatial and spectral resolution observations that allow us to study the kinematics of the emitting gas. In addition we compare the [Ne II] line with optical forbidden lines.
Methods. We obtained high-resolution ground-based observations with VISIR-VLT for 15 stars and UVES-VLT for three of them. The stars were chosen for having bright neon emission lines detected with Spitzer/IRS. The velocity shifts and profiles are used to disentangle the different emitting mechanisms producing the [Ne II] line. A comparison between results from this study and previous high-resolution studies is also presented.
Results. The [Ne II] line was detected in seven stars, among them the first confirmed detection of [Ne II] in a Herbig Be star, V892 Tau. In four cases, the large blueshifted lines indicate an origin in a jet. In two stars, the small shifts and asymmetric profiles indicate an origin in a photo-evaporative wind. CoKu Tau 1, seen close to edge-on, shows a spatially unresolved line centered at the stellar rest velocity, although cross-dispersion centroids move within 10 AU from one side of the star to the other as a function of wavelength. The line profile is symmetric with wings extending up to ~±80 km s-1. The origin of the [Ne II] line is unclear and could either be due to the bipolar jet or to the disk. For the stars with VLT-UVES observations, in several cases, the optical forbidden line profiles and shifts are very similar to the profile of the [Ne II] line, suggesting that the lines are emitted in the same region. A general trend observed with VISIR is a lower line flux when compared with the fluxes obtained with Spitzer. We found no correlation between the line full-width at half maximum and the line peak velocity. The [Ne II] line remains undetected in a large part of the sample, an indication that the emission detected with Spitzer in those stars is likely extended.
Key words: stars: pre-main sequence / stars: formation / ISM: jets and outflows / infrared: stars / protoplanetary disks
Based on observations made with ESO Telescopes Kueyen/UT2 and Melipal/UT3 at the Paranal Observatory under programs ID 083.C-0471, 084.C-1062, 086.C-0911, and 286.C-5038.
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2012
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