First determination of the dynamical mass of a binary L dwarf^*

¹ Max Planck für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching bei München, Germany e-mail: hbouy@mpe.mpg.de
² European Southern Observatory, Karl Schwarzschildstraße 2, 85748 Garching bei München, Germany
³ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 414 rue de la piscine, 38400 Saint Martin d'Hère, France e-mail: [Jerome.Bouvier;Gaspard.Duchene;Xavier.Delfosse]@obs.ujf-grenoble.fr
⁴ Division of Astronomy & Astrophysics, University of California in Los Angeles, Los Angeles, CA90095-1562, USA e-mail: [ghez;mccabe]@astro.ucla.edu
⁵ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: [brandner;koehler]@mpia.de
⁶ Instituto de Astrofisica de Canarias, 38200 La Laguna, Spain e-mail: ege@ll.iac.es
⁷ Canada-France-Hawaii Telescope Corporation, PO Box 1597, Kamuela Hi-96743, USA e-mail: forveille@cfht.hawaii.edu
⁸ Centre de Recherche Astronomique de Lyon (UML 5574), Ecole Normale Supérieure, 69364 Lyon Cedex 07, France e-mail: [fallard;ibaraffe]@ens-lyon.fr
⁹ University of California at Berkeley, Astronomy Department, MC 3411 Berkeley, CA 94720, USA e-mail: basri@astro.berkeley.edu
¹⁰ Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721-0065, USA e-mail: lclose@as.arizona.edu

Received: 29 March 2004
Accepted: 11 May 2004

Abstract

We present here the results of astrometric, photometric and spectroscopic observations leading to the determination of the orbit and dynamical masses of the binary L dwarf 2MASSW J0746425+2000321. High angular resolution observations spread over almost 4 years and obtained with the Hubble Space Telescope (HST), the ESO Very Large Telescope (VLT), and a the W. M. Keck Observatory (Keck) allow us to cover ~36% of the period, corresponding to 60% of the orbit, and, for the first time, to derive a precise estimate of the total and individual masses of such a late-type object. We find an orbital period of 3850.9 days. The corresponding total mass is 0.146 with uncertainties depending on the distance. Spatially resolved low resolution optical (550–1025 nm) spectra have been obtained with HST/STIS, allowing us to measure the spectral types of the two components ( for the primary and for the secondary). We also present precise photometry of the individual components measured on the high angular resolution images obtained with HST/ACS and WFPC2 (visible), VLT/NACO (J, H and K_S bands) and Keck I (K_S band). These spectral and photometric measurements enable us to estimate their effective temperatures and mass ratio, and to place the object accurately in a H–R diagram. The binary system is most likely formed by a primary with a mass of and a secondary with a mass of , thus clearly substellar, for an age of approximately Myr. Hα variability indicates chromospheric and/or magnetic activity.