The Ultracool SpeXtroscopic Survey. I. Volume-limited Spectroscopic Sample and Luminosity Function of M7-L5 Ultracool Dwarfs

October 2019 • 2019ApJ...883..205B

Authors • Bardalez Gagliuffi, Daniella C. • Burgasser, Adam J. • Schmidt, Sarah J. • Theissen, Christopher • Gagné, Jonathan • Gillon, Michael • Sahlmann, Johannes • Faherty, Jacqueline K. • Gelino, Christopher • Cruz, Kelle L. • Skrzypek, Nathalie • Looper, Dagny

Abstract • We present a volume-limited, spectroscopically verified sample of M7-L5 ultracool dwarfs (UCDs) within 25 pc. The sample contains 410 sources, of which 93% have trigonometric distance measurements (80% from Gaia DR2) and 81% have low-resolution (R ∼ 120), near-infrared (NIR) spectroscopy. We also present an additional list of 60 sources that may be M7-L5 dwarfs within 25 pc when distance or spectral-type uncertainties are taken into account. The spectra provide NIR spectral and gravity classifications, and we use these to identify young sources, red and blue J - K _S color outliers, and spectral binaries. We measure very low gravity and intermediate-gravity fractions of {2.1}_{-0.8 %}^{+0.9 %} and {7.8}_{-1.5 %}^{+1.7 %}, respectively; fractions of red and blue color outliers of {1.4}_{-0.5 %}^{+0.6 %} and {3.6}_{-0.9 %}^{+1.0 %}, respectively; and a spectral binary fraction of {1.6}_{-0.5 %}^{+0.5 %}. We present an updated luminosity function for M7-L5 dwarfs continuous across the hydrogen-burning limit that agrees with previous studies. We estimate our completeness to range between 69% and 80% when compared to an isotropic model. However, we find that the literature late-M sample is severely incomplete compared to L dwarfs, with completeness of {62}_{-7 %}^{+8 %} and {83}_{-9 %}^{+10 %}, respectively. This incompleteness can be addressed with astrometric-based searches of UCDs with Gaia to identify objects previously missed by color- and magnitude-limited surveys.

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Chris Gelino

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