A Cross-Match of 2MASS and SDSS: Newly Found L and T Dwarfs and an Estimate of the Space Density of T Dwarfs

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 potentially metal-poor L2 dwarf with unusually blue near-IR colors. 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^(+0.0032)_ (-0.0030) pc^-3 (95% confidence interval), i.e., about one per 140 pc^3. Compared to predictions for the T dwarf space density that depend on various assumptions for the substellar mass function, this result is most consistent with models that assume a flat substellar mass function dN/dM ∝ M^0.0. No >T8 dwarfs were discovered in the present cross-match, although less than one was expected in the limited area (2099 deg^2) of SDSS DR1.

[1]  et al,et al.  Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: The Effects of Temperature, Clouds, and Gravity , 2004, astro-ph/0402451.

[2]  D. A. Golimowski,et al.  Preliminary Parallaxes of 40 L and T Dwarfs from the US Naval Observatory Infrared Astrometry Program , 2004 .

[3]  Bhasker K. Moorthy,et al.  The First Data Release of the Sloan Digital Sky Survey , 2003, astro-ph/0305492.

[4]  D. Schlegel,et al.  Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds , 1997, astro-ph/9710327.

[5]  et al,et al.  L Dwarfs Found in Sloan Digital Sky Survey Commissioning Imaging Data , 2000 .

[6]  M. Raddick,et al.  The Fifth Data Release of the Sloan Digital Sky Survey , 2007, 0707.3380.

[7]  J. Davy Kirkpatrick,et al.  New spectral types L and T , 2005 .

[8]  Michael C. Liu,et al.  Characterizing Young Brown Dwarfs Using Low-Resolution Near-Infrared Spectra , 2006, astro-ph/0611408.

[9]  S. Megeath,et al.  Spectroscopic Confirmation of the Least Massive Known Brown Dwarf in Chamaeleon , 2004, astro-ph/0411445.

[10]  Discovery of a Second L Subdwarf in the Two Micron All Sky Survey , 2004, astro-ph/0409179.

[11]  James Liebert,et al.  Spitzer IRAC Photometry of M, L, and T Dwarfs , 2006, astro-ph/0606432.

[12]  David G. Monet,et al.  Dwarfs Cooler than “M”: The Definition of Spectral Type “L” Using Discoveries from the 2-Micron All-Sky Survey (2MASS) , 1999 .

[13]  D. Burrows,et al.  Determination of Confidence Limits for Experiments with Low Numbers of Counts , 1991 .

[14]  D. Schlegel,et al.  Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds , 1998 .

[15]  Patrick J. Lowrance,et al.  Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574 , 2005 .

[16]  S. Seager,et al.  Clouds and chemistry: Ultracool dwarf atmospheric properties from optical and infrared colors , 2002 .

[17]  Pennsylvania State University,et al.  Seventy-One New L and T Dwarfs from the Sloan Digital Sky Survey , 2006, astro-ph/0601089.

[18]  John T. Rayner,et al.  An Infrared Spectroscopic Sequence of M, L, and T Dwarfs , 2004, astro-ph/0412313.

[19]  David G. Monet,et al.  � 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. DISCOVERY OF FOUR FIELD METHANE (T-TYPE) DWARFS WITH THE TWO MICRON ALL-SKY SURVEY 1 , 1999 .

[20]  N. Gehrels Confidence limits for small numbers of events in astrophysical data , 1986 .

[21]  Spectroscopy of New High Proper Motion Stars in the Northern Sky. I. New Nearby Stars, New High-Velocity Stars, and an Enhanced Classification Scheme for M Dwarfs , 2002, astro-ph/0209284.

[22]  Uomoto,et al.  The Missing Link: Early Methane (“T”) Dwarfs in the Sloan Digital Sky Survey , 2000, The Astrophysical journal.

[23]  M. Skrutskie,et al.  The Two Micron All Sky Survey (2MASS) , 2006 .

[24]  E. al.,et al.  The Sloan Digital Sky Survey: Technical summary , 2000, astro-ph/0006396.

[25]  Dagny L. Looper,et al.  Discovery of 11 New T Dwarfs in the Two Micron All Sky Survey, Including a Possible L/T Transition Binary , 2007, 0706.1601.

[26]  A. Burgasser Binaries and the L Dwarf/T Dwarf Transition , 2006, astro-ph/0611505.

[27]  F. Allard,et al.  Infrared Spectroscopy of Substellar Objects in Orion , 2001, astro-ph/0105154.

[28]  M. Fukugita,et al.  The Sloan Digital Sky Survey Photometric System , 1996 .

[29]  D. E. Trilling,et al.  The Substellar Mass Function: A Bayesian Approach , 2005, astro-ph/0502189.

[30]  S. Schmidt,et al.  Activity and Kinematics of Ultracool Dwarfs, Including an Amazing Flare Observation , 2007, astro-ph/0701055.

[31]  Howard Raiffa,et al.  Applied Statistical Decision Theory. , 1961 .

[32]  David A. Golimowski,et al.  SDSS J1534+1615AB: A Novel T Dwarf Binary Found with Keck Laser Guide Star Adaptive Optics and the Potential Role of Binarity in the L/T Transition , 2006, astro-ph/0605037.

[33]  John E. Gizis M-subdwarfs: spectroscopic classification and the metallicity scale , 1997 .

[34]  A. Burgasser T Dwarfs and the Substellar Mass Function. I. Monte Carlo Simulations , 2004, astro-ph/0407624.

[35]  H Germany,et al.  A Method of Correcting Near‐Infrared Spectra for Telluric Absorption , 2002, astro-ph/0211255.

[36]  Alexander S. Szalay,et al.  Sloan digital sky survey: Early data release , 2002 .

[37]  I. Reid,et al.  Hubble Space Telescope NICMOS Observations of T Dwarfs: Brown Dwarf Multiplicity and New Probes of the L/T Transition , 2006, astro-ph/0605577.

[38]  et al,et al.  Sloan Digital Sky Survey Imaging of Low Galactic Latitude Fields: Technical Summary and Data Release , 2004, astro-ph/0409700.

[39]  John T. Rayner,et al.  SpeX: A Medium‐Resolution 0.8–5.5 Micron Spectrograph and Imager for the NASA Infrared Telescope Facility , 2003 .

[40]  Marco Bonati,et al.  The Automated Palomar 60 Inch Telescope , 2006, astro-ph/0608323.

[41]  G. Fazio,et al.  The Infrared Array Camera (IRAC) for the Spitzer Space Telescope , 2004, astro-ph/0405616.

[42]  et al,et al.  A Survey of z > 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z ∼ 6 , 2001, astro-ph/0108063.

[43]  M. Irwin,et al.  The UKIRT Infrared Deep Sky Survey (UKIDSS) , 2006, astro-ph/0604426.

[44]  Wm. A. Wheaton,et al.  2MASS All Sky Catalog of point sources. , 2003 .

[45]  Johns Hopkins University,et al.  Characterization of M, L, and T Dwarfs in the Sloan Digital Sky Survey , 2002, astro-ph/0204065.

[46]  A. A. Henden,et al.  Astrometry and photometry for cool dwarfs and brown dwarfs , 2002 .

[47]  et al,et al.  Toward Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis , 2001, astro-ph/0108443.

[48]  John T. Rayner,et al.  Spextool: A Spectral Extraction Package for SpeX, a 0.8–5.5 Micron Cross‐Dispersed Spectrograph , 2004 .

[49]  Robert Lupton,et al.  A Modified Magnitude System that Produces Well-Behaved Magnitudes, Colors, and Errors Even for Low Signal-to-Noise Ratio Measurements , 1999, astro-ph/9903081.

[50]  James Liebert,et al.  Meeting the Cool Neighbors. IX. The Luminosity Function of M7-L8 Ultracool Dwarfs in the Field , 2006, astro-ph/0609648.

[51]  M. Skrutskie,et al.  L Dwarfs and the Substellar Mass Function , 1999, astro-ph/9905170.

[52]  Adam J. Burgasser,et al.  Optical Spectroscopy of 2MASS Color-Selected Ultracool Subdwarfs , 2006 .

[53]  Brown Dwarf Companions to G-Type Stars. I. Gliese 417B and Gliese 584C , 2001, astro-ph/0103218.

[54]  Sandy K. Leggett,et al.  JHK Magnitudes for L and T Dwarfs and Infrared Photometric Systems , 2004 .

[55]  H. Raiffa,et al.  Applied Statistical Decision Theory. , 1961 .

[56]  et al,et al.  Infrared Photometry of Late-M, L, and T Dwarfs , 2001, astro-ph/0108435.

[57]  R. Paul Butler,et al.  Four New Planets Orbiting Metal-enriched Stars* , 2002 .

[58]  I. Reid,et al.  The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics , 2003, astro-ph/0304174.

[59]  Robert Jedicke,et al.  Pan-STARRS: A Large Synoptic Survey Telescope Array , 2002, SPIE Astronomical Telescopes + Instrumentation.

[60]  Adam Burrows,et al.  Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs , 2003, astro-ph/0304226.

[61]  Gibor BasriAnsgar Reiners A Survey for Spectroscopic Binaries among Very Low Mass Stars , 2006 .

[62]  Edwin A. Valentijn,et al.  Survey and other telescope technologies and discoveries , 2002 .

[63]  Adam J. Burgasser,et al.  The NIRSPEC Brown Dwarf Spectroscopic Survey. I. Low-Resolution Near-Infrared Spectra , 2003, astro-ph/0309257.

[64]  J. Mather,et al.  Space Telescopes and Instrumentation I: Optical, Infrared, and Millimeter , 2006 .

[65]  et al,et al.  The Discovery of a Field Methane Dwarf from Sloan Digital Sky Survey Commissioning Data , 1999, astro-ph/9905391.

[66]  Harland W. Epps,et al.  THE KECK LOW-RESOLUTION IMAGING SPECTROMETER , 1995 .

[67]  David A. Golimowski,et al.  A Unified Near-Infrared Spectral Classification Scheme for T Dwarfs , 2006 .

[68]  David G. Monet,et al.  67 Additional L Dwarfs Discovered by the Two Micron All Sky Survey , 2000, astro-ph/0003317.

[69]  A Search for Binary Systems among the Nearest L Dwarfs , 2006, astro-ph/0606331.

[70]  Uomoto,et al.  The Discovery of a Second Field Methane Brown Dwarf from Sloan Digital Sky Survey Commissioning Data , 2000, The Astrophysical journal.