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Shin I.-G.,Chungbuk National University | Sumi T.,Osaka University | Udalski A.,University of Warsaw | Choi J.Y.,Chungbuk National University | And 38 more authors.
Astrophysical Journal | Year: 2013

Microlensing can provide an important tool to study binaries, especially those composed of faint or dark objects. However, accurate analysis of binary-lens light curves is often hampered by the well-known degeneracy between close (s < 1) and wide (s > 1) binaries, which can be very severe due to an intrinsic symmetry in the lens equation. Here, s is the normalized projected binary separation. In this paper, we propose a method that can resolve the close/wide degeneracy using the effect of a lens orbital motion on lensing light curves. The method is based on the fact that the orbital effect tends to be important for close binaries while it is negligible for wide binaries. We demonstrate the usefulness of the method by applying it to an actually observed binary-lens event MOA-2011-BLG-040/OGLE-2011-BLG-0001, which suffers from severe close/wide degeneracy. From this, we are able to uniquely specify that the lens is composed of K- and M-type dwarfs located ≳3.5 kpc from the Earth. © 2013. The American Astronomical Society. All rights reserved..

Koshimoto N.,Osaka University | Udalski A.,University of Warsaw | Sumi T.,Osaka University | Bennett D.P.,University of Notre Dame | And 34 more authors.
Astrophysical Journal | Year: 2014

We report the discovery of a massive planet, OGLE-2008-BLG-355Lb. The light curve analysis indicates a planet:host mass ratio of q = 0.0118 ± 0.0006 at a separation of 0.877 ± 0.010 Einstein radii. We do not measure a significant microlensing parallax signal and do not have high angular resolution images that could detect the planetary host star. Therefore, we do not have a direct measurement of the host star mass. A Bayesian analysis, assuming that all host stars have equal probability to host a planet with the measured mass ratio, implies a host star mass of and a companion of mass , at a projected separation of AU. The implied distance to the planetary system is D L = 6.8 ± 1.1 kpc. A planetary system with the properties preferred by the Bayesian analysis may be a challenge to the core accretion model of planet formation, as the core accretion model predicts that massive planets are far more likely to form around more massive host stars. This core accretion model prediction is not consistent with our Bayesian prior of an equal probability of host stars of all masses to host a planet with the measured mass ratio. Thus, if the core accretion model prediction is right, we should expect that follow-up high angular resolution observations will detect a host star with a mass in the upper part of the range allowed by the Bayesian analysis. That is, the host would probably be a K or G dwarf. © 2014. The American Astronomical Society. All rights reserved..

If a star moves in front of an another star, the light from the distant star is bent by the gravitational pull of the nearer star and the more distant star is magnified. Microlensing does not rely on the light from the host stars; thus, it can detect planets, even when the host stars cannot be detected. This technique is very useful for detecting alien worlds in the inner galactic disk and bulge, where it is difficult to search for planets with other methods. An international team of researchers, led by Aparna Bhattacharyaha of the University of Notre Dame used the gravitational microlensing method to detect a gas giant planet orbiting the lens stars of a microlensing event. This gravity lens, discovered in August 2014, was designated OGLE-2014-BLG-1760 and is the 1,760th microlensing event detected by the Optical Gravitational Lensing Experiment (OGLE) collaboration. OGLE is a Polish astronomical project based at the University of Warsaw, searching for dark matter and extrasolar planets. It utilizes the 1.3 meter Warsaw telescope mounted at the Las Campanas observatory in Chile. Follow-up observations were carried out by the Microlensing Observation in Astrophysics (MOA) collaboration, the Microlensing Follow-Up Network (μFUN) and the RoboNet project. MOA uses the 1.8 meter MOA-II telescope at the Mount John Observatory at Lake Tekapo, New Zealand, while μFUN and RoboNet are global groups employing a network of telescopes worldwide. The scientists have detected a strong light curve signal coming from OGLE-2014-BLG-1760. They assume that it must be caused by the presence of a gas giant planet. "One unusual feature of this event is that the source star is quite blue (…). This is marginally consistent with source star in the galactic bulge, but it could possibly indicate a young source star in the far side of the disk. Assuming a bulge source, we perform a Bayesian analysis assuming a standard galactic model, and this indicates that the planetary system resides in or near the galactic bulge," the paper reads. According to the research, the planet has a mass of about 180 Earth masses and is orbiting its parent star at a distance of approximately 1.75 AU. The host star is less massive than the sun, with a mass of some 0.51 solar masses. The lens system distance equals 22,000 light years, which suggests that the system is very likely to be in the Milky Way's bulge. The team notes that currently the lens is too faint to be detected in high resolution images unless the lens and source are partially resolved. It is expected to occur in 2020-2022, when the source will be resolvable by the James Webb Space Telescope (JWST), the Hubble Space Telescope (HST) or adaptive optics imaging. "Follow-up observations with JWST, HST or adaptive optics in 2020-2022 should be able to measure the lens brightness and determine the planetary mass and distance," the researchers concluded. Explore further: Planets that have no stars: New class of planets discovered More information: Discovery of a Gas giant Planet in Microlensing Event OGLE-2014-BLG-1760, arXiv:1603.05677 [astro-ph.EP], arxiv.org/abs/1603.05677 Abstract We present the analysis of the planetary microlensing event OGLE-2014-BLG-1760, which shows a strong light curve signal due to the presence of a Jupiter mass-ratio planet. One unusual feature of this event is that the source star is quite blue, with V−I=1.48±0.08. This is marginally consistent with source star in the Galactic bulge, but it could possibly indicate a young source star in the far side of the disk. Assuming a bulge source, we perform a Bayesian analysis assuming a standard Galactic model, and this indicates that the planetary system resides in or near the Galactic bulge at DL=6.9±1.1 kpc. It also indicates a host star mass of M∗=0.51±0.44M⊙, a planet mass of mp=180±110M⊕, and a projected star-planet separation of a⊥=1.7±0.3AU. The lens-source relative proper motion is μrel=6.5±1.1 mas/yr. The lens (and stellar host star) is predicted to be very faint, so it is most likely that it can detected only when the lens and source stars are partially resolved. Due to the relatively high relative proper motion, the lens and source will be resolved to about ∼46mas in 6-8 years after the peak magnification. So, by 2020 - 2022, we can hope to detect the lens star with deep, high resolution images.

Ishiguro M.,Seoul National University | Ishiguro M.,Institute Of Mecanique Celeste Et Of Calcul Des Ephemerides | Kuroda D.,Japan National Astronomical Observatory | Hasegawa S.,Japan Aerospace Exploration Agency | And 32 more authors.
Astrophysical Journal | Year: 2014

We investigated the magnitude-phase relation of (162173) 1999 JU3, a target asteroid for the JAXA Hayabusa 2 sample return mission. We initially employed the International Astronomical Union's H-G formalism but found that it fits less well using a single set of parameters. To improve the inadequate fit, we employed two photometric functions: the Shevchenko and Hapke functions. With the Shevchenko function, we found that the magnitude-phase relation exhibits linear behavior in a wide phase angle range (α = 5°-75°) and shows weak nonlinear opposition brightening at α < 5°, providing a more reliable absolute magnitude of H V = 19.25 ± 0.03. The phase slope (0.039 ± 0.001 mag deg-1) and opposition effect amplitude (parameterized by the ratio of intensity at α = 0.°3 to that at α = 5°, I(0.°3)/I(5°) = 1.31 ± 0.05) are consistent with those of typical C-type asteroids. We also attempted to determine the parameters for the Hapke model, which are applicable for constructing the surface reflectance map with the Hayabusa 2 onboard cameras. Although we could not constrain the full set of Hapke parameters, we obtained possible values, w = 0.041, g = -0.38, B 0 = 1.43, and h = 0.050, assuming a surface roughness parameter = 20°. By combining our photometric study with a thermal model of the asteroid, we obtained a geometric albedo of p v = 0.047 ± 0.003, phase integral q = 0.32 ± 0.03, and Bond albedo A B = 0.014 ± 0.002, which are commensurate with the values for common C-type asteroids. © 2014. The American Astronomical Society. All rights reserved..

Bennett D.P.,University of Notre Dame | Rhie S.H.,University of Notre Dame | Nikolaev S.,Lawrence Livermore National Laboratory | Gaudi B.S.,Ohio State University | And 73 more authors.
Astrophysical Journal | Year: 2010

We present a new analysis of the Jupiter+Saturn analog system, OGLE-2006-BLG-109Lb,c, which was the first double planet system discovered with the gravitational microlensing method. This is the only multi-planet system discovered by any method with measured masses for the star and both planets. In addition to the signatures of two planets, this event also exhibits a microlensing parallax signature and finite source effects that provide a direct measure of the masses of the star and planets, and the expected brightness of the host star is confirmed by Keck AO imaging, yielding masses of M * = 0.51+0.05?0.04 M⊙ Mb = 231 ± 19M, and Mc = 86 ± 7M. The Saturn-analog planet in this system had a planetary light-curve deviation that lasted for 11 days, and as a result, the effects of the orbital motion are visible in the microlensing light curve. We find that four of the six orbital parameters are tightly constrained and that a fifth parameter, the orbital acceleration, is weakly constrained. No orbital information is available for the Jupiter-analog planet, but its presence helps to constrain the orbital motion of the Saturn-analog planet. Assuming co-planar orbits, we find an orbital eccentricity of ε = 0.15+0.17?0.10 and an orbital inclination of i = 64° +4°?7° . The 95% confidence level lower limit on the inclination of i> 49° implies that this planetary system can be detected and studied via radial velocity measurements using a telescope of ≳30m aperture. © 2010. The American Astronomical Society.

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