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Register a new userOn the Nearby Binary Brown Dwarf WISE J104915.57-531906.1 (Luhman 16)
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Eric E. Mamajek
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I report some observations and calculations related to the new nearby brown dwarf at d = 2 pc discovered by Luhman (2013, ApJ Letters, in press; arXiv:1303.2401). I report archival astrometry and photometry of the new object from IRAS (epoch 1983.5; IRAS Z10473-5303), AKARI (epoch 2007.0; AKARI J1049166-531907), and the Guide Star Catalog (epoch 1995.304; GSC2.2 S11132026703, GSC2.3 S4BM006703). A SuperCOSMOS scan of a plate taken with the ESO Schmidt Telescope (epoch 1984.169) shows the source as elongated (PA = 138 deg). Membership of the binary to any of the known nearby young groups within 100 pc appears unlikely based on the available astrometry and photometry. Based on the proper motion and parallax, a Monte Carlo simulation of thin disk/thick disk/halo stars is suggestive that the binary is, unsurprisingly, most likely a thin disk star (~96%), with a ~4% chance that it is a thick disk (and negligible chance that it is a halo star). I suggest that this important new nearby binary be called by either its provisional Washington Double Star catalog identifier (Luhman 16), or perhaps Luhman-WISE 1, either of which is easier to remember than the WISE identifier.
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WISE J104915.57$-$531906.1 is a L/T brown dwarf binary located 2pc from the Sun. The pair contains the closest known brown dwarfs and is the third closest known system, stellar or sub-stellar. We report comprehensive follow-up observations of this newly uncovered system. We have determined the spectral types of both components (L8+/-1, for the primary, agreeing with the discovery paper; T1.5+/-2 for the secondary, which was lacking spectroscopic type determination in the discovery paper) and, for the first time, their radial velocities (V_rad~23.1, 19.5 km/s) using optical spectra obtained at the Southern African Large Telescope (SALT) and other facilities located at the South African Astronomical Observatory (SAAO). The relative radial velocity of the two components is smaller than the range of orbital velocities for theoretically predicted masses, implying that they form a gravitationally bound system. We report resolved near-infrared JHK_S photometry from the IRSF telescope at the SAAO which yields colors consistent with the spectroscopically derived spectral types. The available kinematic and photometric information excludes the possibility that the object belongs to any of the known nearby young moving groups or associations. Simultaneous optical polarimetry observations taken at the SAAO 1.9-m give a non-detection with an upper limit of 0.07%. For the given spectral types and absolute magnitudes, 1Gyr theoretical models predict masses of 0.04--0.05 M_odot for the primary, and 0.03--0.05 M_odot for the secondary.
We report upper limits to the radio and X-ray emission from the newly discovered ultracool dwarf binary WISE J104915.57$-$531906.1 (Luhman 16AB). As the nearest ultracool dwarf binary (2 pc), its proximity offers a hefty advantage to studying plasma processes in ultracool dwarfs which are more similar in gross properties (radius, mass, temperature) to the solar system giant planets than stars. The radio and X-ray emission upper limits from the Australia Telescope Compact Array (ATCA) and Chandra observations, each spanning multiple rotation periods, provide the deepest fractional radio and X-ray luminosities to date on an ultracool dwarf, with $log{(L_{rm r, u}/L_{rm bol}) [Hz^{-1}]} < -18.1$ (5.5 GHz), $log{(L_{rm r, u}/L_{rm bol}) [Hz^{-1}]} < -17.9$ (9 GHz), and $log{(L_{rm x}/L_{rm bol})} < -5.7$. While the radio upper limits alone do not allow for a constraint on the magnetic field strength, we limit the size of any coherently emitting region in our line of sight to less than 0.2% of the radius of one of the brown dwarfs. Any source of incoherent emission must span less than about 20% of the brown dwarf radius, assuming magnetic field strengths of a few tens to a few hundred Gauss. The fast rotation and large amplitude photometric variability exhibited by the T dwarf in the Luhman 16AB system are not accompanied by enhanced nonthermal radio emission, nor enhanced heating to coronal temperatures, as observed on some higher mass ultracool dwarfs, confirming the expected decoupling of matter and magnetic field in cool neutral atmospheres.
We present two epochs of MPG/ESO 2.2m GROND simultaneous 6-band ($rizJHK$) photometric monitoring of the closest known L/T transition brown dwarf binary WISE J104915.57-531906.1AB. We report here the first resolved variability monitoring of both the T0.5 and L7.5 components. We obtained 4 hours of focused observations on the night of UT 2013-04-22, as well as 4 hours of defocused (unresolved) observations on the night of UT 2013-04-16. We note a number of robust trends in our light curves. The $r$ and $i$ light curves appear to be anticorrelated with $z$ and $H$ for the T0.5 component and in the unresolved lightcurve. In the defocused dataset, $J$ appears correlated with $z$ and $H$ and anticorrelated with $r$ and $i$, while in the focused dataset we measure no variability for $J$ at the level of our photometric precision, likely due to evolving weather phenomena. In our focused T0.5 component lightcurve, the $K$ band lightcurve displays a significant phase offset relative to both $H$ and $z$. We argue that the measured phase offsets are correlated with atmospheric pressure probed at each band, as estimated from 1D atmospheric models. We also report low-amplitude variability in $i$ and $z$ intrinsic to the L7.5 component.
We report the detection of radio emission and orbital motion from the nearby star-brown dwarf binary WISE J072003.20-084651.2AB. Radio observations across the 4.5-6.5 GHz band with the Very Large Array identify at the position of the system quiescent emission with a flux density of 15$pm$3 $mu$Jy, and a highly-polarized radio source that underwent a 2-3 min burst with peak flux density 300$pm$90 $mu$Jy. The latter emission is likely a low-level magnetic flare similar to optical flares previously observed for this source. No outbursts were detected in separate narrow-band H$alpha$ monitoring observations. We report new high-resolution imaging and spectroscopic observations that confirm the presence of a co-moving T5.5 secondary and provide the first indications of three-dimensional orbital motion. We used these data to revise our estimates for the orbital period (4.1$^{+2.7}_{-1.3}$ yr) and tightly constrain the orbital inclination to be nearly edge-on (93.6deg$^{+1.6deg}_{-1.4deg}$), although robust measures of the component and system masses will require further monitoring. The inferred orbital motion does not change the high likelihood that this radio-emitting very low-mass binary made a close pass to the Sun in the past 100 kyr.
We have observed the eclipsing, post-common envelope white dwarf-brown dwarf binary, SDSS141126.20+200911.1, in the near-IR with the HAWK-I imager, and present here the first direct detection of the dark side of an irradiated brown dwarf in the $H$ band, and a tentative detection in the $K_s$ band. Our analysis of the lightcurves and indicates that the brown dwarf is likely to have an effective temperature of 1300 K, which is not consistent with the effective temperature of 800 K suggested by its mass and radius. As the brown dwarf is already absorbing almost all the white dwarf emission in the $K_s$ band we suggest that this inconsistency may be due to the UV-irradiation from the white dwarf inducing an artificial brightening in the $K_s$ band, similar to that seen for the similar system WD0137-349B, suggesting this brightening may be characteristic of these UV-irradiated binaries.
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