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

Spitzer Team, , OGLE Collaboration, KMTNet Collaboration, MiNDSTEp Collaboration, Yossi Shvartzvald (Lead / Corresponding author), 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 BoltGrant Christie, Dan Maoz, Tim Natusch, Richard W. Pogge, Rachel A. Street, Thiam-Guan Tan, Yiannis Tsapras, Paweł 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, Hyoun-Woo Kim, Seung-Lee Kim, Dong-Joo Lee, Yongseok Lee, Byeong-Gon Park, Yoon-Hyun Ryu, Justyn Campbell-White

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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
Pages (from-to)1-8
Number of pages8
JournalAstronomical Journal
Issue number3
Early online date11 Feb 2019
Publication statusPublished - Mar 2019


  • Galaxy: bulge
  • gravitational lensing: micro

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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