MOA-2015-BLG-337: A Planetary System with a Low-mass Brown Dwarf/Planetary Boundary Host, or a Brown Dwarf Binary

• Published in: The Astronomical journal 2018-09, Vol.156 (3), p.136
• Main Authors: Miyazaki, S., Sumi, T., Bennett, D. P., Gould, A., Udalski, A., Bond, I. A., Koshimoto, N., Nagakane, M., Rattenbury, N., Abe, F., Bhattacharya, A., Barry, R., Donachie, M., Fukui, A., Hirao, Y., Itow, Y., Kawasaki, K., Li, M. C. A., Ling, C. H., Matsubara, Y., Matsuo, T., Muraki, Y., Ohnishi, K., Ranc, C., Saito, T., Sharan, A., Shibai, H., Suematsu, H., Suzuki, D., Sullivan, D. J., Tristram, P. J., Yamada, T., Yonehara, A., Koz owski, S., Mróz, P., Pawlak, M., Poleski, R., Pietrukowicz, P., Skowron, J., Soszy ski, I., Szyma ski, M. K., Ulaczyk, K., Albrow, M. D., Chung, S.-J., Han, C., Jung, Y. K., Hwang, K.-H., Ryu, Y.-H., Shin, I.-G., Shvartzvald, Y., Yee, J. C., Zang, W., Zhu, W., Cha, S.-M., Kim, D.-J., Kim, H.-W., Kim, S.-L., Lee, C.-U., Lee, D.-J., Lee, Y., Park, B.-G., Pogge, R. W.
• Format: Article
• Language: English
• Subjects: Astronomy; Bayesian analysis; Brown dwarf stars; brown dwarfs; Conditional probability; Extrasolar planets; gravitational lensing: micro; Lenses; Light curve; Mass ratios; Microlenses; Planet formation; Planetary models; Planetary systems; Power law; Probability distribution; Space telescopes; Statistical analysis
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/aad5ee

We report on the discovery and analysis of the short-timescale binary-lens microlensing event, MOA-2015-BLG-337. The lens system could be a planetary system with a very low-mass host, around the brown dwarf (BD)/planetary-mass boundary, or a BD binary. We found two competing models that explain the observed light curves with companion/host mass ratios of q ∼ 0.01 and ∼0.17, respectively. A significant finite source effect in the best-fit planetary model (q ∼ 0.01) reveals a small angular Einstein radius of θE 0.03 mas, which favors a low-mass lens. We obtain the posterior probability distribution of the lens properties from a Bayesian analysis. The results for the planetary models strongly depend on a power-law index in planetary-mass regime, pl, in the assumed mass function. In summary, there are two solutions of the lens system: (1) a BD/planetary-mass boundary object orbited by a super-Neptune (the planetary model with pl = 0.49) and (2) a BD binary (the binary model). If the planetary models are correct, this system can be one of a new class of planetary system, having a low host mass and also a planetary-mass ratio (q < 0.03) between the companion and its host. The discovery of the event is important for the study of planetary formation in very low-mass objects. In addition, it is important to consider all viable solutions in these kinds of ambiguous events in order for the future comprehensive statistical analyses of planetary/binary microlensing events.