Spitzer Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-rotating Low-mass Brown Dwarf

Yossi Shvartzvald, Jennifer C. Yee, Jan Skowron, Chung Uk Lee, Andrzej Udalski, Sebastiano Calchi Novati, Valerio Bozza, Charles A. Beichman, Geoffery Bryden, Sean Carey, B. Scott Gaudi, Calen B. Henderson, Wei Zhu, Etienne Bachelet, Greg Bolt, Grant Christie, Dan Maoz, Tim Natusch, Richard W. Pogge, Rachel A. StreetThiam Guan Tan, Yiannis Tsapras, Pawel Pietrukowicz, Igor Soszyński, Michał K. Szymański, Przemek Mróz, Radoslaw Poleski, Szymon Kozłowski, Krzysztof Ulaczyk, Michał Pawlak, Krzysztof A. Rybicki, Patryk Iwanek, Michael D. Albrow, Sang Mok Cha, Sun Ju Chung, Andrew Gould, Cheongho Han, Kyu Ha Hwang, Youn Kil Jung, Dong Jin Kim, Hyoun Woo Kim, Seung Lee Kim, Dong Joo Lee, Yongseok Lee, Byeong Gon Park, Yoon Hyun Ryu, In Gu Shin, Weicheng Zang, Martin Dominik, Christiane Helling, Markus Hundertmark, Uffe G. Jorgensen, Penelope Longa-Pena, Stephen Lowry, Sedighe Sajadian, Martin J. Burgdorf, Justyn Campbell-White, Simona Ciceri, Daniel F. Evans, Yuri I. Fujii, Tobias C. Hinse, Sohrab Rahvar, Markus Rabus, Jesper Skottfelt, Colin Snodgrass, John Southworth

Research output: Contribution to journalArticlepeer-review


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 the 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 ±1 M J or 20 ±1 M J . This is the lowest isolated-object mass measurement to date, only ∼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 ±0.1 kpc or 4.1 ±0.1 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.

Original languageEnglish
Article number106
JournalAstronomical Journal
Issue number3
StatePublished - 2019


FundersFunder number
Not addedST/L004569/1, ST/N000854/1
National Science Foundation1516842, 1500811
National Aeronautics and Space Administration
Jet Propulsion Laboratory2017R1A4A1015178
Science and Technology Facilities CouncilST/P000495/1
National Research Foundation of Korea
Narodowe Centrum NaukiINSF-95843339, MAESTRO 2014/ 14/A/ST9/00121


    • Galaxy: bulge
    • gravitational lensing: micro


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