Shortest Microlensing Event with a Bound Planet: KMT-2016-BLG-2605

TL;DR

This paper analyzes the shortest microlensing event with a bound planet, revealing that such events likely involve very low-mass hosts near the hydrogen-burning limit, and discusses methods to expand the sample for better understanding.

Contribution

It presents the first detailed analysis of the shortest planetary microlensing event, suggesting a population of very low-mass hosts and proposing techniques to identify more such events.

Findings

Six of seven short planetary events have similar Einstein radii and proper motions.

These events are consistent with a population of very low-mass hosts near the hydrogen-burning limit.

The paper discusses methods to expand the sample and distinguish genuine planets from brown dwarfs.

Abstract

KMT-2016-BLG-2605, with planet-host mass ratio $q=0.012\pm 0.001$, has the shortest Einstein timescale, $t_\e = 3.41\pm 0.13\,$days, of any planetary microlensing event to date. This prompts us to examine the full sample of 7 short ($t_\e<7\,$day) planetary events with good $q$ measurements. We find that six have clustered Einstein radii $\theta_\e = 115\pm 20\,\muas$ and lens-source relative proper motions $\mu_\rel\simeq 9.5\pm 2.5\,\masyr$. For the seventh, these two quantities could not be measured. These distributions are consistent with a Galactic-bulge population of very low-mass (VLM) hosts near the hydrogen-burning limit. This conjecture could be verified by imaging at first adaptive-optics light on next-generation (30m) telescopes. Based on a preliminary assessment of the sample, "planetary" companions (i.e., below the deuterium-burning limit) are divided into "genuine planets", formed in their disks by core accretion, and very low-mass brown dwarfs, which form like stars. We discuss techniques for expanding the sample, which include taking account of the peculiar "anomaly dominated" morphology of the KMT-2016-BLG-2605 light curve.