OGLE-2015-BLG-1771Lb: A Microlens Planet Orbiting an Ultracool Dwarf?

TL;DR

This paper reports the discovery of a short-duration planetary microlensing event, OGLE-2015-BLG-1771, with multiple models suggesting a Saturn-mass planet orbiting an ultracool dwarf or a super-Jovian planet with a brown dwarf, analyzed through Bayesian methods.

Contribution

First detailed analysis of a short microlensing event with multiple planetary models, proposing possible planetary system configurations and their likelihoods.

Findings

Best-fit model suggests a Saturn-mass planet orbiting a late M dwarf.

Alternative models include a super-Jovian planet with a brown dwarf.

Source and lens could be resolved with AO in 2021 if lens is luminous.

Abstract

We report the discovery and the analysis of the short (tE < 5 days) planetary microlensing event, OGLE-2015-BLG-1771. The event was discovered by the Optical Gravitational Lensing Experiment (OGLE), and the planetary anomaly (at I ~ 19) was captured by The Korea Microlensing Telescope Network (KMTNet). The event has three surviving planetary models that explain the observed light curves, with planet-host mass ratio q \~ 5.4 * 10^{-3}, 4.5 * 10^{-3} and 4.5 * 10^{-2}, respectively. The first model is the best-fit model, while the second model is disfavored by \Delta\chi^2 ~ 3. The last model is strongly disfavored by \Delta\chi^2 ~ 15 but not ruled out. A Bayesian analysis using a Galactic model indicates that the first two models are probably composed of a Saturn-mass planet orbiting a late M dwarf, while the third one could consist of a super-Jovian planet and a mid-mass brown dwarf. The source-lens relative proper motion is mu_rel ~ 9 mas/yr, so the source and lens could be resolved by current adaptive-optics (AO) instruments in 2021 if the lens is luminous.