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We present a new volume-limited sample of L0-T8 dwarfs out to 25 pc defined entirely by parallaxes, using our recent measurements from UKIRT/WFCAM along with Gaia DR2 and literature parallaxes. With 369 members, our sample is the largest parallax-defined volume-limited sample of L and T dwarfs to date, yielding the most precise space densities for such objects. We find the local L0-T8 dwarf population includes $5.5%pm1.3%$ young objects ($lesssim$200 Myr) and $2.6%pm1.6%$ subdwarfs, as expected from recent studies favoring representative ages $lesssim$4 Gyr for the ultracool field population. This is also the first volume-limited sample to comprehensively map the transition from L to T dwarfs (spectral types $approx$L8-T4). After removing binaries, we identify a previously unrecognized, statistically significant (>4.4$sigma$) gap $approx$0.5 mag wide in $(J-K)_{rm MKO}$ colors in the L/T transition, i.e., a lack of such objects in our volume-limited sample, implying a rapid phase of atmospheric evolution. In contrast, the most successful models of the L/T transition to date $-$ the hybrid models of Saumon & Marley (2008) $-$ predict a pile-up of objects at the same colors where we find a deficit, demonstrating the challenge of modeling the atmospheres of cooling brown dwarfs. Our sample illustrates the insights to come from even larger parallax-selected samples from the upcoming Legacy Survey of Space and Time (LSST) by the Vera Rubin Obsevatory.
We present a volume-limited, spectroscopically-verified sample of M7$-$L5 ultracool dwarfs within 25,pc. The sample contains 410 sources, of which $93%$ have trigonometric distance measurements ($80%$ from textit{Gaia} DR2), and $81%$ have low-resolu
We identify and investigate known ultracool stars and brown dwarfs that are being observed or indirectly constrained by the Gaia mission. These objects will be the core of the Gaia ultracool dwarf sample composed of all dwarfs later than M7 that Gaia
We conducted a volume-limited survey at 4.9 GHz of 32 nearby ultracool dwarfs with spectral types covering the range M7 -- T8. A statistical analysis was performed on the combined data from the present survey and previous radio observations of ultrac
We present the first results from our high-precision infrared (IR) astrometry program at the Canada-France-Hawaii Telescope. We measure parallaxes for 83 ultracool dwarfs (spectral types M6--T9) in 49 systems, with a median uncertainty of 1.1 mas (2.
Brown dwarf spectra are rich in information revealing of the chemical and physical processes operating in their atmospheres. We apply a recently developed atmospheric retrieval tool to an ensemble of late T-dwarf (600-800K) near infrared spectra. Wit