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

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 ${M}_{{\rm{L}}}^{m}{R}_{{\rm{L}}}^{r}$ , 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 . 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.

Keywords

• Planet formation
• Exoplanets
• the Milky Way
• Gravitational microlensing