OGLE-2015-BLG-1649Lb: A Gas Giant Planet around a Low-mass Dwarf

We report the discovery of an exoplanet from the analysis of the gravitational microlensing event OGLE-2015-BLG-1649 that challenges the core accretion model of planet formation and appears to support the disk instability model. The planet/host-star mass ratio is q = 7.2 × 10^-3 and the projected separation normalized to the angular Einstein radius is s = 0.9. We conducted high-resolution follow-up observations using the Infrared Camera and Spectrograph (IRCS) camera on the Subaru telescope and are able to place an upper limit on the lens flux. From these measurements we are able to exclude all host stars greater than or equal in mass to a G-type dwarf. We conducted a Bayesian analysis with these new flux constraints included as priors resulting in estimates of the masses of the host star and planet. These are M _L = 0.34 ± 0.19 M _⊙ and {M}_{{p}}={2.5}_-1.4^+1.5 {M}_Jup}, respectively. The distance to the system is {D}_{{L}}={4.23}_-1.64^+1.51 {kpc}. The projected star-planet separation is {a}_\perp ={2.07}_-0.77^+0.65 {au}. The estimated relative lens-source proper motion, ∼7.1 mas yr^-1, is fairly high and thus the lens can be better constrained if additional follow-up observations are conducted several years after the event.