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CAN THE MASSES OF ISOLATED PLANETARY-MASS GRAVITATIONAL LENSES BE MEASURED BY TERRESTRIAL PARALLAX?
Freeman, M.,Philpott, L. C.,Abe, F.,Albrow, M. D.,Bennett, D. P.,Bond, I. A.,Botzler, C. S.,Bray, J. C.,Cherrie, J. M.,Christie, G. W.,Dionnet, Z.,Gould, A.,Han, C.,Heyrovský,, D.,McCormick, J. IOP Publishing 2015 The Astrophysical journal Vol.799 No.2
<P>Recently Sumi et al. reported evidence for a large population of planetary-mass objects ( PMOs) that are either unbound or orbit host stars in orbits >= 10 AU. Their result was deduced from the statistical distribution of durations of gravitational microlensing events observed by the MOA collaboration during 2006 and 2007. Here we study the feasibility of measuring the mass of an individual PMO through microlensing by examining a particular event, MOA-2011-BLG-274. This event was unusual as the duration was short, the magnification high, the source-size effect large, and the angular Einstein radius small. Also, it was intensively monitored from widely separated locations under clear skies at low air masses. Choi et al. concluded that the lens of the event may have been a PMO but they did not attempt a measurement of its mass. We report here a re-analysis of the event using re-reduced data. We confirm the results of Choi et al. and attempt a measurement of the mass and distance of the lens using the terrestrial parallax effect. Evidence for terrestrial parallax is found at a 3 sigma level of confidence. The best fit to the data yields the mass and distance of the lens as 0.80 +/- 0.30 M-J and 0.80 +/- 0.25 kpc respectively. We exclude a host star to the lens out to a separation similar to 40 AU. Drawing on our analysis of MOA-2011-BLG-274 we propose observational strategies for future microlensing surveys to yield sharper results on PMOs including those down to super-Earth mass.</P>
MOA-2011-BLG-322Lb: a 'second generation survey' microlensing planet
Shvartzvald, Y.,Maoz, D.,Kaspi, S.,Sumi, T.,Udalski, A.,Gould, A.,Bennett, D. P.,Han, C.,Abe, F.,Bond, I. A.,Botzler, C. S.,Freeman, M.,Fukui, A.,Fukunaga, D.,Itow, Y.,Koshimoto, N.,Ling, C. H.,Masuda Oxford University Press 2014 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.439 No.1
MICROLENSING DISCOVERY OF A POPULATION OF VERY TIGHT, VERY LOW MASS BINARY BROWN DWARFS
Choi, J.-Y.,Han, C.,Udalski, A.,Sumi, T.,Gaudi, B. S.,Gould, A.,Bennett, D. P.,Dominik, M.,Beaulieu, J.-P.,Tsapras, Y.,Bozza, V.,Abe, F.,Bond, I. A.,Botzler, C. S.,Chote, P.,Freeman, M.,Fukui, A.,Furu IOP Publishing 2013 The Astrophysical journal Vol.768 No.2
<P>Although many models have been proposed, the physical mechanisms responsible for the formation of low-mass brown dwarfs (BDs) are poorly understood. The multiplicity properties and minimum mass of the BD mass function provide critical empirical diagnostics of these mechanisms. We present the discovery via gravitational microlensing of two very low mass, very tight binary systems. These binaries have directly and precisely measured total system masses of 0.025 M-circle dot and 0.034 M-circle dot, and projected separations of 0.31 AU and 0.19 AU, making them the lowest-mass and tightest field BD binaries known. The discovery of a population of such binaries indicates that BD binaries can robustly form at least down to masses of similar to 0.02 M-circle dot. Future microlensing surveys will measure a mass-selected sample of BD binary systems, which can then be directly compared to similar samples of stellar binaries.</P>
MOA-2010-BLG-328Lb: A SUB-NEPTUNE ORBITING VERY LATE M DWARF?
Furusawa, K.,Udalski, A.,Sumi, T.,Bennett, D. P.,Bond, I. A.,Gould, A.,Jørgensen, U. G.,Snodgrass, C.,Prester, D. Dominis,Albrow, M. D.,Abe, F.,Botzler, C. S.,Chote, P.,Freeman, M.,Fukui, A.,Harris, P IOP Publishing 2013 The Astrophysical journal Vol.779 No.2
<P>We analyze the planetary microlensing event MOA-2010-BLG-328. The best fit yields host and planetary masses of M-h = 0.11 +/- 0.01 M-circle dot and M-p = 9.2 +/- 2.2 M-circle dot, corresponding to a very late M dwarf and sub-Neptune-mass planet, respectively. The system lies at D-L = 0.81 +/- 0.10 kpc with projected separation r(perpendicular to) = 0.92 +/- 0.16 AU. Because of the host's a priori unlikely close distance, as well as the unusual nature of the system, we consider the possibility that the microlens parallax signal, which determines the host mass and distance, is actually due to xallarap (source orbital motion) that is being misinterpreted as parallax. We show a result that favors the parallax solution, even given its close host distance. We show that future high-resolution astrometric measurements could decisively resolve the remaining ambiguity of these solutions.</P>
INTERPRETATION OF A SHORT-TERM ANOMALY IN THE GRAVITATIONAL MICROLENSING EVENT MOA-2012-BLG-486
Hwang, K.-H.,Choi, J.-Y.,Bond, I. A.,Sumi, T.,Han, C.,Gaudi, B. S.,Gould, A.,Bozza, V.,Beaulieu, J.-P.,Tsapras, Y.,Abe, F.,Bennett, D. P.,Botzler, C. S.,Chote, P.,Freeman, M.,Fukui, A.,Fukunaga, D.,Ha IOP Publishing 2013 The Astrophysical journal Vol.778 No.1
<P>A planetary microlensing signal is generally characterized by a short-term perturbation to the standard single lensing light curve. A subset of binary-source events can produce perturbations that mimic planetary signals, thereby introducing an ambiguity between the planetary and binary-source interpretations. In this paper, we present the analysis of the microlensing event MOA-2012-BLG-486, for which the light curve exhibits a short-lived perturbation. Routine modeling not considering data taken in different passbands yields a best-fit planetary model that is slightly preferred over the best-fit binary-source model. However, when allowed for a change in the color during the perturbation, we find that the binary-source model yields a significantly better fit and thus the degeneracy is clearly resolved. This event not only signifies the importance of considering various interpretations of short-term anomalies, but also demonstrates the importance of multi-band data for checking the possibility of false-positive planetary signals.</P>
MOA-2011-BLG-293Lb: A TEST OF PURE SURVEY MICROLENSING PLANET DETECTIONS
Yee, J. C.,Shvartzvald, Y.,Gal-Yam, A.,Bond, I. A.,Udalski, A.,Kozłowski, S.,Han, C.,Gould, A.,Skowron, J.,Suzuki, D.,Abe, F.,Bennett, D. P.,Botzler, C. S.,Chote, P.,Freeman, M.,Fukui, A.,Furusawa, K. IOP Publishing 2012 The Astrophysical journal Vol.755 No.2
<P>Because of the development of large-format, wide-field cameras, microlensing surveys are now able to monitor millions of stars with sufficient cadence to detect planets. These new discoveries will span the full range of significance levels including planetary signals too small to be distinguished from the noise. At present, we do not understand where the threshold is for detecting planets. MOA-2011-BLG-293Lb is the first planet to be published from the new surveys, and it also has substantial follow-up observations. This planet is robustly detected in survey+follow-up data (Delta chi(2) similar to 5400). The planet/host mass ratio is q = (5.3 similar to 0.2) x 10(-3). The best-fit projected separation is s = 0.548 +/- 0.005 Einstein radii. However, due to the s <-> s(-1) degeneracy, projected separations of s-1 are only marginally disfavored at Delta chi(2) = 3. A Bayesian estimate of the host mass gives M-L = 0.43(-0.17)(+0.27) M-circle dot, with a sharp upper limit of M-L < 1.2 M-circle dot from upper limits on the lens flux. Hence, the planet mass is m(p) = 2.4(-0.9)(+1.5) M-Jup, and the physical projected separation is either r(perpendicular to) similar or equal to 1.0 AU or r(perpendicular to) similar or equal to 3.4 AU. We show that survey data alone predict this solution and are able to characterize the planet, but the Delta chi(2) is much smaller (Delta chi(2) similar to 500) than with the follow-up data. The Delta chi(2) for the survey data alone is smaller than for any other securely detected planet. This event suggests a means to probe the detection threshold, by analyzing a large sample of events like MOA-2011-BLG-293, which have both follow-up data and high-cadence survey data, to provide a guide for the interpretation of pure survey microlensing data.</P>
USING ORBITAL EFFECTS TO BREAK THE CLOSE/WIDE DEGENERACY IN BINARY-LENS MICROLENSING EVENTS
Shin, I.-G.,Sumi, T.,Udalski, A.,Choi, J. Y.,Han, C.,Gould, A.,Abe, F.,Bennett, D. P.,Bond, I. A.,Botzler, C. S.,Chote, P.,Freeman, M.,Fukui, A.,Furusawa, K.,Harris, P.,Itow, Y.,Ling, C. H.,Masuda, K. IOP Publishing 2013 The Astrophysical journal Vol.764 No.1
<P>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 similar to 3.5 kpc from the Earth.</P>
A COLD NEPTUNE-MASS PLANET OGLE-2007-BLG-368Lb: Cold neptunes are common
Sumi, T.,Bennett, D. P.,Bond, I. A.,Udalski, A.,Batista, V.,Dominik, M.,Fouqué,, P.,Kubas, D.,Gould, A.,Macintosh, B.,Cook, K.,Dong, S.,Skuljan, L.,Cassan, A.,Abe, F.,Botzler, C. S.,Fukui, A.,Fu IOP Publishing 2010 The Astrophysical journal Vol.710 No.2
<P>We present the discovery of a Neptune-mass planet OGLE-2007-BLG-368Lb with a planet-star mass ratio of q = [9.5 +/- 2.1] x 10(-5) via gravitational microlensing. The planetary deviation was detected in real-time thanks to the high cadence of the Microlensing Observations in Astrophysics survey, real-time light-curve monitoring and intensive follow-up observations. A Bayesian analysis returns the stellar mass and distance at M(l) = 0.64(-0.26)(+0.21) M(circle dot) and D(l) = 5.9(-1.4)(+ 0.9) kpc, respectively, so the mass and separation of the planet are M(p) = 20(-8)(+7) M(circle plus) and a = 3.3(-0.8)(+1.4) AU, respectively. This discovery adds another cold Neptune-mass planet to the planetary sample discovered by microlensing, which now comprises four cold Neptune/super-Earths, five gas giant planets, and another sub-Saturn mass planet whose nature is unclear. The discovery of these 10 cold exoplanets by the microlensing method implies that the mass ratio function of cold exoplanets scales as dN(pl)/d log q alpha q(-0.7+/-0.2) with a 95% confidence level upper limit of n < -0.35 ( where dN(pl)/d log q alpha q(n)). As microlensing is most sensitive to planets beyond the snow-line, this implies that Neptune-mass planets are at least three times more common than Jupiters in this region at the 95% confidence level.</P>