$Spitzer$ Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-Rotating Low-Mass Brown Dwarf
Creators: name: Shvartzvald, Yossi
name: Yee, Jennifer C.
name: Skowron, Jan
name: Lee, Chung-Uk
name: Udalski, Andrzej
name: Novati, Sebastiano Calchi
name: Bozza, Valerio
name: Beichman, Charles A.
name: Bryden, Geoffery
name: Carey, Sean
name: Gaudi, B. Scott
name: Henderson, Calen B.
name: Zhu, Wei
name: Bachelet, Etienne
name: Bolt, Greg
name: Christie, Grant
name: Maoz, Dan
name: Natusch, Tim
name: Pogge, Richard W.
name: Street, Rachel A.
name: Tan, Thiam-Guan
name: Tsapras, Yiannis
name: Pietrukowicz, Paweł
name: Soszyński, Igor
name: Szymański, Michał K.
name: Mróz, Przemek
name: Poleski, Radoslaw
name: Kozłowski, Szymon
name: Ulaczyk, Krzysztof
name: Pawlak, Michał
name: Rybicki, Krzysztof A.
name: Iwanek, Patryk
name: Albrow, Michael D.
name: Cha, Sang-Mok
name: Chung, Sun-Ju
name: Gould, Andrew
name: Han, Cheongho
name: Hwang, Kyu-Ha
name: Jung, Youn Kil
name: Kim, Dong-Jin
name: Kim, Hyoun-Woo
name: Kim, Seung-Lee
name: Lee, Dong-Joo
name: Lee, Yongseok
name: Park, Byeong-Gon
name: Ryu, Yoon-Hyun
name: Shin, In-Gu
name: Zang, Weicheng
name: Dominik, Martin
name: Helling, Christiane
name: Hundertmark, Markus
name: Jørgensen, Uffe G.
name: Longa-Peña, Penelope
name: Lowry, Stephen
name: Sajadian, Sedighe
name: Burgdorf, Martin J.
name: Campbell-White, Justyn
name: Ciceri, Simona
name: Evans, Daniel F.
name: Fujii, Yuri I.
name: Hinse, Tobias C.
name: Rahvar, Sohrab
name: Rabus, Markus
name: Skottfelt, Jesper
name: Snodgrass, Colin
name: Southworth, John
Publisher: name: arXiv
Year: 2018
Subjects: subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: The kinematics of isolated brown dwarfs in the Galaxy, beyond the solar neighborhood, is virtually unknown. Microlensing has the potential to probe this hidden population, as it can measure both the mass and five of the six phase-space coordinates (all except the radial velocity) even of a dark isolated lens. However, the measurements of both the microlens parallax and finite-source effects are needed in order to recover the full information. Here, we combine $Spitzer$ satellite parallax measurement with the ground-based light curve, which exhibits strong finite-source effects, of event OGLE-2017-BLG-0896. We find two degenerate solutions for the lens (due to the known satellite-parallax degeneracy), which are consistent with each other except for their proper motion. The lens is an isolated brown dwarf with a mass of either $18\pm1M_J$ or $20\pm1M_J$. This is the lowest isolated-object mass measurement to date, only $\sim$45\% more massive than the theoretical deuterium-fusion boundary at solar metallicity, which is the common definition of a free-floating planet. The brown dwarf is located at either $3.9\pm0.1$ kpc or $4.1\pm0.1$ kpc toward the Galactic bulge, but with proper motion in the opposite direction of disk stars, with one solution suggesting it is moving within the Galactic plane. While it is possibly a halo brown dwarf, it might also represent a different, unknown population.
description: 17 pages, 3 figures, 2 tables. Accepted for publication in The Astronomical Journal