Two very nearby (d ∼ 5 pc) ultracool brown dwarfs detected by their large proper motions from WISE, 2MASS, and SDSS data

Abstract

Aims. WISE provides an infrared all-sky survey that aims at completing our knowledge on the possibly dramatically increasing number of brown dwarfs with lower temperatures. We search for the nearest representatives of the coolest brown dwarfs, which will be very interesting for detailed follow-up observations, once they have been discovered. Methods. We used the preliminary data release from WISE, then selected bright candidates with colours typical of late-T dwarfs, and tried to match them with faint 2MASS and SDSS objects, to determine their proper motions, and to follow them up spectroscopically. Results. We have identified two new ultracool brown dwarfs with large proper motions, WISE J0254+0223 (2.5 arcsec/yr) and WISE J1741+2553 (1.5 arcsec/yr). With their w1-w2 ∼ 3.0 and J-w2 ∼ 4.0 colour indices, we expect both to have spectral types of ∼T8-T10 and absolute magnitude of Mw2 ∼ 14. We confirm WISE J1741+2553 as a T9-T10 dwarf from near-infrared spectroscopy with LBT/LUCIFER1. From their bright WISE w2 magnitudes of 12.7 and 12.3, we estimate distances of 5.5+2.3-1.6 pc and 4.6+1.2-1.0 pc and tangential velocities of ∼65 km s-1 and ∼34 km s-1 that indicate Galactic thick and thin disk membership, respectively. © 2011 ESO.