OGLE-2016-BLG-1195 AO: Lens, Companion to Lens or Source, or None of the Above?

Abstract

We systematically investigate Vandorou et al.’s claim to have detected the host star of the low-mass-ratio ( q < 10 ^−4 ) microlensing planet OGLE-2016-BLG-1195Lb, via Keck adaptive optics (AO) measurements Δ t = 4.12 yr after the event’s peak ( t _0 ). If correct, this measurement would contradict the microlens-parallax measurement derived from Spitzer observations taken near t _0 . We show that this host identification would be in 4 σ conflict with the original ground-based relative lens–source proper-motion measurements. By contrast, Gould estimated a probability p = 10% that the “other star” resolved by single-epoch late-time AO would be a companion to the host or the microlensed source, which is much more probable than a 4 σ statistical fluctuation. Independent of this proper-motion discrepancy, the kinematics of this host identification are substantially less probable than those of the Spitzer solution. Hence, this identification should not be accepted, pending additional observations that would either confirm or contradict it, which could be taken in 2023. Motivated by this tension, we present two additional investigations. We explore the possibility that Vandorou et al. identified the wrong “star” for their analysis. Astrometry of KMT and Keck images favors a star (or asterism) lying about 175 mas northwest of Vandorou et al.’s star. We also present event parameters from a combined fit to all survey data, which yields a more precise mass ratio, q = (4.6 ± 0.4) × 10 ^−5 . Finally, we discuss the broader implications of minimizing such false positives for the first measurement of the planet mass function, which will become possible when AO on next-generation telescopes are applied to microlensing planets.

Keywords

• High-resolution microlensing event imaging
• Satellite microlensing parallax
• Gravitational microlensing exoplanet detection