MOA-2020-BLG-108Lb: A Giant Planet Beyond the Snow Line of a Low-Mass Lens Near the Lower Boundary of the Mass-Ratio Desert

Abstract

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 $q\sim0.02$ and projected host--companion separations of $s=1.33\pm0.01$ and $s=0.76\pm0.01$, respectively. These solutions improve the fit by $Δχ^2>4430$ compared to the single-lens model. We detected the finite-source effect in the light curve and obtained the angular Einstein radius of $θ_{\rm E} = 0.7\pm0.1\:\mathrm {mas}$, 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 $D_{\rm L}\sim5$ kpc and a giant planet with a mass of $M_{\rm L,C}\sim10\:M_{\rm {Jup}}$ 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.