EDP Sciences
Free access
Volume 484, Number 2, June III 2008
Page(s) 413 - 418
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20078381
Published online 19 March 2008

A&A 484, 413-418 (2008)
DOI: 10.1051/0004-6361:20078381

Confirmation of the binary status of Chamaeleon H$\alpha$ 2 - a very young low-mass binary in Chamaeleon

T. O. B. Schmidt1, R. Neuhäuser1, N. Vogt2, 3, A. Seifahrt1, T. Roell1, and A. Bedalov1

1  Astrophysikalisches Institut und Universitäts-Sternwarte, Universität Jena, Schillergäßchen 2-3, 07745 Jena, Germany
    e-mail: tobi@astro.uni-jena.de
2  Departamento de Física y Astronomía, Universidad de Valparaíso, Avenida Gran Bretaña 1111, Valparaíso, Chile
3  Instituto de Astronomía, Universidad Catolica del Norte, Avda. Angamos 0610, Antofagasta, Chile

Received 30 July 2007 / Accepted 29 January 2008

Context. Neuhäuser & Comerón (1998, Science, 282, 83; 1999, A&A, 350, 612) presented direct imaging evidence, as well as first spectra, of several young stellar and sub-stellar M6- to M8-type objects in the Cha I dark cloud. One of these objects is Cha H$\alpha$ 2, classified as brown dwarf candidate in several publications and suggested as possible binary in Neuhäuser et al. (2002, A&A, 384, 999).
Aims. We have searched around Cha H$\alpha$ 2 for close and faint companions with adaptive optics imaging.
Methods. Two epochs of direct imaging data were taken with the Very Large Telescope (VLT) Adaptive Optics instrument NACO in February 2006 and March 2007 in $K{\rm s}$-band together with a Hipparcos binary for astrometric calibration. Moreover, we took a J-band image in March 2007 to get color information. We retrieved an earlier image from 2005 from the European Southern Observatory (ESO) Science Archive Facility, increasing the available time coverage. After confirmation of common proper motion, we deduce physical parameters of the objects by spectroscopy, like temperature and mass.
Results. We find Cha H$\alpha$ 2 to be a very close binary of ~0.16 arcsec separation, having a flux ratio of ~0.91, thus having almost equal brightness and indistinguishable spectral types within the errors. We show that the two tentative components of Cha H$\alpha$ 2 form a common proper motion pair, and that neither component is a non-moving background object. We even find evidence for orbital motion. A combined spectrum of both stars spanning optical and near-infrared parts of the spectral energy distribution yields a temperature of 3000 $\pm$ 100 K, corresponding to a spectral type of M6 $\pm$ 1 and a surface gravity of $\log{g}$ = 4.0 +0.75-0.5, both from a comparison with GAIA model atmospheres. Furthermore, we obtained an optical extinction of $A_{\rm V}$ $\simeq$ 4.3 mag from this comparison.
Conclusions. We derive masses of ~0.110 $M_{\odot}$ ($\geq$0.070 $M_{\odot}$) and ~0.124 $M_{\odot}$ ($\geq$0.077 $M_{\odot}$) for the two components of Cha H$\alpha$ 2, i.e., probably low-mass stars, but one component could possibly be a brown dwarf.

Key words: stars: low-mass, brown dwarfs -- stars: binaries: close -- stars: binaries: visual -- stars: individual: Cha H$\alpha$ 2

© ESO 2008