宮﨑 翔太

OGLE-2014-BLG-0962 (OB140962) is a stellar binary microlensing event that was well covered by observations from the Spitzer satellite as well as ground-based surveys. Modeling yields a unique physical solution: a mid-M+M-dwarf binary with M _prim = 0.20 ± 0.01 M _⊙ and M _sec = 0.16 ± 0.01 M _⊙, with projected separation of 2.0 ± 0.3 au. The lens is only D _LS = 0.41 ± 0.06 kpc in front of the source, making OB140962 a bulge lens and the most distant Spitzer binary lens to date. In contrast, because the Einstein radius (θ _E = 0.143 ± 0.007 mas) is unusually small, a standard Bayesian analysis, conducted in the absence of parallax information, would predict a brown dwarf binary. We compare the results of Bayesian analysis using two commonly used Galactic model priors to the measured values for a set of Spitzer lenses. We find all models tested predict lens properties consistent with the Spitzer data. Furthermore, we illustrate the methodology for probing the Galactic distribution of planets by comparing the cumulative distance distribution of the Spitzer two-body lenses to that of the Spitzer single lenses....

Planet formation theories predict the existence of free-floating planets that have been ejected from their parent systems. Although they emit little or no light, they can be detected during gravitational microlensing events. Microlensing events caused by rogue planets are characterized by very short timescales t_E (typically below two days) and small angular Einstein radii θ_E (up to several μas). Here we present the discovery and characterization of two ultra-short microlensing events identified in data from the Optical Gravitational Lensing Experiment (OGLE) survey, which may have been caused by free-floating or wide-orbit planets. OGLE-2012-BLG-1323 is one of the shortest events discovered thus far (t_E = 0.155 ± 0.005 d, θ_E = 2.37 ± 0.10μas) and was caused by an Earth-mass object in the Galactic disk or a Neptune-mass planet in the Galactic bulge. OGLE-2017-BLG-0560 (t_E = 0.905 ± 0.005 d, θ_E = 38.7 ± 1.6μas) was caused by a Jupiter-mass planet in the Galactic disk or a brown dwarf in the bulge. We rule out stellar companions up to a distance of 6.0 and 3.9 au, respectively. We suggest that the lensing objects, whether located on very wide orbits or free-floating, may originate from the same physical mechanism. Although the sample of ultrashort microlensing events is small, these detections are consistent with low-mass wide-orbit or unbound planets being more common than stars in the Milky Way....

We analyze the binary microlensing event MOA-2016-BLG-231, which was observed from the ground and from Spitzer. The lens is composed of very-low-mass brown dwarfs (BDs) with {M}₁={21}_-5⁺¹² {M}_J and {M}₂={9}_-2⁺⁵ {M}_J, and it is located in the Galactic disk {D}_{{ { L} } }={2.85}_-0.50^+0.88 {kpc}. This is the fifth binary brown dwarf discovered by microlensing, and the BD binary is moving counter to the orbital motion of disk stars. Constraints on the lens physical properties come from late-time, non-caustic-crossing features of the Spitzer light curve. Thus, MOA-2016-BLG-231 shows how Spitzer plays a crucial role in resolving the nature of BDs in binary BD events with short timescales (≲10 days)....

We analyze the gravitational binary-lensing event OGLE-2016-BLG-0156, for which the lensing light curve displays pronounced deviations induced by microlens-parallax effects. The light curve exhibits three distinctive widely separated peaks and we find that the multiple-peak feature provides a very tight constraint on the microlens-parallax effect, enabling us to precisely measure the microlens parallax {π }_{{ { E} } }. All the peaks are densely and continuously covered from high-cadence survey observations using globally located telescopes and the analysis of the peaks leads to the precise measurement of the angular Einstein radius {θ }_{{ { E} } }. From the combination of the measured {π }_{{ { E} } } and {θ }_{{ { E} } }, we determine the physical parameters of the lens. It is found that the lens is a binary composed of two M dwarfs with masses M ₁ = 0.18 ± 0.01 M _⊙ and M ₂ = 0.16 ± 0.01 M _⊙ located at a distance {D}_{{ { L} } }=1.35+/- 0.09 {kpc}. According to the estimated lens mass and distance, the flux from the lens comprises an important fraction, ∼25%, of the blended flux. The bright nature of the lens combined with the high relative lens-source motion, μ = 6.94 ± 0.50 mas yr^-1, suggests that the lens can be directly observed from future high-resolution follow-up observations....

We present an analysis of the microlensing event OGLE-2015-BLG-0232. This event is challenging to characterize for two reasons. First, the light curve is not well sampled during the caustic crossing due to the proximity of the full Moon impacting the photometry quality. Moreover, the source brightness is difficult to estimate because this event is blended with a nearby K dwarf star. We found that the light-curve deviations are likely due to a close brown dwarf companion (i.e., s = 0.55 and q = 0.06), but the exact nature of the lens is still unknown. We finally discuss the potential of follow-up observations to estimate the lens mass and distance in the future....

We compare the planet-to-star mass-ratio distribution measured by gravitational microlensing to core accretion theory predictions from population synthesis models. The core accretion theory’s runaway gas accretion process predicts a dearth of intermediate-mass giant planets that is not seen in the microlensing results. In particular, the models predict ∼10 × fewer planets at mass ratios of {10}^-4≤slant q≤slant 4× {10}^-4 than inferred from microlensing observations. This tension implies that gas giant formation may involve processes that have hitherto been overlooked by existing core accretion models or that the planet-forming environment varies considerably as a function of host-star mass. Variation from the usual assumptions for the protoplanetary disk viscosity and thickness could reduce this discrepancy, but such changes might conflict with microlensing results at larger or smaller mass ratios, or with other observations. The resolution of this discrepancy may have important implications for planetary habitability because it has been suggested that the runaway gas accretion process may have triggered the delivery of water to our inner solar system. So, an understanding of giant planet formation may help us to determine the occurrence rate of habitable planets....

We present the analysis of the planetary microlensing event MOA-2016-BLG-319. The event light curve is characterized by a brief (∼3 days) anomaly near the peak produced by minor-image perturbations. From modeling, we find two distinct solutions that describe the observed light curve almost equally as well. From the investigation of the lens-system configurations, we find that the confusion in the lensing solution is caused by the degeneracy between the two solutions resulting from the source passages on different sides of the planetary caustic. These degeneracies can be severe for major-image perturbations, but it is known that they are considerably less severe for minor-image perturbations. From the comparison of the lens-system configuration with those of two previously discovered planetary events, for which similar degeneracies were reported, we find that the degeneracies are caused by the special source trajectories that passed the star-planet axes at approximately right angles. By conducting a Bayesian analysis, it is estimated that the lens is a planetary system in which a giant planet with a mass {M}_{{ { p} } }={0.62}_-0.33^+1.16 {M}_{{ { J} } } ({0.65}_-0.35^+1.21 {M}_{{ { J} } }) is orbiting a low-mass M-dwarf host with a mass {M}_{{ { h} } }={0.15}_-0.08^+0.28 {M}_⊙ . Here the planet masses in and out of the parentheses represent the masses for the individual degenerate solutions. The projected host-planet separations are a _⊥ ∼ 0.95 and ∼1.05 au for the two solutions. The identified degeneracy indicates the need to check similar degeneracies in future analyses of planetary lensing events with minor-image perturbations....

We present an analysis of the caustic-crossing binary microlensing event OGLE-2017-BLG-0039. Thanks to the very long duration of the event, with a time scale t _E ∼ 130 days, the microlens parallax is measured precisely despite its low value of {π }_{{ { E} } }∼ 0.06. Analysis of the well-resolved caustic crossings during the source star’s entrance and exit of the caustic yields an angular Einstein radius of {θ }_{{ { E} } }∼ 0.6 mas. The measured {π }_{{ { E} } } and {θ }_{{ { E} } } indicate that the lens is a binary composed of two stars with masses ∼ 1.0 {M}_⊙ and ∼0.15 M _⊙, and is located at a distance of ∼6 kpc. From the color and brightness of the lens estimated from its determined mass and distance, it is expected that ∼2/3 of the I-band blended flux comes from the lens. Therefore, the event is a rare case of a bright lens event for which high-resolution follow-up observations can confirm its nature....

We present the analysis of the planetary microlensing event MOA-2011-BLG-291, which has a mass ratio of q = (3.8 ± 0.7) × 10^-4 and a source star that is redder (or brighter) than the bulge main sequence. This event is located at a low Galactic latitude in the survey area that is currently planned for NASA’s Wide Field Infrared Survey Telescope (WFIRST) exoplanet microlensing survey. This unusual color for a microlensed source star implies that we cannot assume that the source star is in the Galactic bulge. The favored interpretation is that the source star is a lower main-sequence star at a distance of D _S = 4.9 ± 1.3 kpc in the Galactic disk. However, the source could also be a turn-off star on the far side of the bulge or a subgiant in the far side of the Galactic disk if it experiences significantly more reddening than the bulge red clump stars. However, these possibilities have only a small effect on our mass estimates for the host star and planet. We find host star and planet masses of {M}_host}={0.15}_-0.10^+0.27 {M}_⊙ and {m}_p={18}_-12⁺³⁴ {M}_\oplus from a Bayesian analysis with a standard Galactic model, under the assumption that the planet hosting probability does not depend on the host mass or distance. However, if we attempt to measure the host and planet masses with host star brightness measurements from high angular resolution follow-up imaging, the implied masses will be sensitive to the host star distance. The WFIRST exoplanet microlensing survey is expected to use this method to determine the masses for many of the planetary systems that it discovers, so this issue has important design implications for the WFIRST exoplanet microlensing survey....

We report on the discovery and analysis of the short-timescale binary-lens microlensing event, MOA-2015-BLG-337. The lens system could be a planetary system with a very low-mass host, around the brown dwarf (BD)/planetary-mass boundary, or a BD binary. We found two competing models that explain the observed light curves with companion/host mass ratios of q ∼ 0.01 and ∼0.17, respectively. A significant finite source effect in the best-fit planetary model (q ∼ 0.01) reveals a small angular Einstein radius of θ _E ≃ 0.03 mas, which favors a low-mass lens. We obtain the posterior probability distribution of the lens properties from a Bayesian analysis. The results for the planetary models strongly depend on a power-law index in planetary-mass regime, α _pl, in the assumed mass function. In summary, there are two solutions of the lens system: (1) a BD/planetary-mass boundary object orbited by a super-Neptune (the planetary model with α _pl = 0.49) and (2) a BD binary (the binary model). If the planetary models are correct, this system can be one of a new class of planetary system, having a low host mass and also a planetary-mass ratio (q < 0.03) between the companion and its host. The discovery of the event is important for the study of planetary formation in very low-mass objects. In addition, it is important to consider all viable solutions in these kinds of ambiguous events in order for the future comprehensive statistical analyses of planetary/binary microlensing events....

We show that dense OGLE and KMTNet I-band survey data require four bodies (sources plus lenses) to explain the microlensing light curve of OGLE-2015-BLG-1459. However, these can equally well consist of three lenses and one source (3L1S), two lenses and two sources (2L2S), or one lens and three sources (1L3S). In the 3L1S and 2L2S interpretations, the host is a brown dwarf and the dominant companion is a Neptune-class planet, with the third body (in the 3L1S case) being a Mars-class object that could have been a moon of the planet. In the 1L3S solution, the light curve anomalies are explained by a tight (five stellar radii) low-luminosity binary source that is offset from the principal source of the event by ∼ 0.17 {au}. These degeneracies are resolved in favor of the 1L3S solution by color effects derived from comparison to MOA data, which are taken in a slightly different (R/I) passband. To enable current and future (WFIRST) surveys to routinely characterize exo-moons and distinguish among such exotic systems requires an observing strategy that includes both a cadence faster than 9 minute^-1 and observations in a second band on a similar timescale....

We report the discovery of a planetary system in which a super-Earth orbits a late M-dwarf host. The planetary system was found from the analysis of the microlensing event OGLE-2017-BLG-0482, wherein the planet signal appears as a short-term anomaly to the smooth lensing light curve produced by the host. Despite its weak signal and short duration, the planetary signal was firmly detected from the dense and continuous coverage by three microlensing surveys. We find a planet/host mass ratio of q ∼ 1.4 × 10^-4. We measure the microlens parallax {π }_{{ { E} } } from the long-term deviation in the observed lensing light curve, but the angular Einstein radius {θ }_{{ { E} } } cannot be measured because the source trajectory did not cross the planet-induced caustic. Using the measured event timescale and the microlens parallax, we find that the masses of the planet and the host are {M}_{{ { p} } }={9.0}_-4.5^+9.0 {M}_\oplus and {M}_host}={0.20}_-0.10^+0.20 {M}_⊙ , respectively, and the projected separation between them is {a}_\perp ={1.8}_-0.7^+0.6 au. The estimated distance to the lens is {D}_{{ { L} } }={5.8}_-2.1^+1.8 kpc. The discovery of the planetary system demonstrates that microlensing provides an important method to detect low-mass planets orbiting low-mass stars....

We report the discovery of OGLE-2016-BLG-1190Lb, which is likely to be the first Spitzer microlensing planet in the Galactic bulge/bar, an assignation that can be confirmed by two epochs of high-resolution imaging of the combined source-lens baseline object. The planet’s mass, M _p = 13.4 ± 0.9 M _J , places it right at the deuterium-burning limit, i.e., the conventional boundary between “planets” and “brown dwarfs.” Its existence raises the question of whether such objects are really “planets” (formed within the disks of their hosts) or “failed stars” (low-mass objects formed by gas fragmentation). This question may ultimately be addressed by comparing disk and bulge/bar planets, which is a goal of the Spitzer microlens program. The host is a G dwarf, M _host = 0.89 ± 0.07 M _⊙, and the planet has a semimajor axis a ∼ 2.0 au. We use Kepler K2 Campaign 9 microlensing data to break the lens-mass degeneracy that generically impacts parallax solutions from Earth-Spitzer observations alone, which is the first successful application of this approach. The microlensing data, derived primarily from near-continuous, ultradense survey observations from OGLE, MOA, and three KMTNet telescopes, contain more orbital information than for any previous microlensing planet, but not quite enough to accurately specify the full orbit. However, these data do permit the first rigorous test of microlensing orbital-motion measurements, which are typically derived from data taken over <1% of an orbital period....

We present the analysis of the binary-microlensing event OGLE-2014-BLG-0289. The event light curve exhibits five very unusual peaks, four of which were produced by caustic crossings and the other by a cusp approach. It is found that the quintuple-peak features of the light curve provide tight constraints on the source trajectory, enabling us to precisely and accurately measure the microlensing parallax {π }_{{ { E} } }. Furthermore, the three resolved caustics allow us to measure the angular Einstein radius {θ }_{{ { E} } }. From the combination of {π }_{{ { E} } } and {θ }_{{ { E} } }, the physical lens parameters are uniquely determined. It is found that the lens is a binary composed of two M dwarfs with masses {M}₁=0.52+/- 0.04 {M}_⊙ and {M}₂=0.42+/- 0.03 {M}_⊙ separated in projection by {a}_\perp =6.4+/- 0.5 {au}. The lens is located in the disk with a distance of {D}_{{ { L} } }=3.3+/- 0.3 {kpc}. The reason for the absence of a lensing signal in the Spitzer data is that the time of observation corresponds to the flat region of the light curve....

We present the result of microlensing event MOA-2016-BLG-290, which received observations from the two-wheel Kepler (K2), Spitzer, as well as ground-based observatories. A joint analysis of data from K2 and the ground leads to two degenerate solutions of the lens mass and distance. This degeneracy is effectively broken once the (partial) Spitzer light curve is included. Altogether, the lens is found to be an extremely low-mass star or brown dwarf ({77}_-23⁺³⁴ {M}_{{ { J} } }) located in the Galactic bulge (6.8+/- 0.4 kpc). MOA-2016-BLG-290 is the first microlensing event for which we have signals from three well-separated (∼1 au) locations. It demonstrates the power of two-satellite microlensing experiment in reducing the ambiguity of lens properties, as pointed out independently by S. Refsdal and A. Gould several decades ago....

We present the analysis of the binary gravitational microlensing event MOA-2015-BLG-020. The event has a fairly long timescale (∼63 days) and thus the light curve deviates significantly from the lensing model that is based on the rectilinear lens-source relative motion. This enables us to measure the microlensing parallax through the annual parallax effect. The microlensing parallax parameters constrained by the ground-based data are confirmed by the Spitzer observations through the satellite parallax method. By additionally measuring the angular Einstein radius from the analysis of the resolved caustic crossing, the physical parameters of the lens are determined. It is found that the binary lens is composed of two dwarf stars with masses {M}₁=0.606+/- 0.028 {M}_⊙ and {M}₂=0.125 +/- 0.006 {M}_⊙ in the Galactic disk. Assuming that the source star is at the same distance as the bulge red clump stars, we find the lens is at a distance {D}_L=2.44+/- 0.10 {kpc}. We also provide a summary and short discussion of all of the published microlensing events in which the annual parallax effect is confirmed by other independent observations....