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 which 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 haven been discovered. Methods: We have used the preliminary data release from WISE, selected bright candidates with colours typical of late-T dwarfs, tried to match them with faint 2MASS and SDSS objects, to determine their proper motions, and to follow-up them spectroscopically. Results: We have identified two new ultracool brown dwarfs, WISE J0254+0223 and WISE J1741+2553, with large proper motions of about 2.5 and 1.5 arcsec/yr, respectively. With their w1-w2~3.0 and J-w2~4.0 colour indices we expect both to have a spectral type of ~T8-T10 and absolute magnitude of M_{w2}~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${}_{-1.6}^{+2.3}$ pc and 4.6${}_{-1.0}^{+1.2}$ pc and tangential velocities of ~65 km/s and ~34 km/s indicating Galactic thick and thin disk membership, respectively.

revised (accepted) version with new Fig.4, more comparison objects and references, slightly modified classification and more conservative accuracies in the photometric distance estimates

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