October
2026
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2026PASJ...78.1716S
Authors
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Satoh, Yuki K.
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Bennett, David P.
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Sumi, Takahiro
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Bond, Ian A.
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Rattenbury, Nicholas J.
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Suzuki, Daisuke
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Koshimoto, Naoki
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Miyazaki, Shota
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Kirikawa, Rintaro
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Abe, Fumio
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Bhattacharya, Aparna
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Hamada, Ryusei
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Ishitani Silva, Stela
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Hirao, Yuki
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Matsubara, Yutaka
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Muraki, Yasushi
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Nagai, Tutumi
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Nunota, Kansuke
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Olmschenk, Greg
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Ranc, Clément
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Terry, Sean K.
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Tristram, Paul J.
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Vandorou, Aikaterini
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Yama, Hibiki
Abstract
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We present an analysis of the microlensing event MOA-2020-BLG-108, which was discovered in June 2020 by the MOA collaboration toward the Galactic bulge. The observed light curve shows significant deviations from the standard single-lens single-source model. We find two degenerate binary-lens single-source solutions, corresponding to the wide and close configurations, with a companion-to-host mass ratio of $q\sim 0.02$ and projected host--companion separations of $s=1.33\pm 0.01$ and $s=0.76\pm 0.01$, respectively. These solutions improve the fit by $\Delta \chi ^2\gt 4430$ compared to the single-lens model. We detected the finite-source effect in the light curve and obtained the angular Einstein radius of $\theta _{\rm E} = 0.7\pm 0.1\:\mathrm{mas}$, which provides a mass--distance relation for the lens. We conducted a Bayesian analysis to estimate the physical parameters of the lens system. The results indicate that the lens system consists of a host star with a mass of $M_{\rm L,H} \sim 0.6\:M_\odot$ at a distance of $D_{\rm L}\sim 5$ kpc and a giant planet with a mass of $M_{\rm L,C}\sim 10\:M_{\rm {Jup}}$ orbiting beyond the snow line. Conventional planet formation theories suggest that giant planets are unlikely to form around low-mass stars. Furthermore, several statistical studies have suggested the existence of a companion-to-host mass-ratio desert in the range $0.02 \lesssim q \lesssim 0.05$, and the companion in the lens system discovered in this work lies near the lower boundary of this desert. Objects near the planet--brown dwarf boundary may form through multiple pathways, and this discovery provides an additional data point for understanding their formation mechanisms.
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