The Spectra of T Dwarfs. I. Near-Infrared Data and Spectral Classification

By A. Burgasser, J. D. Kirkpatrick, Michael E. Brown, I. Neill Reid, Adam Burrows, J. Liebert, K. Matthews, J. Gizis, C. Dahn, D. Monet and 2 more. 2001

357 citations recorded by the source corpus.

We present near-infrared spectra for a sample of T dwarfs, including 11 new discoveries made using the 2 Micron All Sky Survey. These objects are distinguished from warmer (L-type) brown dwarfs by the presence of methane absorption bands in the 1-2.5 μm spectral region. A first attempt at a near-infrared classification scheme for T dwarfs is made, based on the strengths of CH4 and H2O bands and the shapes of the 1.25, 1.6, and 2.1 μm flux peaks. Subtypes T1 V-T8 V are defined, and spectral indices useful for classification are presented. The subclasses appear to follow a decreasing Teff scale, based on the evolution of CH4 and H2O bands and the properties of L and T dwarfs with known distances. However, we speculate that this scale is not linear with spectral type for cool dwarfs, due to the settling of dust layers below the photosphere and subsequent rapid evolution of spectral morphology around Teff ~ 1300-1500 K. Similarities in near-infrared colors and continuity of spectral features suggest that the gap between the latest L dwarfs and earliest T dwarfs has been nearly bridged. This argument is strengthened by the possible role of CH4 as a minor absorber, shaping the K-band spectra of the latest L dwarfs. Finally, we discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infrared colors (J - Ks = -0.89 ± 0.24) due to suppression of the 2.1 μm peak. The feature is likely caused by enhanced collision-induced H2 absorption in a high-pressure or low-metallicity photosphere.

Subfields: Astronomy and Astrophysics, Education, Instrumentation

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Citations in the corpus

MID-INFRARED PHOTOMETRY OF COLD BROWN DWARFS: DIVERSITY IN AGE, MASS, AND METALLICITY (2010, background): “…on-line material; for completeness we list here the discovery and spectral classification references: Becklin & Zuckerman (1988); Bidelman (1985); Burgasser et al. (1999, 2000a, 2000b, 2002, 2003b, 2004b, 2006b); Burningham et al. (2008, 2009, 2010); Chiu et al. (2006, 2008); Cruz et al. (2003);…”
The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics (2003, methodology): “2MASS 0532+8246 was identified in a search for T dwarfs using the 2MASS working database (Burgasser et al. 2002).”
The UKIRT Infrared Deep Sky Survey ZY JHK photometric system: passbands and synthetic colours (2006, background): “…temperatures the colours reverse and track back up the colour sequence, becoming successively bluer, until the coolest T dwarfs are bluer than O stars (due to the presence of strong absorption bands of water, methane and pressureinduced H2; see e.g. Burgasser et al. 2002; Geballe et al. 2002).”
THE FIRST HUNDRED BROWN DWARFS DISCOVERED BY THE WIDE-FIELD INFRARED SURVEY EXPLORER (WISE) (2011, background): “Whereas there were 16 objects known previously with types of T8 or later (Burgasser et al. 2002, Tinney et al. 2005, Looper et al. 2007, Warren et al. 2007, Delorme et al. 2008, Burningham et al. 2008, Burningham et al. 2009, Burningham et al. 2010b, Goldman et al. 2010, Lucas et al. 2010, Delorme…”
The 2MASS Wide-Field T Dwarf Search. III. Seven New T Dwarfs and Other Cool Dwarf Discoveries (2004, methodology): “These objects exhibit a broad range of CH4 band strengths and near-infrared colors (−0.7 < J − Ks < 0.85), encompassing early, mid-, and late-type T dwarf spectral morphologies (Burgasser et al. 2002; Geballe et al. 2002).”
New spectral types L and T (2005, methodology): “5-T0 (optical/near-infrared mismatch noted by Cruz et al. 2003; duplicity noted by Burgasser, priv. comm.), and 2MASS J0937 + 2931 at T6-T7 (spectroscopic peculiarity noted by Burgasser et al. 2002a).”
ERRATUM: “THE LUMINOSITY AND MASS FUNCTIONS OF LOW-MASS STARS IN THE GALACTIC DISK. II. THE FIELD” (2010, AJ, 139, 2679) (2010, methodology): “In particular, 2MASS photometry led to the discovery of two new spectral classes, L and T (Kirkpatrick et al. 1999; Burgasser et al. 2002), and was used to trace the structure of the Sagittarius dwarf galaxy (Majewski et al. 2003) with M giants.”
WEATHER ON OTHER WORLDS. II. SURVEY RESULTS: SPOTS ARE UBIQUITOUS ON L AND T DWARFS (2014, background): “Ackerman & Marley (2001) proposed that such heterogeneous cloud structures may be present in brown dwarf atmospheres, leading to rotationally modulated flux variations and explaining many of the unusual characteristics of the L dwarf/T dwarf transition (Burgasser et al. 2002; Marley et al. 2010).”
SpeX SPECTROSCOPY OF UNRESOLVED VERY LOW MASS BINARIES. I. IDENTIFICATION OF 17 CANDIDATE BINARIES STRADDLING THE L DWARF/T DWARF TRANSITION (2009, background): “Careful tuning of cloud parameters (e.g, settling efficiency, cloud coverage or cloud top temperature) can reproduce the characteristics of the L dwarf/T dwarf transition, but in an arguably ad-hoc manner (e.g., Burgasser et al. 2002b; Tsuji 2005; Cushing et al. 2008).”
Evidence of Cloud Disruption in the L/T Dwarf Transition (2002, methodology): “First, the Teff scale between the late-type L and early-type T dwarfs should be very narrow; this has already been surmised by a number of studies (Kirkpatrick et al. 2000; Burgasser et al. 2002a).”
DISCOVERIES FROM A NEAR-INFRARED PROPER MOTION SURVEY USING MULTI-EPOCH TWO MICRON ALL-SKY SURVEY DATA (2010, background): “Because of its blue color (J-Ks=0.475±0.156), it was observed as a candidate T dwarf in the survey of Burgasser et al. (2002) and was not found in deeper infrared images obtained on 2000 May 20 (UT).”
A STUDY OF THE DIVERSE T DWARF POPULATION REVEALED BY WISE (2013, background): “T dwarfs are classified by increased methane, water, and molecular hydrogen collisioninduced absorption (CIA) in the near-infrared (Burgasser et al. 2002, 2006b; Geballe et al. 2002), and have inferred temperatures between ∼500 and ∼1300 K (Cushing et al. 2008, 2011; Stephens et al. 2009).”
Infrared Parallaxes for Methane T Dwarfs (2003, methodology): “Burgasser et al. (2002c) suggested that a possible resolution for this discrepancy could be the appearance of uneven cloud cover on the surface of early T dwarfs.”
Hubble Space Telescope Observations of Binary Very Low Mass Stars and Brown Dwarfs (2003, background): “…Skrutskie et al. 1997; Cutri et al. 2000),3 and Sloan Digital Sky Survey (SDSS; York et al. 2000) have enabled the discovery of numerous cool dwarfs and led to the definition of new spectral types L (Kirkpatrick et al. 1999; Martı́n et al. 1999b) and T (Burgasser et al. 2002a; Geballe et al. 2002).”
Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: The Effects of Temperature, Clouds, and Gravity (2004, methodology): “Indeed, Burgasser et al. (2002b) found that a relatively small fraction of holes would appreciably move a late L-type object toward the blue in J K.”