7 research outputs found
THE HAWAII INFRARED PARALLAX PROGRAM. II. YOUNG ULTRACOOL FIELD DWARFS
(Abridged) We present a large, uniform analysis of young (~10-150 Myr)
ultracool dwarfs, based on new high-precision IR parallaxes for 68 objects. We
find that low-gravity (VL-G) late-M and L dwarfs form a continuous sequence in
IR color-magnitude diagrams, separate from field objects and current
theoretical models. VL-G objects also appear distinct from young substellar
(brown dwarf and exoplanet) companions, suggesting the two populations have a
different range of physical properties. In contrast, at the L/T transition,
young, old, and peculiar objects all span a narrow range in near-IR absolute
magnitudes. At a given spectral type, the IR absolute magnitudes of young
objects can be offset from ordinary field dwarfs, with the largest offsets
occurring in the Y and J bands for late-M dwarfs (brighter than the field) and
mid/late-L dwarfs (fainter than the field). Overall, low-gravity (VL-G) objects
have the most uniform photometric behavior while intermediate-gravity (INT-G)
objects are more diverse, suggesting a third governing parameter beyond
spectral type and gravity class. We examine the moving group memberships for
all young ultracool dwarfs with parallaxes, changing/refuting the status of 23
objects and fortifying the status of another 28 objects. We use our resulting
age-calibrated sample to establish empirical young isochrones and find a
declining frequency of VL-G objects relative to INT-G objects with increasing
age. Notable objects in our sample include high-velocity INT-G objects; very
red, late-L dwarfs with high surface gravities; candidate disk-bearing members
of the MBM20 cloud and beta Pic moving group; and very young distant
interlopers. Finally, we provide a comprehensive summary of the absolute
magnitudes and spectral classifications of 102 young ultracool dwarfs, found in
the field and as substellar companions to young stars.Comment: ApJ, in press, 138 pages including 33 figures and 15 tables.
Compilation of young ultracool dwarfs and young substellar (brown dwarf and
exoplanet) companions available at the Database of Ultracool Parallaxes (see
http://www.as.utexas.edu/~tdupuy/plx