We report the gravitational microlensing discovery of a sub-Saturn mass
planet, MOA-2009-BLG-319Lb, orbiting a K or M-dwarf star in the inner
Galactic disk or Galactic bulge. The high cadence observations of the
MOA-II survey discovered this microlensing event and enabled its
identification as a high magnification event approximately 24 hours
prior to peak magnification. As a result, the planetary signal at the
peak of this light curve was observed by 20 different telescopes, which
is the largest number of telescopes to contribute to a planetary
discovery to date. The microlensing model for this event indicates a
planet-star mass ratio of q = (3.95 +/- 0.02) x 10^{-4} and a separation
of d = 0.97537 +/- 0.00007 in units of the Einstein radius. A Bayesian
analysis based on the measured Einstein radius crossing time, t_E, and
angular Einstein radius, \theta_E, along with a standard Galactic model
indicates a host star mass of M_L = 0.38^{+0.34}_{-0.18} M_{Sun} and a
planet mass of M_p = 50^{+44}_{-24} M_{Earth}, which is half the mass of
Saturn. This analysis also yields a planet-star three-dimensional
separation of a = 2.4^{+1.2}_{-0.6} AU and a distance to the planetary
system of D_L = 6.1^{+1.1}_{-1.2} kpc. This separation is ~ 2 times the
distance of the snow line, a separation similar to most of the other
planets discovered by microlensing
