A Volume-Limited Sample of Ultracool Dwarfs. I. Construction, Space Density, and a Gap in the L/T Transition. (arXiv:2010.15853v3 [astro-ph.SR] UPDATED)

A Volume-Limited Sample of Ultracool Dwarfs. I. Construction, Space Density, and a Gap in the L/T Transition. (arXiv:2010.15853v3 [astro-ph.SR] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Best_W/0/1/0/all/0/1">William M. J. Best</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Liu_M/0/1/0/all/0/1">Michael C. Liu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Magnier_E/0/1/0/all/0/1">Eugene A. Magnier</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dupuy_T/0/1/0/all/0/1">Trent J. Dupuy</a>

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 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.

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