OGLE-2011-BLG-1303Lb and MOA-2020-BLG-108Lb: Two Giant microlensing Planets Around Low Mass Host Stars

The gravitational microlensing method is an exoplanet detection method that uses the phenomenon of light being bent by gravity. In the gravitational microlensing, when a lensing object crosses in front of a source star, the brightness of the source star changes with time owing to the gravitational effect of the lensing object. When the lens object is a single star, the light magnifies most when the source, lensing object, and observer are in line. If the lensing object is accompanied by a companion, the gravity of this companion causes a secondary magnification. The gravitational microlensing method does not use the light from the lensing object, but only the time-dependent variations arising from the gravitational effect of the lensing object or objects on the light from the source. Therefore, the gravitational microlensing method can detect planets around faint stars at great distances from Earth and free-floating planets that do not orbit a particular star, which are difficult to detect by other planet detection methods. In addition, the gravitational microlensing method has the advantage of being sensitive to planets beyond the snowline. Many giant planets are thought to form outside the snowline. We report on the analysis of two gravitational microlensing events. In the analysis of OGLE-2011-BLG-1303, as many as 15 solutions were degenerate, but Bayesian estimation using the Galactic model and microlensing parameters obtained from the analysis showed that all solutions are a giant planet around a late-type star or a brown dwarf. The distance from Earth to the lensing system is 6-8 kpc and is located near the Galactic bulge. The analysis of MOA-2020-BLG-108 also had multiple degenerate solutions, but Bayesian analysis yielded the result of a giant planet around a late-type star. Thus, both OGLE-2011-BLG-1303 and MOA-2020-BLG-108 have lens systems composed of low-mass host stars and giant planets. Planets around low-mass host stars are unlikely to form in the standard core accretion model. Planet samples like our results are important in improving planetary formation models. In this presentation, further details of the analysis will be reported.