T dwarfs [Issues] [T Dwarf Compendium] [Spectral Repository] [Links]

T dwarfs are a class of low mass, low temperature, low luminosity brown dwarfs that exhibit spectral signatures of CH₄ and H₂O in the near-infrared. These objects are the coolest "stars" so far detected, and likely have atmospheres not unlike those of hot extrasolar planets. The first T dwarf identified, in 1995, was also the first widely accepted brown dwarf, Gliese 229B. The faintest known T dwarf, 2MASS 0415-0935, has an effective temperature T_eff ~ 700 K and luminosity L ~ 10^-6 L_sun.

There are roughly 60 T dwarfs currently known, most identified as nearby field sources in the 2MASS and SDSS surveys. T dwarfs have also been identified as companions to other stars and brown dwarfs, in deep pointed surveys, and possibly in young star forming regions. A current sample is given in Table 1 below.

This page is still being completed. Please check back for updates.

Current issues

• The 2MASS Wide-field Search for T dwarfs: We have initiated a wide-field search for T dwarfs using the recently released 2MASS All Sky Data Release, with broad color constraints and areal coverage. So far this search has identified 37 field T dwarfs, the largest sample to date. We have also identified a handful of ultracool subdwarfs, including the prototype L subdwarf 2MASS 0532+8246
  • Paper I: Burgasser et al. 2003, AJ 125, 850 [AJ] [NASA ADS] [astro-ph]
  • Paper II: Burgasser, McElwain, & Kirkpatrick 2003, AJ 126, 2487 [AJ] [NASA ADS] [astro-ph]
  • Paper III: Burgasser et al. 2004, AJ 127, 2856 [AJ] [NASA ADS] [astro-ph]
  • Paper IV (Tinney et al.) is in preparation.

• Spectral Classification: There are currently two schemes defined to classify T dwarfs in the near-infrared, by Burgasser et al. 2002 and Geballe et al. 2002. Currently Tom Geballe, Sandy Leggett, Davy Kirkpatrick, Dave Golimowski, Xiaohui Fan, and I are working on a unified scheme for classification. We should be submitting an article for publication before the end of 2004.
  • Spectral standards and classification indices are listed below.
  • Preliminary results for the revised scheme are given in our conference proceedings for IAU Symposium 211, "Brown Dwarfs".
  • An optical scheme has also been defined by Burgasser et al. 2003, ApJ, 594, 510 [ApJ] [NASA ADS] [astro-ph]
  • Look at the spectral data.
  • An older classification webpage.

• Clouds, Weather and the L/T Transition: We have explained the unusual brightening of mid-type T dwarfs like 2MASS 0559-1404, as revealed by parallax observations of Dahn et al. (2002), and the reappearance of FeH absorption by the dynamic disruption of condensate clouds across this transition. By modifying the cloud models of Ackerman and Marley (2001) to include partial cloud clearing, we are able to fit the parallax data very well. Essentially, it is (very bad) weather that causes the rapid L/T transition, not decreasing temperatures!
  • Burgasser, A.J., et al. 2002, ApJ 571, L151 [ApJ] [astro-ph]
  • Related Publications:
    Ackerman & Marley 2001 ApJ 556, 872 [ApJ] [astro-ph]
    Dahn et al. 2002, AJ 124, 1170 [AJ] [astro-ph]
    

• The S Ori 70 Controversy: Is S Ori 70 a "planetary-mass" cluster brown dwarf? Or is it a foreground interloper?
  • Zapatero Osorio et al. 2002, ApJ 578, 536 identified an T dwarf in their deep survey of the Sigma Orionis cluster which they claim to be the lowest-mass brown dwarf (~3 Jupiter masses) ever discovered. Martin et al. 2003, ApJ 593, L113 further strengthened their argument through NIRSPEC spectroscopy, where they showed the object, S Ori 70, to be quite different than the known field T dwarf 2MASS 0559-1404.
  • In 2004, I wrote a criticism (Burgasser et al. 2004, ApJ 604, 827) of these papers, citing that (1) the comparison spectrum of 2MASS 0559-1404 from Martin et al. 2002 was in error, and that in fact the spectrum of S Ori 70 is not unlike those of field T dwarfs; (2) the spectral model fits indicating low surface gravity also predict low surface gravity for field T dwarfs, including the 2-5 Gyr Gliese 570D, suggesting that the models may be inadequate and are being overinterpretted; and (3) the statistical likelihood of finding a foreground T dwarf in the field observed by Zapatero Osorio 2002 is merely a 1 sigma event. On these arguments, I concluded that S Ori 70 may simply be a foreground brown dwarf, and that the authors' conclusions were not yet substantiated.
  • Eduardo Martin has recently written a rebuttal in his conference proceedings for Moriond YLU 2004. However, I will point out that the title of the article states that S Ori 70 is a strong candidate cluster member, which I claim to be a success on my part!
  How would you resolve this controversy...?
  

• T Dwarf Binaries: A number of groups have investigated brown dwarf binary systems with high-resolution imaging, afforded through HST and ground-based adaptive optics systems. Overall, roughly 20% of late-type M, L and T dwarf systems observed appear to be binary, and in all cases they are closely separated (< 20 AU) high mass ratio (q ~ 1) systems. We identified two binary T dwarf systems out of ten in an HST program during Cycle 9, and we are currently completing analysis on a larger sample to improve the statistics and look for very low-temperature brown dwarfs. These systems are also amenable to mass measurement through orbital monitoring, and are excellent laboratories for studying mass/temperature effects independent of age or metallicity variations.
  • The first HST sample is described in Burgasser et al. 2003, ApJ 586, 512 [ApJ] [astro-ph]
  • An interesting likely binary, comprised of a late L and early T dwarf, was discovered by Cruz et al. 2004, ApJ 604, L61 [ApJ] [astro-ph]

• The Active T dwarf 2MASS 1237+6527 Incidence of H alpha emission drops precipitously amongst the late-type M and L dwarfs, so that little emission is expected in a cool T dwarfs. However, 2MASS 1237+65 has exhibited a constant level of emission over a long period. A possible mechanism could be a close interacting brown dwarf binary; or this could be an isolated, young, and very low mass brown dwarf. This object remains a mystery.
  • The original discovery of H alpha emission was reported in Burgasser et al. 2000, AJ 120, 473 [AJ] [astro-ph]
  • A follow-up paper failed to detect any photometric or spectroscopic variability that would support an interacting binary hypothesis: Burgasser et al. 2002, AJ 123, 2744 [AJ] [astro-ph]

There are 59 T dwarf systems including 3 binary systems with T dwarf primaries currently known.