Precise Mass Measurement of OGLE-2013-BLG-0132/MOA-2013-BLG-148: A Saturn-mass Planet Orbiting an M Dwarf

We revisit the planetary microlensing event OGLE-2013-BLG-0132/MOA-2013-BLG-148 using Keck adaptive optics imaging in 2013 with NIRC2 and in 2020, 7.4 yr after the event, with OSIRIS. The 2020 observations yield a source and lens separation of 56.91 ± 0.29 mas, which provides us with a precise measurement of the heliocentric proper motion of the event μrel,hel = 7.695 ± 0.039 mas yr−1. We measured the magnitude of the lens in the K band as Klens = 18.69 ± 0.04. Using these constraints, we refit the microlensing light curve and undertake a full reanalysis of the event parameters including the microlensing parallax πE and the distance to the source DS. We confirm the results obtained in the initial study by Mróz et al. and improve significantly upon the accuracy of the physical parameters. The system is an M dwarf of 0.495 ± 0.054 M⊙ orbited by a cold, Saturn-mass planet of 0.26 ± 0.028 MJup at projected separation r⊥ = 3.14 ± 0.28 au. This work confirms that the planetary system is at a distance of 3.48 ± 0.36 kpc, which places it in the Galactic disk and not the Galactic bulge.