Fifteen new T dwarfs discovered in the UKIDSS Large Area Survey

Fifteen new T dwarfs discovered in the UKIDSS Large Area Survey,10.1111/j.1365-2966.2008.13729.x,Monthly Notices of The Royal Astronomical Society,D.

Fifteen new T dwarfs discovered in the UKIDSS Large Area Survey   (Citations: 11)
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D. J. Pinfield, B. Burningham, M. Tamura, S. K. Leggett, N. Lodieu, P. W. Lucas, D. J. Mortlock, S. J. Warren, D. Homeier, M. Ishii, N. R. Deacon, R. G. McMahon
We present the discovery of 15 new T2.5-T7.5 dwarfs (with estimated distances ~24-93pc), identified in the first three main data releases of the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey. This brings the total number of T dwarfs discovered in the Large Area Survey (LAS) (to date) to 28. These discoveries are confirmed by near-infrared spectroscopy, from which we derive spectral types on the unified scheme of Burgasser et al. Seven of the new T dwarfs have spectral types of T2.5-T4.5, five have spectral types of T5-T5.5, one is a T6.5p and two are T7-7.5. We assess spectral morphology and colours to identify T dwarfs in our sample that may have non-typical physical properties (by comparison to solar neighbourhood populations), and find that three of these new T dwarfs may have unusual metallicity, two may have low surface gravity, and one may have high surface gravity. The colours of the full sample of LAS T dwarfs show a possible trend to bluer Y - J with decreasing effective temperature, and some interesting colour changes in J - H and z - J (deserving further investigation) beyond T8. The LAS T dwarf sample from the first and second main data releases show good evidence for a good level of completion to J = 19. By accounting for the main sources of incompleteness (selection, follow-up and spatial) as well as the effects of unresolved binarity, Malmquist and Eddington bias, we estimate that there are 17 +/- 4 >= T 4 dwarfs in the J <= 19 volume of the LAS second data release. This value is most consistent with theoretical predictions if the substellar mass function exponent alpha (dN/dm ~ m-alpha) lies between -1.0 and 0. This is consistent with the latest 2-Micron All Sky Survey (2MASS)/Sloan Digital Sky Survey (SDSS) constraint (which is based on lower number statistics) and is significantly lower than the alpha ~ 1.0 suggested by L dwarf field populations, which is possibly a result of the lower mass range probed by the T dwarf class.
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