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Kelu-1 AB
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Title: Kelu-1 AB - A possible brown dwarf triple system
Authors: M.B. Stumpf, W. Brandner, Th. Henning, H. Bouy, R. Koehler, F. Hormuth, V. Joergens, M. Kasper

We have monitored the benchmark L dwarf binary Kelu-1AB over the past 3 years to derive improved spectral types and luminosities for the individual components. The astrometric measurements enable us to compute the orbital parameters and result in the first dynamical mass estimate for the system. We obtained resolved high angular resolution, near-IR images with HST and the VLT/NACO adaptive optics instrument in the J, H and Ks bands. In addition we used NACO to achieve the first spatially resolved mid-resolution spectra in the H- and K-band for a precise spectral type determination. The near-IR spectrum of Kelu-1A reveals a distinct dip in the H-band providing evidence that Kelu-1 A itself is a spectroscopic binary. We derive spectral types of L0.5 0.5 for Kelu- 1 Aa and T7.5 1 for Kelu-1 Ab. Kelu-1B is classified as spectral type L3pec 1.5. From the relative orbit, we derive an orbital period of 38 +8 -6 years and a semi-major axis of 6.4+2.4 -1.3 AU. This yields the first dynamically determined total system mass of 177 +113 -55 Mjup for the Kelu-1AB system, with the uncertainties mainly attributed to the error of the system distance measurement and the yet missing apastron passage. The derived properties of Kelu-1AB allow us to test common theoretical models. The comparison of our results with colour-magnitude diagrams based on evolutionary models yields a slightly revised age estimate (0.3 - 0.5 Gyr) and a discrepancy between dynamically and theoretically derived masses, stressing the importance for further dynamical mass determinations of brown dwarf binaries.

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L dwarf binary Kelu-1
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Title: A Chandra X-ray detection of the L dwarf binary Kelu-1: Simultaneous Chandra and Very Large Array observations
Authors: M. Audard (1,2), R.A. Osten (3), A. Brown (4), K.R. Briggs (5), M. Guedel (5), E. Hodges-Kluck (3,4), J.E. Gizis (6) ((1) ISDC, (2) Observatoire de Geneve, (3) Univ. of Maryland, (4) Univ. of Colorado, (5) Paul Scherrer Institut, (6) Univ. of Delaware)

Magnetic activity in ultracool dwarfs, as measured in X-rays and H\alpha, shows a steep decline after spectral type M7-M8. So far, no L dwarf has been detected in X-rays. In contrast, L dwarfs may have higher radio activity than M dwarfs. We observe L and T dwarfs simultaneously in X-rays and radio to determine their level of magnetic activity in the context of the general decline of magnetic activity with cooler effective temperatures. The field L dwarf binary Kelu-1 was observed simultaneously with Chandra and the Very Large Array. Kelu-1AB was detected in X-rays with L_{
m X} = 2.9_{-1.3}^{+1.8} \times 10^{25} erg/s, while it remained undetected in the radio down to a 3 \sigma limit of L_{
m R} \leq 1.4 x 10^{13} erg/s/Hz. We argue that, whereas the X-ray and H\alpha emissions decline in ultracool dwarfs with decreasing effective temperature, the radio luminosity stays (more or less) constant across M and early-L dwarfs. The radio surface flux or the luminosity may better trace magnetic activity in ultracool dwarfs than the ratio of the luminosity to the bolometric luminosity. Deeper radio observations (and at short frequencies) are required to determine if and when the cut-off in radio activity occurs in L and T dwarfs, and what kind of emission mechanism takes place in ultracool dwarfs.

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Posts: 131433
Date:
Kelu-1
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Title: Kelu-1 is a Binary L Dwarf: First Brown Dwarf Science from Laser Guide
Star Adaptive Optics
Authors: Michael C. Liu (IfA/Hawaii), Sandy K. Leggett (UKIRT/JAC)

Researchers have presented near-IR imaging of the nearby L2 dwarf Kelu-1 obtained with the Keck sodium laser guide star adaptive optics (LGS AO) system as part of a high angular resolution survey for substellar binaries.

Kelu-1 was one of the first free-floating L dwarfs identified, and the origin of its overluminosity compared to other similar objects has been a long-standing question. Their images clearly resolve Kelu-1 into a 0.29'' (5.4 AU) binary, and a previous non-detection by HST demonstrates that the system is a true physical pair.



Binarity explains the properties of Kelu-1 that were previously noted to be anomalous compared to other early-L dwarfs.
They estimate spectral types of L1.5-L3 and L3-L4.5 for the two components, giving model-derived masses of 0.05-0.07 solar masses and 0.045-0.065 solar masses for an estimated age of 0.3-0.8 Gyr.
More distant companions are not detected to a limit of 5-9 Jupiter masses. The presence of lithium absorption indicates that both components are substellar, but the weakness of this feature relative to other L dwarfs may arise from the fact that only Kelu-1B is lithium bearing.
Determining if both or if only one of the components possesses lithium could constrain the age of Kelu-1 (and other Li-bearing L binaries) with higher precision than is possible for most ultracool field objects. These results are the first LGS AO observations of brown dwarfs and demonstrate the potential of this new instrumental capability for substellar astronomy.


Position(2000): R.A. 13h 05m 40.2s , Dec -25° 41' 06"

Kelu-1(CE 298) is located at a distance of 33 light-years from the Sun in the constellation Hydra, with a proper motion of 0.35" per year.

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