A sub-Saturn mass planet, MOA-2009-BLG-319LB

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 thismicrolensing event and enabled its identification as a high-magnification event approximately 24 hr 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, tE, and angular Einstein radius, θE, along with a standard Galactic model indicates a host star mass of ML = 0.38+0.34 -0.18M and a planet mass of Mp = 50+44 -24 M, 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 DL = 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.