THE BROWN DWARF KINEMATICS PROJECT (BDKP). III. PARALLAXES FOR 70 ULTRACOOL DWARFS

Abstract

We report parallax measurements for 70 ultracool dwarfs (UCDs). Using both\nliterature values and our sample, we report new polynomial relations between\nspectral type and M$_{JHK}$. Including resolved L/T transition binaries in the\nrelations, we find no reason to differentiate between a "bright" (unresolved\nbinary) and "faint" (single source) sample across the L/T boundary. Isolating\nearly T dwarfs, we find that the brightening of T0-T4 sources is prominent in\nM$_{J}$ where there is a [1.2 - 1.4] magnitude difference. A similar yet\ndampened brightening of [0.3 - 0.5] magnitude happens at M$_{H}$ and a plateau\nor dimming of [-0.2 - -0.3] magnitude is seen in M$_{K}$. Comparing with\nevolutionary models that vary gravity, metallicity, and cloud thickness we find\nthat a near constant temperature of 1200 $\\pm$100 K along a narrow spectral\nsubtype of T0-T4 is required to account for the brightening and color magnitude\ndiagram of the L-dwarf/T-dwarf transition. Furthermore, there is a significant\npopulation of both L and T dwarfs which are red or potentially "ultra-cloudy"\ncompared to the models, many of which are known to be young indicating a\ncorrelation between enhanced photospheric dust and youth. For the low\nsurface-gravity or young companion L dwarfs we find that 8 out of 10 are at\nleast [0.2-1.0] magnitude underluminous in M$_{JH}$ and/or M$_{K}$ compared to\nequivalent spectral type objects. We speculate that this is a consequence of\nincreased dust opacity and conclude that low-surface gravity L dwarfs require a\ncompletely new spectral-type/absolute magnitude polynomial for analysis.\n