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Register a new userKMT-2016-BLG-1820 and KMT-2016-BLG-2142: Two Microlensing Binaries Composed of Planetary-mass Companions and Very-Low-Mass Primaries
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We present the analyses of two short-timescale $(t_{rm E} sim 5~{rm days})$ 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 $theta_{rm E} = 0.12~{rm mas}$, 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 $0.043_{-0.018}^{+0.043}~M_{odot}$ and $0.088_{-0.041}^{+0.120}~M_{odot}$, implying that they are well within the very-low-mass regime. The estimated lens-component masses indicate that the binary lenses consist of a giant-planet/brown-dwarf 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 $6$ kpc and $7$ kpc. The projected lens-components separations are $1.1$ AU 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.
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KMT-2016-BLG-2605, with planet-host mass ratio $q=0.012pm 0.001$, has the shortest Einstein timescale, $t_e = 3.41pm 0.13,$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 $theta_e = 115pm 20,muas$ and lens-source relative proper motions $mu_relsimeq 9.5pm 2.5,masyr$. 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.
We report the discovery of a binary composed of two brown dwarfs, based on the analysis of the microlensing event OGLE-2016-BLG-1469. Thanks to detection of both finite-source and microlens-parallax effects, we are able to measure both the masses $M_1sim 0.05 M_odot$, $M_2sim 0.01 M_odot$, and distance $D_{rm L} sim 4.5$ kpc, as well as the projected separation $a_perp sim 0.33$ au. This is the third brown-dwarf binary detected using the microlensing method, demonstrating the usefulness of microlensing in detecting field brown-dwarf binaries with separations less than 1 au.
We report the discovery of a super-Jovian planet in the microlensing event KMT-2016-BLG-1836, which was found by the Korea Microlensing Telescope Networks high-cadence observations (Gamma ~ 4~{hr}^{-1}). The planet-host mass ratio q ~ 0.004. A Bayesian analysis indicates that the planetary system is composed of a super-Jovian M_{planet} = 2.2_{-1.1}^{+1.9} M_{J} planet orbiting an M or K dwarf M_{rm host} = 0.49_{-0.25}^{+0.38} M_{Sun}, at a distance of D_{L} = 7.1_{-2.4}^{+0.8} kpc. The projected planet-host separation is 3.5^{+1.1}_{-0.9} AU, implying that the planet is located beyond the snowline of the host star. Future high-resolution images can potentially strongly constrain the lens brightness and thus the mass and distance of the planetary system. Without considering detailed detection efficiency, selection or publication biases, we find a potential mass ratio desert at -3.7 lesssim log q lesssim -3.0 for the 31 published KMTNet planets.
At $q=1.81pm 0.20 times 10^{-5}$, KMT-2018-BLG-0029Lb has the lowest planet-host mass ratio $q$ of any microlensing planet to date by more than a factor of two. Hence, it is the first planet that probes below the apparent pile-up at $q=5$--10 $times 10^{-5}$. The event was observed by {it Spitzer}, yielding a microlens-parallax $pi_{rm E}$ measurement. Combined with a measurement of the Einstein radius $theta_{rm E}$ from finite-source effects during the caustic crossings, these measurements imply masses of the host $M_{rm host}=1.14^{+0.10}_{-0.12}, M_odot$ and planet $M_{rm planet} = 7.59^{+0.75}_{-0.69},M_oplus$, system distance $D_L = 3.38^{+0.22}_{-0.26},,{rm kpc}$ and projected separation $a_perp = 4.27^{+0.21}_{-0.23},{rm au}$. The blended light, which is substantially brighter than the microlensed source, is plausibly due to the lens and could be observed at high resolution immediately.
We show that microlensing event KMT-2016-BLG-1107 displays a new type of degeneracy between wide-binary and close-binary Hollywood events in which a giant-star source envelops the planetary caustic. The planetary anomaly takes the form of a smooth, two-day bump far out on the falling wing of the light curve, which can be interpreted either as the source completely enveloping a minor-image caustic due to a close companion with mass ratio $q=0.036$, or partially enveloping a major-image caustic due to a wide companion with $q=0.004$. The best estimates of the companion masses are both in the planetary regime ($3.3^{+3.5}_{-1.8},M_{rm jup}$ and $0.090^{+0.096}_{-0.037},M_{rm jup}$) but differ by an even larger factor than the mass ratios due to different inferred host masses. We show that the two solutions can be distinguished by high-resolution imaging at first light on next-generation (30m) telescopes. We provide analytic guidance to understand the conditions under which this new type of degeneracy can appear.
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