Follow-up observations of binary ultra-cool dwarfs

¹ Astronomy Dpt, 601 Campbell Hall, UC Berkeley, Berkeley, CA 94720, USA e-mail: [hbouy;basri]@astro.berkeley.edu
² Instituto de Astrofísica de Canarias, c/ vía Láctea S/N, 38200 San Cristobal de La Laguna, Tenerife, Spain e-mail: [ege;lval;mosorio]@iac.es
³ University of Central Florida, Department of Physics, PO Box 162385, Orlando, 32816-2385, USA
⁴ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: [brandner;stumpf]@mpia.de
⁵ Laboratoire d'Astrophysique, Observatoire de Grenoble, UJF, CNRS, BP 53, 38041 Grenoble Cedex 9, France e-mail: [Thierry.Forveille;Xavier.Delfosse]@obs.ujf-grenoble.fr
⁶ Laboratorio de Astrofísca Espacial y Física Fundamental (LAEFF-INTA), Apdo 50727, 28080 Madrid, Spain e-mail: nhuelamo@laeff.inta.es
⁷ Universidad Nacional Autnónoma de México, Apartado Postal 72-3, 58089 Morelia, Michoacán, Mexico e-mail: girard@astroscu.unam.mx
⁸ Department of Physics and Astronomy, UCLA, Los Angeles, 90095-1547, USA e-mail: ghez@astro.ucla.edu
⁹ MIT Kavli Institute for Astrophysics & Space Research, 77 Massachusetts Ave., Building 37-664B, Cambridge, MA 02139, USA e-mail: ajb@mit.edu
¹⁰ Californian Institute of Technology, MC 105-24. 1200 East California Blvd, Pasadena 91125, USA e-mail: kelle@astro.caltech.edu

Received: 5 October 2007
Accepted: 31 January 2008

Abstract

Context. Astrometric observations of resolved binaries provide estimates of orbital periods and will eventually lead to measurement of dynamical masses. Only a few very low mass star and brown dwarf masses have been measured to date, and the mass-luminosity relation still needs to be calibrated.

Aims. We have monitored 14 very low mass multiple systems for several years to confirm their multiplicity and, for those with a short period, derive accurate orbital parameters and dynamical mass estimates.

Methods. We have used high spatial resolution images obtained at the Paranal, Lick and HST observatories to obtain astrometric and photometric measurements of the multiple systems at several epochs. The targets have periods ranging from 5 to 200 years, and spectral types in the range M7.5–T5.5.

Results. All of our 14 multiple systems are confirmed as common proper motion pairs. One system (2MASSW J0920122+351742) is not resolved in our new images, probably because the discovery images were taken near maximum elongation. Six systems have periods short enough to allow dynamical mass measurements within the next 15 to 20 years. We estimate that only 8% of the ultracool dwarfs in the solar neighborhood are binaries with separations large enough to be resolved, and yet periods short enough to derive astrometric orbital fits over a reasonable time frame with current instrumentation. A survey that doubles the number of ultracool dwarfs observed with high angular resolution is called for to discover enough binaries for a first attempt to derive the mass-luminosity relationship for very low-mass stars and brown dwarfs.