MOA-2009-BLG-387Lb: A massive planet orbiting an M dwarf

TL;DR

This paper reports the discovery of a massive planet orbiting an M dwarf through microlensing, providing detailed measurements of the planet's mass, orbit, and host star characteristics, and demonstrating the use of microlensing parameters and Galactic models.

Contribution

The study precisely measures the planet-to-star mass ratio and host star properties using microlensing, including resolving degeneracies with Galactic and orbital priors.

Findings

Planet mass approximately 2.6 Jupiter masses

Host star mass between 0.07 and 0.49 solar masses

Planet's orbital period estimated between 3.8 and 7.6 years

Abstract

We report the discovery of a planet with a high planet-to-star mass ratio in the microlensing event MOA-2009-BLG-387, which exhibited pronounced deviations over a 12-day interval, one of the longest for any planetary event. The host is an M dwarf, with a mass in the range 0.07 M_sun < M_host < 0.49M_sun at 90% confidence. The planet-star mass ratio q = 0.0132 +- 0.003 has been measured extremely well, so at the best-estimated host mass, the planet mass is m_p = 2.6 Jupiter masses for the median host mass, M = 0.19 M_sun. The host mass is determined from two "higher order" microlensing parameters. One of these, the angular Einstein radius \theta_E = 0.31 +- 0.03 mas, is very well measured, but the other (the microlens parallax \pi_E, which is due to the Earth's orbital motion) is highly degenate with the orbital motion of the planet. We statistically resolve the degeneracy between Earth and planet orbital effects by imposing priors from a Galactic model that specifies the positions and velocities of lenses and sources and a Kepler model of orbits. The 90% confidence intervals for the distance, semi-major axis, and period of the planet are 3.5 kpc < D_L < 7.9 kpc, 1.1 AU < a < 2.7AU, and 3.8 yr < P < 7.6 yr, respectively.