A Late-type L Dwarf at 11 pc Hiding in the Galactic Plane Characterized Using Gaia DR2

Abstract: We report on the characterization of a nearby (d = 11.2 pc) ultracool L dwarf (WISE J192512.78+070038.8; hereafter W1925) identified as a faint (G = 20.0) high proper motion (0.22"/yr) object in the Gaia Data Releases 1 and 2. A Palomar/TripleSpec near-infrared spectrum of W1925 confirms a previously estimated photometric L7 spectral type by Scholz et al. 2018, and its infrared colors and absolute magnitudes are consistent with a single object of this type. We constructed a spectral energy distribution using the Gaia parallax, literature photometry and near infrared spectrum and find a luminosity log(Lbol/LSun) = -4.443+/-0.008. Applying evolutionary models, we infer that W1925 is likely a 53+/-18 MJup brown dwarf with Teff=1404+/-71 K and log g =5.1+/-0.4 dex (cgs). While W1925 was detected in both the 2MASS and WISE infrared sky surveys, it was not detected in photographic plate sky surveys. Its combination of extreme optical-infrared colors, high proper motion, and location near the crowded Galactic plane (b = -4.2 deg) likely contributed to it having evaded detection in pre-Gaia surveys.