Presentation #102.237 in the session Poster Session.
Planetary transits of ultracool dwarfs (UCDs, Teff < 3000 K) provide our only opportunity to characterize terrestrial exoplanets in detail in the next 20 years. The SPECULOOS Transit Survey is leading the effort to discover these worlds, using a network of six 1-meter telescopes on three continents to search for planetary transits of nearby UCDs. Of the 366 highest-priority SPECULOOS targets, those for which JWST observations would best probe the atmospheres of any detected transiting planets, 103 are candidate UCDs that have been recently discovered with Gaia and lack archival spectra. Here we detail the first results of a large survey to spectroscopically confirm this newly discovered population of low-mass neighbors using moderate-resolution near-infrared spectra from Magellan/FIRE and IRTF/SpeX and red-optical spectra from Magellan/LDSS3 and Lick/KAST. Our spectral library allows us to probe binarity, assign spectral types, and measure effective temperatures, surface gravities, radial velocities, and metallicities for dozens of targets. The combination of red-optical and NIR spectra also allows us to identify unusual objects with unique compositions or magnetic activity levels. We also present a new approach for constraining stellar parameters using a nested-sampling retrieval built on the PHOENIX stellar models grid. We compare the results from our retrieval approach to index-derived parameter estimates to highlight the consequences following from current state-of-the-art, index-based approaches in terms of stellar characterization and, thus, planetary characterization. As uncertainties in stellar parameters drive uncertainties in derived planetary parameters, new approaches that afford refinements in the precision of stellar parameters like that presented here will greatly advance planetary characterization efforts.