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Post Info TOPIC: T dwarfs


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Title: Fifteen new T dwarfs discovered in the UKIDSS Large Area Survey
Authors: D. J. Pinfield (1), B. Burningham (1), M. Tamura (2), S. K. Leggett (3), N. Lodieu (4), P. W. Lucas (1), D. J. Mortlock (5), S. J. Warren (5), D. Homeier (6), M. Ishi (7), N. R. Deacon (8), R. G. McMahon (9), P. C. Hewett (9), M. R. Zapatero Osorio (4), E. L. Martin (4), H. R. A. Jones (1), B.P. Venemans (9), A. Day-Jones (1), P. D. Dobbie (10), S. L. Folkes (1), S. Dye (11), F. Allard (12), I. Baraffe (13), D. Barrado y Navascues (14), S. L. Casewell (15), K. Chiu (16), G. Chabrier (13), F. Clarke (17), S. T. Hodgkin (9), A. Magazzu (18), M. J. McCaughrean (16), E. Moraux (19), T. Nakajima (2), Y. Pavlenko (20), C. G. Tinney (21) ((1) University of Hertfordshire, UK, (2) National Astronomical Observatory, Japan, (3) Gemini Observatory, USA, (4) Instituto de Astrofisica de Canarias, Spain, (5) Imperial College London, UK, (6) Georg-August-Universitat, Germany, (7) Subaru Telescope, USA, (8) Radboud University, The Netherlands, (9) Cambridge University, UK, (10) Anglo-Australian Observatory, Australia, (11) Cardif University, UK, (12) Universite de Lyon, France, (13) C.R.A.L., France, (14) Laboratorio de Astrofisica Espacial y Fisica Fundamental, Spain, (15) University of Leicester, UK, (16) University of Exeter, UK, (17) European Southern Observatory, Chile, (18) Fundacion Galileo Galilei-INAF, Spain, (19) Laboratoire d'Astrophysique de Grenoble, France, (20) Main Astronomical Observatory, Ukraine, (21) University of New South Wales, Australia)

We present the discovery of fifteen new T2.5-T7.5 dwarfs (with estimated distances between ~24-93pc, identified in the first three main data releases of the UKIRT Infrared Deep Sky Survey. This brings the total number of T dwarfs discovered in the Large Area Survey (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. (2006). 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). The colours of the full sample of LAS T dwarfs show a possible trend to bluer Y-J with decreasing effective temperature beyond T8. By accounting for the main sources of incompleteness (selection, follow-up and spatial) as well as the effects of unresolved binarity and Malmquist bias, we estimate that there are 174 >=T4 dwarfs in the J<=19 volume of the LAS second data release. Comparing this to theoretical predictions is most consistent with a sub-stellar mass function exponent alpha between -1.0 and 0. This is consistent with the latest 2MASS/SDSS constraint (which is based on lower number statistics), and is significantly lower than the alpha~1.0 suggested by L dwarf field populations, possibly a result of the lower mass range probed by the T dwarf class.

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-- Edited by Blobrana at 08:51, 2008-06-03

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Title: A Cross-Match of 2MASS and SDSS: Newly-Found L and T Dwarfs and an Estimate of the Space Densitfy of T Dwarfs
Authors: Stanimir Metchev (1), J. Davy Kirkpatrick (2), G. Bruce Berriman (2), Dagny Looper (3) ((1) UCLA, (2) Caltech/IPAC, (3) U Hawaii/IfA)

We report new L and T dwarfs found in a cross-match of the SDSS Data Release 1 and 2MASS. Our simultaneous search of the two databases effectively allows us to relax the criteria for object detection in either survey and to explore the combined databases to a greater completeness level. We find two new T dwarfs in addition to the 13 already known in the SDSS DR1 footprint. We also identify 22 new candidate and bona-fide L dwarfs, including a new young L2 dwarf and a peculiar L2 dwarf with unusually blue near-IR colors: potentially the result of mildly sub-solar metallicity. These discoveries underscore the utility of simultaneous database cross-correlation in searching for rare objects. Our cross-match completes the census of T dwarfs within the joint SDSS and 2MASS flux limits to the 97% level. Hence, we are able to accurately infer the space density of T dwarfs. We employ Monte Carlo tools to simulate the observed population of SDSS DR1 T dwarfs with 2MASS counterparts and find that the space density of T0-T8 dwarf systems is 0.0070 (-0.0030; +0.0032) per cubic parsec (95% confidence interval), i.e., about one per 140 cubic parsecs. Compared to predictions for the T dwarf space density that depend on various assumptions for the sub-stellar mass function, this result is most consistent with models that assume a flat sub-stellar mass function dN/dM ~ M^0. No >T8 dwarfs were discovered in the present cross-match, though less than one was expected in the limited area (2099 sq. degrees) of SDSS DR1.

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Title: Two T dwarfs from the UKIDSS Early Data Release
Authors: T. R. Kendall, M. Tamura, C. G. Tinney, E. L. Martin, M. Ishii, D. J. Pinfield, P. W. Lucas, H. R. A. Jones, S. K. Leggett, S. Dye, P. C. Hewett, F. Allard, I. Baraffe, D. Barrado y Navascues, G. Carraro, S. L. Casewell, G. Chabrier, R. J. Chappelle, F. Clarke, A. Day-Jones, N. Deacon, P. D. Dobbie, S. Folkes, N. C. Hambly, S. T. Hodgkin, T. Nakajima, R. F. Jameson, N. Lodieu, A. Magazzu, M. J. McCaughrean, Y. V. Pavlenko, N. Tadashi, M. R. Zapatero Osorio

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. We aim to show that our methodologies for searching the ~27 sq degs 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. Ultracool dwarfs are selected by combinations of their YJH(K) UKIDSS colours and SDSS DR4 z-J and i-z 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 4m and 8m-class facilities. Our main result is the discovery following CH4 imaging and spectroscopy of a T4.5 dwarf, ULASJ 1452+0655, lying ~80pc distant. A further T dwarf candidate, ULASJ 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. 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.

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