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Astron. Astrophys. 360, L39-L42 (2000)

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Letter to the Editor

Spectrum and proper motion of a brown dwarf companion of the T Tauri star CoD-33°7795 *

R. Neuhäuser 1, E.W. Guenther 2, M.G. Petr 3, W. Brandner 4, N. Huélamo 1 and J. Alves 5

1 MPI für extraterrestrische Physik, 85740 Garching, Germany
2 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
3 MPI für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
4 University of Hawaii, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
5 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

Received 26 June 2000 / Accepted 15 July 2000

Abstract

We present optical and infrared spectra as well as the proper motion of an H=12 mag object [FORMULA] off the [FORMULA] mag brighter spectroscopic binary star CoD-33°7795 (=TWA-5), a member of the TW Hya association of T Tauri stars at [FORMULA] pc. It was suggested as companion candidate by Lowrance et al. (1999) and Webb et al. (1999), but neither a spectrum nor the proper motion of the faint object were available before. Our spectra taken with FORS2 and ISAAC at the ESO-VLT reveal that the companion candidate has spectral type M8.5 to M9. It shows strong H[FORMULA] emission and weak Na I absorption, both indicative of a young age. The faint object is clearly detected and resolved in our optical and infrared images, with a FWHM of [FORMULA] in the FORS2 image. The faint object's proper motion, based on two year epoch difference, is consistent with the proper motion of CoD-33°7795 by 5 Gaussian [FORMULA] significance. From three different theoretical pre-main sequence models, we estimate the companion mass to be between [FORMULA] and 40 [FORMULA], assuming the distance and age of the primary. A slight offset between the VLT and HST images with an epoch difference of two years can be interpreted as orbital motion. The probability for chance alignment of such a late-type object that close to CoD-33°7795 with the correct proper motion is below [FORMULA]. Hence, the faint object is physically associated with CoD-33°7795, the 4th brown dwarf companion around a normal star confirmed by both spectrum and proper motion, the first around a pre-main sequence star.

Key words: stars: binaries: visual – stars: individual: CoD-33 7795 – stars: late-type – stars: pre-main sequence

* Based on observations obtained at the European Southern Observatory, Cerro Paranal, partly from program 65.L-0144 and partly based on public data released from FORS2 technical observations at the VLT Kueyen telescope

Send offprint requests to: R. Neuhäuser (rne@mpe.mpg.de)

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000
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