Astrophys. J., 752, 56 (2012/June-2)
The Brown Dwarf Kinematics Project (BDKP). III. Parallaxes for 70 ultracool dwarfs.
FAHERTY J.K., BURGASSER A.J., WALTER F.M., VAN DER BLIEK N., SHARA M.M., CRUZ K.L., WEST A.A., VRBA F.J. and ANGLADA-ESCUDE G.
Abstract (from CDS):
We report parallax measurements for 70 ultracool dwarfs (UCDs) including 11 late-M, 32 L, and 27 T dwarfs. In this sample, 14 M and L dwarfs exhibit low surface gravity features, 6 are close binary systems, and 2 are metal-poor subdwarfs. We combined our new measurements with 114 previously published UCD parallaxes and optical-mid-IR photometry to examine trends in spectral-type/absolute magnitude, and color-color diagrams. We report new polynomial relations between spectral type and MJHK. Including resolved L/T transition binaries in the relations, we find no reason to differentiate between a "bright" (unresolved binary) and a "faint" (single source) sample across the L/T boundary. Isolating early T dwarfs, we find that the brightening of T0-T4 sources is prominent in MJ where there is a [1.2-1.4] mag difference. A similar yet dampened brightening of [0.3-0.5] mag happens at MH and a plateau or dimming of [-0.2 to -0.3] mag is seen in MK. Comparison with evolutionary models that vary gravity, metallicity, and cloud thickness verifies that for L into T dwarfs, decreasing cloud thickness reproduces brown dwarf near-IR color-magnitude diagrams. However we find that a near constant temperature of 1200 ±100 K along a narrow spectral subtype of T0-T4 is required to account for the brightening and color-magnitude diagram of the L-dwarf/T-dwarf transition. There is a significant population of both L and T dwarfs which are red or potentially "ultra-cloudy" compared to the models, many of which are known to be young indicating a correlation between enhanced photospheric dust and youth. For the low surface gravity or young companion L dwarfs we find that 8 out of 10 are at least [0.2-1.0] mag underluminous in MJHand/or MKcompared to equivalent spectral type objects. We speculate that this is a consequence of increased dust opacity and conclude that low surface gravity L dwarfs require a completely new spectral-type/absolute magnitude polynomial for analysis.
astrometry - brown dwarfs - stars: low-mass
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<Available at CDS (J/ApJ/752/56): table1.dat table2.dat table5.dat refs.dat>
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