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Bensby T.,Lund Observatory | Yee J.C.,Ohio State University | Feltzing S.,Lund Observatory | Johnson J.A.,Ohio State University | And 18 more authors.
Astronomy and Astrophysics | Year: 2013

Based on high-resolution spectra obtained during gravitational microlensing events we present a detailed elemental abundance analysis of 32 dwarf and subgiant stars in the Galactic bulge. Combined with the sample of 26 stars from the previous papers in this series, we now have 58 microlensed bulge dwarfs and subgiants that have been homogeneously analysed. The main characteristics of the sample and the findings that can be drawn are: (i) the metallicity distribution (MDF) is wide and spans all metallicities between [Fe/H] = -1.9 to +0.6; (ii) the dip in the MDF around solar metallicity that was apparent in our previous analysis of a smaller sample (26 microlensed stars) is no longer evident; instead it has a complex structure and indications of multiple components are starting to emerge. A tentative interpretation is that there could be different stellar populations at interplay, each with a different scale height: the thin disk, the thick disk, and a bar population; (iii) the stars with [Fe/H] ≤ -0.1 are old with ages between 10 and 12 Gyr; (iv) the metal-rich stars with [Fe/H] ≥ -0.1 show a wide variety of ages, ranging from 2 to 12 Gyr with a distribution that has a dominant peak around 4-5 Gyr and a tail towards higher ages; (v) there are indications in the [α/Fe]-[Fe/H] abundance trends that the "knee" occurs around [Fe/H] = -0.3to -0.2, which is a slightly higher metallicity as compared to the "knee" for the local thick disk. This suggests that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk. The results from the microlensed bulge dwarf stars in combination with other findings in the literature, in particular the evidence that the bulge has cylindrical rotation, indicate that the Milky Way could be an almost pure disk galaxy. The bulge would then just be a conglomerate of the other Galactic stellar populations (thin disk, thick disk, halo, and.?), residing together in the central parts of the Galaxy, influenced by the Galactic bar. © 2013 ESO.

Hwang K.-H.,Chungbuk National University | Udalski A.,University of Warsaw | Han C.,Chungbuk National University | Han C.,Korea Astronomy and Space Science Institute | And 82 more authors.
Astrophysical Journal | Year: 2010

The mass function and statistics of binaries provide important diagnostics of the star formation process. Despite this importance, the mass function at low masses remains poorly known due to observational difficulties caused by the faintness of the objects. Here we report the microlensing discovery and characterization of a binary lens composed of very low mass stars just above the hydrogen-burning limit. From the combined measurements of the Einstein radius and microlens parallax, we measure the masses of the binary components of 0.10±0.01 M⊙ and 0.09±0.01 M⊙. This discovery demonstrates that microlensing will provide a method to measure the mass function of all Galactic populations of very low mass binaries that is independent of the biases caused by the luminosity of the population. © 2010. The American Astronomical Society.

Hwang K.-H.,Chungbuk National University | Han C.,Chungbuk National University | Udalski A.,University of Warsaw | Sumi T.,Nagoya University | And 39 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We report the result of the analysis of the light curve of a caustic-crossing binary-lens microlensing event OGLE-2009-BLG-023/MOA-2009-BLG-028. Even though the event was observed solely by survey experiments, we could uniquely determine the mass of the lens and distance to it by simultaneously measuring the Einstein radius and lens parallax. From this, we find that the lens system is composed of M-type dwarfs with masses (0.50 ± 0.07) and (0.15 ± 0.02)M⊙ located in the Galactic disc with a distance of ∼1.8kpc toward the Galactic bulge direction. The event demonstrates that physical lens parameters of binary-lens events can be routinely determined from future high-cadence lensing surveys and thus microlensing can provide a new way to study Galactic binaries. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Fukui A.,Nagoya University | Narita N.,Japan National Astronomical Observatory | Tristram P.J.,Mt. John Observatory | Sumi T.,Nagoya University | And 34 more authors.
Publications of the Astronomical Society of Japan | Year: 2011

We observed 7 new transits of the "hot Jupiter" WASP-5b using a 61 cm telescope located in New Zealand, in order to search for transit timing variations (TTVs), which can be induced by additional bodies existing in the system. Combining them with other available photometric and radial velocity (RV) data, we find that its transit timings do not match a linear ephemeris; the best-fit x2 value is 32.2 with 9 degrees of freedom, which corresponds to a confidence level of 99.982% or 3.7 σ. This result indicates that excess variations of transit timings have been observed, either due to unknown systematic effects, or possibly due to real TTVs. The TTV amplitude is as large as 50 s, and if this is real it cannot be explained by some effect other than an additional body, or bodies. From RV data, we put an upper limit on the RV amplitude caused by a possible secondary body (planet) as 21ms-1, which corresponds to its mass of 22-70M over the orbital period ratio of the two planets from 0.2 to 5.0. From the TTV data, using numerical simulations, we narrowed the limits down to 2M near 1:2 and 2:1 mean-motion resonances (MMRs) with WASP-5b at the 3+ level, assuming that the two planets are co-planer. We also put an upper limit of 43M (3 σ) on excess of Trojan mass using both RV and photometric data. We also find that if the orbit of the possible secondary planet is a circle or an ellipse of small eccentricity, it would be likely an orbit near that of low-order MMRs. © 2011. Astronomical Society of Japan.

Hwang K.-H.,Chungbuk National University | Han C.,Chungbuk National University | Bond I.A.,Massey University | Miyake N.,Nagoya University | And 37 more authors.
Astrophysical Journal | Year: 2010

We report the result of the analysis of the light curve of the microlensing event MOA-2009-BLG-016. The light curve is characterized by a short-duration anomaly near the peak and an overall asymmetry. We find that the peak anomaly is due to a binary companion to the primary lens and the asymmetry of the light curve is explained by the parallax effect caused by the acceleration of the observer over the course of the event due to the orbital motion of the Earth around the Sun. In addition, we detect evidence for the effect of the finite size of the source near the peak of the event, which allows us to measure the angular Einstein radius of the lens system. The Einstein radius combined with the microlens parallax allows us to determine the total mass of the lens and the distance to the lens. We identify three distinct classes of degenerate solutions for the binary lens parameters, where two are manifestations of the previously identified degeneracies of close/wide binaries and positive/negative impact parameters, while the third class is caused by the symmetric cycloid shape of the caustic. We find that, for the best-fit solution, the estimated mass of the lower-mass component of the binary is (0.04±0.01) M ⊙, implying a brown-dwarf companion. However, there exists a solution that is worse only by Δχ2 ∼ 3 for which the mass of the secondary is above the hydrogen-burning limit. Unfortunately, resolving these two degenerate solutions will be difficult as the relative lens-source proper motions for both are similar and small (∼1 mas yr -1) and thus the lens will remain blended with the source for the next several decades. © 2010. The American Astronomical Society.

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