Photometric brown-dwarf classification

I. A method to identify and accurately classify large samples of brown dwarfs without spectroscopy

¹ Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
e-mail: sjw4@imperial.ac.uk
² Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
³ Department of Mathematics, Imperial College London, London SW7 2AZ, UK
⁴ Department of Physics, University of California, San Diego, CA 92093, USA
⁵ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

Received: 9 July 2014
Accepted: 27 November 2014

Abstract

Aims. We present a method, named photo-type, to identify and accurately classify L and T dwarfs onto the standard spectral classification system using photometry alone. This enables the creation of large and deep homogeneous samples of these objects efficiently, without the need for spectroscopy.

Methods. We created a catalogue of point sources with photometry in 8 bands, ranging from 0.75 to 4.6 μm, selected from an area of 3344 deg², by combining SDSS, UKIDSS LAS, and WISE data. Sources with 13.0 <J< 17.5, and Y − J> 0.8, were then classified by comparison against template colours of quasars, stars, and brown dwarfs. The L and T templates, spectral types L0 to T8, were created by identifying previously known sources with spectroscopic classifications, and fitting polynomial relations between colour and spectral type.

Results. Of the 192 known L and T dwarfs with reliable photometry in the surveyed area and magnitude range, 189 are recovered by our selection and classification method. We have quantified the accuracy of the classification method both externally, with spectroscopy, and internally, by creating synthetic catalogues and accounting for the uncertainties. We find that, brighter than J = 17.5, photo-type classifications are accurate to one spectral sub-type, and are therefore competitive with spectroscopic classifications. The resultant catalogue of 1157 L and T dwarfs will be presented in a companion paper.