Letter to the Editor

The widest ultracool binary^*

Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: caballero@mpia.de

Received: 24 November 2006
Accepted: 12 December 2006

Abstract

Aims.I test the ejection scenario for the formation of brown dwarfs and low-mass stars through the detection of a very wide ultracool binary.

Methods.LEHPM 494 (M6.0 ± 1.0 V) and DENIS-P J0021.0–4244 (M9.5 ± 0.5 V) are separated by 1.3 arcmin and are high proper motion co-moving ultracool stars. I have used six astrometric epochs spaced by 22 years to confirm their common tangential velocity.

Results.The angular separation between both low-mass stars remains constant with an uncertainty of less than 0.1%. I have also derived their most probable heliocentric distance (23 ± 2 pc), age interval (2–10 Ga) and masses (0.103 ± 0.006 and 0.079 ± 0.004 ). The pair, with a projected physical separation of 1800 ± 170 AU, is by far the widest ultracool binary yet found in the field.

Conclusions.This serendipitous and simple detection is inconsistent with ultra low-mass formation ejection scenarios and complements current searches for low-mass tight binaries.