Lake, New Zealand
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Rattenbury N.J.,University of Auckland | Bennett D.P.,University of Notre Dame | Sumi T.,Osaka University | Koshimoto N.,Osaka University | And 31 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

We report the discovery of a possible planet in microlensing event MOA-2010-BLG-353. This event was only recognized as having a planetary signal after the microlensing event had finished, and following a systematic analysis of all archival data for binary lens microlensing events collected to date. Data for event MOA-2010-BLG-353 were only recorded by the highcadence observations of the OGLE and MOA survey groups. If we make the assumptions that the probability of the lens star hosting a planet of the measured mass ratio is independent of the lens star mass or distance, and that the source star is in the Galactic bulge, a probability density analysis indicates the planetary system comprises a 0.9-0.53 +1.6 MSaturn mass planet orbiting a 0.18-0.11 +0.32 M⊙ red dwarf star, 6.43-1.15 +1.09 kpc away. The projected separation of the planet from the host star is 1.72-0.48 +0.56 au. Under the additional assumption that the source is on the far side of the Galactic bulge, the probability density analysis favours a lens system comprising a slightly lighter planet. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

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.

Sumi T.,Nagoya University | Bennett D.P.,University of Notre Dame | Bond I.A.,Massey University | Udalski A.,University of Warsaw | And 106 more authors.
Astrophysical Journal | Year: 2010

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] × 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 Ml = 0.64+0.21 -0.26 M· and Dl = 5.9 +0.9 -1.4 kpc, respectively, so the mass and separation of the planet are Mp = 20+7 -8 M ⊕ and a = 3.3+1.4 -0.8AU, 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/dlog qαq -0.70.2 with a 95% confidence level upper limit of n < -0.35 (where dN pl/dlog qαqn ). 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. © 2010. The American Astronomical Society. All rights reserved..

Miyake N.,Nagoya University | Udalski A.,University of Warsaw | Sumi T.,Osaka University | Bennett D.P.,University of Notre Dame | And 83 more authors.
Astrophysical Journal | Year: 2012

We report the extremely high-magnification (A > 1000) binary microlensing event OGLE-2007-BLG-514. We obtained good coverage around the double peak structure in the light curve via follow-up observations from different observatories. The binary lens model that includes the effects of parallax (known orbital motion of the Earth) and orbital motion of the lens yields a binary lens mass ratio of q = 0.321 ± 0.007 and a projected separation of s = 0.072 ± 0.001 in units of the Einstein radius. The parallax parameters allow us to determine the lens distance DL = 3.11 ± 0.39kpc and total mass ML = 1.40 ± 0.18 M ⊙; this leads to the primary and secondary components having masses of M 1 = 1.06 ± 0.13 M ⊙ and M 2 = 0.34 ± 0.04 M ⊙, respectively. The parallax model indicates that the binary lens system is likely constructed by the main-sequence stars. On the other hand, we used a Bayesian analysis to estimate probability distributions by the model that includes the effects of xallarap (possible orbital motion of the source around a companion) and parallax (q = 0.270 ± 0.005, s = 0.083 ± 0.001). The primary component of the binary lens is relatively massive, with M 1 = 0.9+4.6 -0.3 M ⊙ and it is at a distance of D L = 2.6+3.8 -0.9kpc. Given the secure mass ratio measurement, the companion mass is therefore M 2 = 0.2+1.2 -0.1 M ⊙. The xallarap model implies that the primary lens is likely a stellar remnant, such as a white dwarf, a neutron star, or a black hole. © 2012. The American Astronomical Society. All rights reserved.

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.

Bennett D.P.,University of Notre Dame | Sumi T.,Osaka University | Bond I.A.,Massey University | Kamiya K.,Nagoya University | And 21 more authors.
Astrophysical Journal | Year: 2012

We present the analysis of four candidate short-duration binary microlensing events from the 2006-2007 MOA Project short-event analysis. These events were discovered as a by-product of an analysis designed to find short-timescale single-lens events that may be due to free-floating planets. Three of these events are determined to be microlensing events, while the fourth is most likely caused by stellar variability. For each of the three microlensing events, the signal is almost entirely due to a brief caustic feature with little or no lensing attributable mainly to the lens primary. One of these events, MOA-bin-1, is due to a planet, and it is the first example of a planetary event in which the stellar host is only detected through binary microlensing effects. The mass ratio and separation are q = (4.9 ± 1.4) × 10-3 and s = 2.10 ± 0.05, respectively. A Bayesian analysis based on a standard Galactic model indicates that the planet, MOA-bin-1Lb, has a mass of mp = 3.7 ± 2.1 M Jup and orbits a star of M*=0.75-0.41+0.33M ⊙at a semimajor axis of AU. This is one of the most massive and widest separation planets found by microlensing. The scarcity of such wide-separation planets also has implications for interpretation of the isolated planetary mass objects found by this analysis. If we assume that we have been able to detect wide-separation planets with an efficiency at least as high as that for isolated planets, then we can set limits on the distribution of planets in wide orbits. In particular, if the entire isolated planet sample found by Sumi et al. consists of planets bound in wide orbits around stars, we find that it is likely that the median orbital semimajor axis is >30AU. © © 2012. The American Astronomical Society. All rights reserved.

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.

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.

Janczak J.,Ohio State University | Fukui A.,Nagoya University | Dong S.,Ohio State University | Monard L.A.G.,Bronberg Observatory | And 65 more authors.
Astrophysical Journal | Year: 2010

We report the detection of sub-Saturn-mass planet MOA-2008-BLG-310Lb and argue that it is the strongest candidate yet for a bulge planet. Deviations from the single-lens fit are smoothed out by finite-source effects and therefore are not immediately apparent from the light curve. Nevertheless, we find that a model in which the primary has a planetary companion is favored over the single-lens model by Δχ2 ∼ 880 for an additional 3 degrees of freedom. Detailed analysis yields a planet/star mass ratio q = (3.3 ± 0.3) × 10-4 and an angular separation between the planet and star within 10% of the angular Einstein radius. The small angular Einstein radius, θE = 0.155 ± 0.011 mas, constrains the distance to the lens to be DL >6.0kpc if it is a star (M L >0.08 M). This is the only microlensing exoplanet host discovered so far that must be in the bulge if it is a star. By analyzing VLT NACO adaptive optics images taken near the baseline of the event, we detect additional blended light that is aligned to within 130mas of the lensed source. This light is plausibly from the lens, but could also be due to a companion to the lens or source, or possibly an unassociated star. If the blended light is indeed due to the lens, we can estimate the mass of the lens, ML = 0.67 0.14 M ⊙, planet mass m = 74 ± 17 M ⊕, and projected separation between the planet and host, 1.25 ± 0.10AU, putting it right on the "snow line." If not, then the planet has lower mass, is closer to its host and is colder. To distinguish among these possibilities on reasonable timescales would require obtaining Hubble Space Telescope images almost immediately, before the source-lens relative motion of causes them to separate substantially. © 2010. The American Astronomical Society.

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