  We present the analysis of high-resolution images of MOA-2013-BLG-220, taken
with the Keck adaptive optics system 6 years after the initial observation,
identifying the lens as a solar-type star hosting a super-Jupiter mass planet.
The masses of planets and host-stars discovered by microlensing are often not
determined from light curve data, while the star-planet mass-ratio and
projected separation in units of Einstein ring radius are well measured.
High-resolution follow-up observations after the lensing event is complete can
resolve the source and lens. This allows direct measurements of flux, and the
amplitude and direction of proper motion, giving strong constraints on the
system parameters. Due to the high relative proper motion, $\mu_{\rm rel,Geo} =
12.62\pm0.11$ mas/yr, the source and lens were resolved in 2019, with a
separation of $77.1\pm0.5$ mas. Thus, we constrain the lens flux to $K_{\rm
Keck,lens}= 17.92\pm0.02$. By combining constraints from the model and Keck
flux, we find the lens mass to be $M_L = 0.88\pm0.05\ M_\odot$ at $D_L =
6.72\pm0.59$ kpc. With a mass-ratio of $q=(3.00\pm0.03)\times10^{-3}$ the
planet's mass is determined to be $M_P = 2.74\pm0.17\ M_{J}$ at a separation of
$r_\perp = 3.03\pm0.27$ AU. The lens mass is much higher than the prediction
made by the Bayesian analysis that assumes all stars have an equal probability
to host a planet of the measured mass ratio, and suggests that planets with
mass ratios of a few 10$^{-3}$ are more common orbiting massive stars. This
demonstrates the importance of high-resolution follow-up observations for
testing theories like these.
