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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>
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>
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>
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>