MOA-2016-BLG-227Lb: A Massive Planet Characterized by Combining Light-curve Analysis and Keck AO Imaging

Abstract

We report the discovery of a microlensing planet-MOA-2016-BLG-227Lb-with a large planet/host mass ratio of q ; 9×10 −3 . This event was located near the K2 Campaign 9 field that was observed by a large number of telescopes. As a result, the event was in the microlensing survey area of a number of these telescopes, and this enabled good coverage of the planetary light-curve signal. High angular resolution adaptive optics images from the Keck telescope reveal excess flux at the position of the source above the flux of the source star, as indicated by the light-curve model. This excess flux could be due to the lens star, but it could also be due to a companion to the source or lens star, or even an unrelated star. We consider all these possibilities in a Bayesian analysis in the context of a standard Galactic model. Our analysis indicates that it is unlikely that a large fraction of the excess flux comes from the lens, unless solar-type stars are much more likely to host planets of this mass ratio than lower mass stars. We recommend that a method similar to the one developed in this paper be used for other events with high angular resolution follow-up observations when the follow-up observations are insufficient to measure the lenssource relative proper motion.

Combining Spitzer Parallax and Keck II Adaptive Optics Imaging to Measure the Mass of a Solar-like Star Orbited by a Cold Gaseous Planet Discovered by Microlensing

“…The same calculation has also been done following the approach of Koshimoto et al (2017b); these authors also use the multiplicity estimates from Duchêne & Kraus (2013), but the treatment of the surface density distribution of field stars is slightly different. The two approaches also slightly differ in their a priori distributions: Koshimoto et al (2017b) use a continuous law that is a function of the primary mass, whereas V. Batista et al 2017 in preparation use a set of distinct laws associated to different mass bins.…”

“…The two approaches also slightly differ in their a priori distributions: Koshimoto et al (2017b) use a continuous law that is a function of the primary mass, whereas V. Batista et al 2017 in preparation use a set of distinct laws associated to different mass bins.…”

“…Finally, the treatment of the Keck measurement in their Bayesian analysis slightly differs, as Koshimoto et al (2017b) use it as a selection criteria of their flux combinations, while V. Batista et al 2017 in preparation use it as an a priori distribution.…”

“…It is also a cautionary tale showing that it is important to carefully estimate the potential contribution of source and lens companions that may potentially bias the inferred host properties if they are not accounted for. We note that for fainter lenses, these contributions will be more dramatic, like the case of MOA-2016-BLG-227 (Koshimoto et al 2017b); a dedicated study will have to be performed in the framework of Euclid and WFIRST. Not accounting for these potential companions might lead to a bias toward higher inferred lens masses.…”

“…Moreover, Koshimoto et al (2017b) use a Galactic model in their calculation, while V. Batista et al 2017 in preparation use the best-fit parameters from Udalski et al (2015) for M,D L ,Π E , and θ E , and the OGLE calculator for D S . Finally, the treatment of the Keck measurement in their Bayesian analysis slightly differs, as Koshimoto et al (2017b) use it as a selection criteria of their flux combinations, while V. Batista et al 2017 in preparation use it as an a priori distribution.…”