KMT-2016-BLG-1820 and KMT-2016-BLG-2142: Two Microlensing Binaries Composed of Planetary-mass Companions and Very-low-mass Primaries

We present the analyses of two short-timescale (t(E) similar to 5 days) microlensing events, KMT-2016-BLG-1820 and KMT-2016-BLG-2142. In both light curves, the brief anomalies were clearly captured and densely covered by the Korea Microlensing Telescope Network survey. From these analyses, we find that both events have small Einstein radii of theta(E) = 0.12 mas, suggesting that the binary-lens systems are composed of very-low-mass components and/ or are located much closer to the lensed stars than to Earth. From Bayesian analyses, we find that these binaries have total system masses of 0.043(0.018)(+0.043) M-circle dot and 0.088(0.041)(+0.120) M-circle dot, implying that they are well within the very-lowmass regime. The estimated lens-component masses indicate that the binary lenses consist of a giant-planet/browndwarf pair (KMT-2016-BLG-1820) and a dark/faint object pair (KMT-2016-BLG-2140) that are located near the deuterium-burning and hydrogen-burning mass limits, respectively. Both lens systems are likely to be in the Galactic disk with estimated distances of about 6 and 7 kpc. The projected lens-components separations are 1.1 and 0.8 au, and the mass ratios are 0.11 and 0.20. These prove that the microlensing method is effective to identify these closely separated very-low-mass binaries having low mass-ratios.