Measurement of Dependence of Microlensing Planet Frequency on the Host Star Mass and Galactocentric Distance by Using a Galactic Model

Abstract

Abstract We measure the dependence of planet frequency on host star mass, M L, and distance from the Galactic center, R L, using a sample of planets discovered by gravitational microlensing. We compare the two-dimensional distribution of the lens-source proper motion, μ rel, and the Einstein radius crossing time, t E, measured for 22 planetary events from Suzuki et al. with the distribution expected from Galactic model. Assuming that the planet-hosting probability of a star is proportional to M L m R L r , we calculate the likelihood distribution of (m,r). We estimate that r = 0.10 − 0.37 + 0.51 and m = 0.50 − 0.70 + 0.90 under the assumption that the planet-hosting probability is independent of the mass ratio. We also divide the planet sample into subsamples based on their mass ratio, q, and estimate that m = − 0.08 − 0.65 + 0.95 for q < 10−3 and 1.25 − 1.14 + 1.07 for q > 10−3. Although uncertainties are still large, this result implies a possibility that, in orbits beyond the snowline, massive planets are more likely to exist around more massive stars whereas low-mass planets exist regardless of their host star mass.