The Hawaii Infrared Parallax Program. VI. The Fundamental Properties of 1000+ Ultracool Dwarfs and Planetary-mass Objects Using Optical to Mid-infrared Spectral Energy Distributions and Comparison to BT-Settl and ATMO 2020 Model Atmospheres

Abstract We derive the bolometric luminosities ( L bol ) of 865 field-age and 189 young ultracool dwarfs (spectral types M6–T9, including 40 new discoveries presented here) by directly integrating flux-calibrated optical to mid-infrared (MIR) spectral energy distributions (SEDs). The SEDs consist of low-resolution ( R ∼ 150) near-infrared (NIR; 0.8–2.5 μ m) spectra (including new spectra for 97 objects), optical photometry from the Pan-STARRS1 survey, and MIR photometry from the CatWISE2020 survey and Spitzer/IRAC. Our L bol calculations benefit from recent advances in parallaxes from Gaia, Spitzer, and UKIRT, as well as new parallaxes for 19 objects from CFHT and Pan-STARRS1 presented here. Coupling our L bol measurements with a new uniform age analysis for all objects, we estimate substellar masses, radii, surface gravities, and effective temperatures ( T eff ) using evolutionary models. We construct empirical relationships for L bol and T eff as functions of spectral type and absolute magnitude, determine bolometric corrections in optical and infrared bandpasses, and study the correlation between evolutionary model-derived surface gravities and NIR gravity classes. Our sample enables a detailed characterization of BT-Settl and ATMO 2020 atmospheric model systematics as a function of spectral type and position in the NIR color–magnitude diagram. We find the greatest discrepancies between atmospheric and evolutionary model-derived T eff (up to 800 K) and radii (up to 2.0 R Jup ) at the M/L spectral type transition boundary. With 1054 objects, this work constitutes the largest sample to date of ultracool dwarfs with determinations of their fundamental parameters.