Astrometric planet search around southern ultracool dwarfs

III. Discovery of a brown dwarf in a 3-year orbit around DE0630−18^⋆

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 Main Astronomical Observatory, National Academy of Sciences of the Ukraine, Zabolotnogo 27, 03680 Kyiv, Ukraine
³ Observatoire de Genève, Université de Genève, 51 chemin Des Maillettes, 1290 Versoix, Switzerland
⁴ INTA-CSIC Centro de Astrobiología, 28850 Torrejón de Ardoz, Madrid, Spain
⁵ University of Cambridge, Cavendish Laboratory, J J Thomson Avenue, Cambridge, CB3 0HE, UK

Received: 27 January 2015
Accepted: 26 February 2015

Abstract

Using astrometric measurements obtained with the FORS2/VLT camera, we are searching for low-mass companions around 20 nearby ultracool dwarfs. With a single-measurement precision of ~0.1 milli-arcsec, our survey is sensitive to a wide range of companion masses from planetary companions to binary systems. Here, we report the discovery and orbit characterisation of a new ultracool binary at a distance of 19.5 pc from Earth that is composed of the M8.5-dwarf primary DE0630−18 and a substellar companion. The nearly edge-on orbit is moderately eccentric (e = 0.23) with an orbital period of 1120 d, which corresponds to a relative separation in semimajor axis of approximately 1.1 AU. We obtained a high-resolution optical spectrum with UVES/VLT and measured the system’s heliocentric radial velocity. The spectrum does not exhibit lithium absorption at 670.8 nm, indicating that the system is not extremely young. A preliminary estimate of the binary’s physical parameters tells us that it is composed of a primary at the stellar-substellar limit and a massive brown-dwarf companion. DE0630−18 is a new very low-mass binary system with a well-characterised orbit.