Two T dwarfs from the UKIDSS early data release

¹ Centre for Astrophysics Research, Science & Technology Research Institute, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK e-mail: trkendall@googlemail.com, d.j.pinfield@herts.ac.uk
² National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 Japan
³ Anglo-Australian Observatory, PO Box 296, Epping 1710, Australia
⁴ Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain
⁵ University of Central Florida, Physics Dept., PO Box 162385, Orlando, FL 32816-2385, USA
⁶ Subaru Telescope, 650 North A'ohoku Place, Hilo, Hi 96720, USA
⁷ Gemini Observatory, 670 North A'oholu Place, Hilo, Hawai'i 96720, USA
⁸ School of Physics & Astronomy, Cardiff University, 5, The Parade, Cardiff, CF24 3AA, Wales, UK
⁹ Inst. of Astronomy, Madingley Rd., Cambridge CB3 0HA, UK
¹⁰ C.R.A.L. (UMR 5574 CNRS), École Normale Supérieure, 69364 Lyon Cedex 07, France
¹¹ Laboratorio de Astrofísica Espacial y Física Fundamental, INTA, PO Box 50727, 28080 Madrid, Spain
¹² Dipartmento di Astronomia, Universitá di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
¹³ Dept. of Physics & Astronomy, University of Leicester, LE1 7RH, UK
¹⁴ Astronomical Institute, Academy of Sciences of the Czech Republic, Bocni II/1401a, 141 31 Prague, Czech Republic
¹⁵ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001 Santiago 19, Chile
¹⁶ Department of Astrophysics, Faculty of Science, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁷ Scottish Universities Physics Alliance, Institute for Astronomy, School of Physics, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹⁸ Fundación Galileo Galilei-INAF, Apartado 565, 38700 Santa Cruz de La Palma, Spain
¹⁹ University of Exeter, School of Physics, Stocker Road, Exeter, EX4 4QL, UK
²⁰ Main Astronomical Observatory, National Academy of Sciences, Zabolotnoho, Kyiv-127, 03680 Ukraine

Received: 15 September 2006
Accepted: 5 February 2007

Abstract

Context.We report on the first ultracool dwarf discoveries from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release (LAS EDR), in particular the discovery of T dwarfs which are fainter and more distant than those found using the 2MASS and SDSS surveys.

Aims.We aim to show that our methodologies for searching the ~27 deg² of the LAS EDR are successful for finding both L and T dwarfs via cross-correlation with the Sloan Digital Sky Survey (SDSS) DR4 release. While the area searched so far is small, the numbers of objects found shows great promise for near-future releases of the LAS and great potential for finding large numbers of such dwarfs.

Methods.Ultracool dwarfs are selected by combinations of their UKIDSS colours and SDSS DR4 and  colours, or, lower limits on these red optical/infrared colours in the case of DR4 dropouts. After passing visual inspection tests, candidates have been followed up by methane imaging and spectroscopy at 4 m and 8 m-class facilities.

Results.Our main result is the discovery following CH₄ imaging and spectroscopy of a T4.5 dwarf, ULAS J 1452+0655, lying ~80 pc distant. A further T dwarf candidate, ULAS J 1301+0023, has very similar CH₄ colours but has not yet been confirmed spectroscopically. We also report on the identification of a brighter L0 dwarf, and on the selection of a list of LAS objects designed to probe for T-like dwarfs to the survey J-band limit.

Conclusions.Our findings indicate that the combination of the UKIDSS LAS and SDSS surveys provide an excellent tool for identifying L and T dwarfs down to much fainter limits than previously possible. Our discovery of one confirmed and one probable T dwarf in the EDR is consistent with expectations from the previously measured T dwarf density on the sky.