DE0823−49 is a juvenile binary brown dwarf at 20.7 pc^⋆

¹ European Space Agency, European Space Astronomy Centre, PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain
e-mail: Johannes.Sahlmann@esa.int
² Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA 92093, USA
³ INTA-CSIC Centro de Astrobiología, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Main Astronomical Observatory, National Academy of Sciences of the Ukraine, Zabolotnogo 27, 03680 Kyiv, Ukraine
⁵ Observatoire de Genève, Université de Genève, 51 chemin Des Maillettes, 1290 Versoix, Switzerland
⁶ University of Cambridge, Cavendish Laboratory, J J Thomson Avenue, Cambridge, CB3 0HE, UK

Received: 18 December 2014
Accepted: 28 May 2015

Abstract

Astrometric monitoring of the nearby early-L dwarf DE0823−49 has revealed a low-mass companion in a 248-day orbit that was announced in an earlier work. Here, we present new astrometric and spectroscopic observations that allow us to characterise the system in detail. The optical spectrum shows Li i-absorption indicative of a young age and/or substellar mass for the primary component. The near-infrared spectrum is best reproduced by a binary system of brown dwarfs with spectral types of L1.5 + L5.5 and effective temperatures of 2150 ± 100 K and 1670 ± 140 K. To conform with the photocentric orbit size measured with astrometry and the current understanding of substellar evolution, the system must have an age in the 80−500 Myr range. Evolutionary models predict component masses in the ranges of M₁ ≃ 0.028−0.063 M_⊙ and M₂ ≃ 0.018−0.045 M_⊙ with a mass ratio of q ≃ 0.64−0.74. Multi-epoch radial velocity measurements unambiguously establish the three-dimensional orbit of the system and allow us to investigate its kinematic properties. DE0823−49 emerges as a rare example of a nearby brown dwarf binary with orbit, component properties, and age that are characterised well. It is a juvenile resident of the solar neighbourhood, but does not appear to belong to a known young association or moving group.