科研之友
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摘要
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-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 0.034-0.104M(circle dot) 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 infer differences in effective temperatures and surface gravities between components of no more than similar to 150 K and similar to 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 similar to 20 x 10(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(-8)(+10) % for unresolved spectral types of L9-T4 at separations greater than or similar to 0 %26apos;%26apos;.1. This translates into a volume-corrected frequency of 13(-6)(+7) %, which is similar to values of similar to 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(-4)(+6) -%); however, this increase is not yet statistically significant and a larger sample is required to settle the issue.
- 出版日期2013-11-20
