The Effect of a Binary Source Companion on the Astrometric Microlensing Behavior

If gravitational microlensing occurs in a binary-source system, both source components are magnified, and the resulting light curve deviates from the standard one of a single source event. However, in most cases only one source component is highly magnified and the other component (the companion) can be treated as a simple blending source: blending approximation. In this paper, we show that, unlike the light curves, the astrometric curves, representing the trajectories of the source image centroid, of an important fraction of binary-source events will not be sufficiently well modeled by the blending effect alone. This is because the centroid shift induced by the source companion endures to considerable distances from the lens. Therefore, in determining the lens parameters from astrometric curves to be measured by future high-precision astrometric instruments, it will be important to take the full effect of the source companion into consideration.Comment: total 6 pages, including 5 figures and no table, MNRAS, submitted, better quality pdf file is avalilable at http://astroph.chungbuk.ac.kr/~cheongho/publication.htm

Microlensing Search for Planets with Two Simultaneously Rising Suns

Among more than 200 extrasolar planet candidates discovered to date, there is no known planet orbiting around normal binary stars. In this paper, we demonstrate that microlensing is a technique that can detect such planets. Microlensing discoveries of these planets are possible because the planet and host binary stars produce perturbations at a common region around center of mass of the binary stars and thus the signatures of both planet and binary can be detected in the light curves of high-magnification microlensing events. The ranges of the planetary and binary separations of systems for optimal detection vary depending on the planet mass. For a Jupiter-mass planet, we find that high detection efficiency is expected for planets located in the range of $\sim$ 1 AU -- 5 AU from the binary stars which are separated by $\sim$ 0.15 AU -- 0.5 AUComment: 4 pages, 4 figure

On the Intrinsic Bias in detecting Caustic Crossings between Galactic Halo and Self-lensing Events in the Magellanic Clouds

In this paper, we investigate the intrinsic bias in detecting caustic crossings between Galactic halo and self-lensing events in the Magellanic Clouds. For this, we determine the region for optimal caustic-crossing detection in the parameter space of the physical binary separations, $\ell$, and the total binary lens mass, $M$, and find that the optimal regions for both populations of events are similar to each other. In particular, if the Galactic halo is composed of lenses with the claimed average mass of $\sim 0.5 M_\odot$, the optimal binary separation range of Galactic halo events of $3.5 AU\lesssim \ell\lesssim 14 AU$ matches well with that of a Magellanic Cloud self-lensing event caused by a binary lens with a total mass $M\sim 1 M_\odot$; well within the mass range of the most probable lens population of stars in the Magellanic Clouds. Therefore, our computation implies that if the binary fractions and the distributions of binary separations of the two populations of lenses are not significantly different from each other, there is no strong detection bias against Galactic halo caustic-crossing events.Comment: total 4 pages, including 3 Figures and no Table, accepted for publication in MNRA