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Title: Discovery of a visual T-dwarf triple system and binarity at the L/T transition

We present new high contrast imaging of eight L/T transition brown dwarfs (BDs) using the NIRC2 camera on the Keck II telescope. One of our targets, the T3.5 dwarf 2MASS J08381155+1511155, was resolved into a hierarchal triple with projected separations of 2.5 ± 0.5 AU and 27 ± 5 AU for the BC and A(BC) components, respectively. Resolved OSIRIS spectroscopy of the A(BC) components confirms that all system members are T dwarfs. The system therefore constitutes the first triple T-dwarf system ever reported. Using resolved photometry to model the integrated-light spectrum, we infer spectral types of T3 ± 1, T3 ± 1, and T4.5 ± 1 for the A, B, and C components, respectively. The uniformly brighter primary has a bluer J – K{sub s} color than the next faintest component, which may reflect a sensitive dependence of the L/T transition temperature on gravity, or alternatively divergent cloud properties among components. Relying on empirical trends and evolutionary models we infer a total system mass of 0.034-0.104 M {sub ☉} for the BC components at ages of 0.3-3 Gyr, which would imply a period of 12-21 yr assuming the system semimajor axis to be similar to its projection. We also infermore » differences in effective temperatures and surface gravities between components of no more than ∼150 K and ∼0.1 dex. Given the similar physical properties of the components, the 2M0838+15 system provides a controlled sample for constraining the relative roles of effective temperature, surface gravity, and dust clouds in the poorly understood L/T transition regime. For an age of 3 Gyr we estimate a binding energy of ∼20 × 10{sup 41} erg for the wide A(BC) pair, which falls above the empirical minimum found for typical BD binaries, and suggests that the system may have been able to survive a dynamical ejection during formation. Combining our imaging survey results with previous work we find an observed binary fraction of 4/18 or 22{sub −8}{sup +10}% for unresolved spectral types of L9-T4 at separations ≳ 0.''1. This translates into a volume-corrected frequency of 13{sub −6}{sup +7}%, which is similar to values of ∼9%-12% reported outside the transition. Our reported L/T transition binary fraction is roughly twice as large as the binary fraction of an equivalent L9-T4 sample selected from primary rather than unresolved spectral types (6{sub −4}{sup +6}%); however, this increase is not yet statistically significant and a larger sample is required to settle the issue.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4] ;  [5] ;  [6]
  1. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
  2. Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada)
  3. Département de Physique, Université de Montréal, C.P. 6128 Succ. Centre-Ville, Montréal, QC H3C 3J7 (Canada)
  4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
  5. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States)
  6. School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY169SS (United Kingdom)
Publication Date:
OSTI Identifier:
22342031
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 778; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BINARY STARS; BINDING ENERGY; COLOR; DUSTS; DWARF STARS; EV RANGE; GRAVITATION; MASS; NEAR INFRARED RADIATION; PHOTOMETRY; SPECTRA; SPECTROSCOPY; STARS; SURFACES; TELESCOPES; TRANSITION TEMPERATURE; VISIBLE RADIATION