  We present an adaptive optics (AO) analysis of images from the Keck-II
telescope NIRC2 instrument of the planetary microlensing event
MOA-2009-BLG-319. The $\sim$10 year baseline between the event and the Keck
observations allows the planetary host star to be detected at a separation of
$66.5\pm 1.7\,$mas from the source star, consistent with the light curve model
prediction. The combination of the host star brightness and light curve
parameters yield host star and planet masses of M_host = 0.514 $\pm$ 0.063M_Sun
and m_p = 66.0 $\pm$ 8.1M_Earth at a distance of $D_L = 7.0 \pm 0.7\,$kpc. The
star-planet projected separation is $2.03 \pm 0.21\,$AU. The planet-star mass
ratio of this system, $q = (3.857 \pm 0.029)\times 10^{-4}$, places it in the
predicted "planet desert" at $10^{-4} < q < 4\times 10^{-4}$ according to the
runaway gas accretion scenario of the core accretion theory. Seven of the 30
planets in the Suzuki et al. (2016) sample fall in this mass ratio range, and
this is the third with a measured host mass. All three of these host stars have
masses of 0.5 $\leq$ M_host/M_Sun $\leq$ 0.7, which implies that this predicted
mass ratio gap is filled with planets that have host stars within a factor of
two of 1M_Sun. This suggests that runaway gas accretion does not play a major
role in determining giant planet masses for stars somewhat less massive than
the Sun. Our analysis has been accomplished with a modified DAOPHOT code that
has been designed to measure the brightness and positions of closely blended
stars. This will aid in the development of the primary method that the Nancy
Grace Roman Space Telescope mission will use to determine the masses of
microlens planets and their hosts.
