Mt John University Observatory

Lake, New Zealand

Mt John University Observatory

Lake, New Zealand
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Henderson C.B.,Ohio State University | Park H.,Chungbuk National University | Sumi T.,Osaka University | Udalski A.,University of Warsaw | And 84 more authors.
Astrophysical Journal | Year: 2014

The mass of the lenses giving rise to Galactic microlensing events can be constrained by measuring the relative lens-source proper motion and lens flux. The flux of the lens can be separated from that of the source, companions to the source, and unrelated nearby stars with high-resolution images taken when the lens and source are spatially resolved. For typical ground-based adaptive optics (AO) or space-based observations, this requires either inordinately long time baselines or high relative proper motions. We provide a list of microlensing events toward the Galactic bulge with high relative lens-source proper motion that are therefore good candidates for constraining the lens mass with future high-resolution imaging. We investigate all events from 2004 to 2013 that display detectable finite-source effects, a feature that allows us to measure the proper motion. In total, we present 20 events with μ ≳ 8 mas yr-1. Of these, 14 were culled from previous analyses while 6 are new, including OGLE-2004-BLG-368, MOA-2005-BLG-36, OGLE-2012-BLG-0211, OGLE-2012-BLG-0456, MOA-2012-BLG-532, and MOA-2013-BLG-029. In ≲12 yr from the time of each event the lens and source of each event will be sufficiently separated for ground-based telescopes with AO systems or space telescopes to resolve each component and further characterize the lens system. Furthermore, for the most recent events, comparison of the lens flux estimates from images taken immediately to those estimated from images taken when the lens and source are resolved can be used to empirically check the robustness of the single-epoch method currently being used to estimate lens masses for many events. © 2014. The American Astronomical Society. All rights reserved..

Fukui A.,Japan National Astronomical Observatory | Gould A.,Ohio State University | Sumi T.,Osaka University | Bennett D.P.,University of Notre Dame | And 65 more authors.
Astrophysical Journal | Year: 2015

We report the discovery of a microlensing exoplanet OGLE-2012-BLG-0563Lb with the planet-star mass ratio of ∼1 ×10-3. Intensive photometric observations of a high-magnification microlensing event allow us to detect a clear signal of the planet. Although no parallax signal is detected in the light curve, we instead succeed at detecting the flux from the host star in high-resolution JHK-band images obtained by the Subaru/AO188 and Infrared Camera and Spectrograph instruments, allowing us to constrain the absolute physical parameters of the planetary system. With the help of spectroscopic information about the source star obtained during the high-magnification state by Bensby et al., we find that the lens system is located at 1.3 0.6 0.8 kpc from us, and consists of an M dwarf (0.34 0.12 0.20 M) orbited by a Saturn-mass planet (0.39 0.14 0.23 MJup) at the projected separation of 0.74 0.26 0.42 AU (close model) or 4.3 1.5 2.5 AU (wide model). The probability of contamination in the host star's flux, which would reduce the masses by a factor of up to three, is estimated to be 17%. This possibility can be tested by future high-resolution imaging. We also estimate the (J - Ks ) and (H - Ks ) colors of the host star, which are marginally consistent with a low metallicity mid-to-early M dwarf, although further observations are required for the metallicity to be conclusive. This is the fifth sub-Jupiter-mass (0.2 ≤ mp MJup <1) microlensing planet around an M dwarf with the mass well constrained. The relatively rich harvest of sub-Jupiters around M dwarfs is contrasted with a possible paucity of 1-2 Jupiter-mass planets around the same type of star, which can be explained by the planetary formation process in the core-accretion scheme. © 2015. The American Astronomical Society. All rights reserved.

Novati S.C.,California Institute of Technology | Novati S.C.,University of Salerno | Novati S.C.,Instituto Internazionale per Gli Alti Studi Scientifici IIASS | Gould A.,Ohio State University | And 97 more authors.
Astrophysical Journal | Year: 2015

We present microlens parallax measurements for 21 (apparently) isolated lenses observed toward the Galactic bulge that were imaged simultaneously from Earth and Spitzer, which was ∼1 AU west of Earth in projection. We combine these measurements with a kinematic model of the Galaxy to derive distance estimates for each lens, with error bars that are small compared to the Sun's galactocentric distance. The ensemble therefore yields a well-defined cumulative distribution of lens distances. In principle, it is possible to compare this distribution against a set of planets detected in the same experiment in order to measure the Galactic distribution of planets. Since these Spitzer observations yielded only one planet, this is not yet possible in practice. However, it will become possible as larger samples are accumulated. © 2015. The American Astronomical Society. All rights reserved.

Han C.,Chungbuk National University | Jung Y.K.,Chungbuk National University | Udalski A.,University of Warsaw | Sumi T.,Osaka University | And 70 more authors.
Astrophysical Journal | Year: 2013

Observations of accretion disks around young brown dwarfs (BDs) have led to the speculation that they may form planetary systems similar to normal stars. While there have been several detections of planetary-mass objects around BDs (2MASS 1207-3932 and 2MASS 0441-2301), these companions have relatively large mass ratios and projected separations, suggesting that they formed in a manner analogous to stellar binaries. We present the discovery of a planetary-mass object orbiting a field BD via gravitational microlensing, OGLE-2012-BLG-0358Lb. The system is a low secondary/primary mass ratio (0.080 ± 0.001), relatively tightly separated (0.87 AU) binary composed of a planetary-mass object with 1.9 ± 0.2 Jupiter masses orbiting a BD with a mass 0.022 M. The relatively small mass ratio and separation suggest that the companion may have formed in a protoplanetary disk around the BD host in a manner analogous to planets. © 2013. The American Astronomical Society. All rights reserved..

Yee J.C.,Ohio State University | Shvartzvald Y.,Tel Aviv University | Gal-Yam A.,Weizmann Institute of Science | Bond I.A.,Massey University | And 76 more authors.
Astrophysical Journal | Year: 2012

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 (Δχ2 5400). The planet/host mass ratio is q = (5.3 ± 0.2) × 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 Δχ2 = 3. A Bayesian estimate of the host mass gives ML = 0.43+0.27 - 0.17 M, with a sharp upper limit of ML < 1.2 M from upper limits on the lens flux. Hence, the planet mass is mp = 2.4+1.5 - 0.9 M Jup, and the physical projected separation is either r ≃ 1.0AU or r ≃ 3.4AU. We show that survey data alone predict this solution and are able to characterize the planet, but the Δχ2 is much smaller (Δχ2 500) than with the follow-up data. The Δχ2 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. © © 2012. The American Astronomical Society. All rights reserved..

Shvartzvald Y.,Tel Aviv University | Maoz D.,Tel Aviv University | Kaspi S.,Tel Aviv University | Sumi T.,Osaka University | And 38 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

Global 'second-generation' microlensing surveys aim to discover and characterize extrasolar planets and their frequency, by means of round-the-clock high-cadence monitoring of a large area of the Galactic bulge, in a controlled experiment. We report the discovery of a giant planet in microlensing event MOA-2011-BLG-322. This moderate-magnification event, which displays a clear anomaly induced by a second lensing mass, was inside the footprint of our second-generation microlensing survey, involving MOA, OGLE and the Wise Observatory. The event was observed by the survey groups, without prompting alerts that could have led to dedicated follow-up observations. Fitting a microlensing model to the data, we find that the time-scale of the event was tE=23.2 ± 0.8 d, and the mass ratio between the lens star and its companion is q=0.028 ± 0.001. Finite-source effects are marginally detected, and upper limits on them help break some of the degeneracy in the system parameters. Using a Bayesian analysis that incorporates a Galactic structure model, we estimate the mass of the lens at 0.39+0.45-0.19M⊙ at a distance of 7.56 ± 0.91 kpc. Thus, the companion is likely a planet of mass 11.6+13.4-5.6 at a projected separation of 4.3+1.5-1.2 au, rather far beyond the snow line. This is the first pure-survey planet reported from a second-generation microlensing survey, and shows that survey data alone can be sufficient to characterize a planetary model. With the detection of additional survey-only planets, we will be able to constrain the frequency of extrasolar planets near their systems' snow lines. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Freeman M.,University of Auckland | Philpott L.C.,University of British Columbia | Abe F.,Nagoya University | Albrow M.D.,University of Canterbury | And 21 more authors.
Astrophysical Journal | Year: 2015

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 3σ level of confidence. The best fit to the data yields the mass and distance of the lens as 0.80 ± 0.30 MJ and 0.80 ± 0.25 kpc respectively. We exclude a host star to the lens out to a separation ∼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. © 2015. The American Astronomical Society. All rights reserved.

Jung Y.K.,Chungbuk National University | Udalski A.,University of Warsaw | Sumi T.,Osaka University | Han C.,Chungbuk National University | And 54 more authors.
Astrophysical Journal | Year: 2015

We present an analysis of the gravitational microlensing event OGLE-2013-BLG-0102. The light curve of the event is characterized by a strong short-term anomaly superposed on a smoothly varying lensing curve with a moderate magnification A max ∼ 1.5. It is found that the event was produced by a binary lens with a mass ratio between the components of q = 0.13 and the anomaly was caused by the passage of the source trajectory over a caustic located away from the barycenter of the binary. Based on the analysis of the effects on the light curve due to the finite size of the source and the parallactic motion of the Earth, we determine the physical parameters of the lens system. The measured masses of the lens components are M 1 = 0.096 ± 0.013 M ⊙ and M 2 = 0.012 ± 0.002 M ⊙, which correspond to near the hydrogen-burning and deuterium-burning mass limits, respectively. The distance to the lens is 3.04 ± 0.31 kpc and the projected separation between the lens components is 0.80 ± 0.08 AU. © 2015. The American Astronomical Society. All rights reserved..

Shvartzvald Y.,Tel Aviv University | Maoz D.,Tel Aviv University | Udalski A.,University of Warsaw | Sumi T.,Osaka University | And 34 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2016

We present a statistical analysis of the first four seasons from a 'second-generation' microlensing survey for extrasolar planets, consisting of near-continuous time coverage of 8 deg2 of the Galactic bulge by the Optical Gravitational Lens Experiment (OGLE), Microlensing Observations in Astrophysics (MOA), and Wise microlensing surveys. During this period, 224 microlensing events were observed by all three groups. Over 12 per cent of the events showed a deviation from single-lens microlensing, and for ~one-third of those the anomaly is likely caused by a planetary companion. For each of the 224 events, we have performed numerical ray-tracing simulations to calculate the detection efficiency of possible companions as a function of companion-to-host mass ratio and separation. Accounting for the detection efficiency, we find that 55+34 -22 per cent of microlensed stars host a snowline planet. Moreover, we find that Neptune-mass planets are ~10 times more common than Jupiter-mass planets. The companion-to-host mass-ratio distribution shows a deficit at q ~ 10-2, separating the distribution into two companion populations, analogous to the stellar-companion and planet populations, seen in radial-velocity surveys around solar-like stars.Our survey, however,which probes mainly lower mass stars, suggests a minimum in the distribution in the super-Jupiter mass range, and a relatively high occurrence of brown-dwarf companions. © 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Beaulieu J.-P.,CNRS Paris Institute of Astrophysics | Beaulieu J.-P.,University of Paris Descartes | Beaulieu J.-P.,University of Tasmania | Bennett D.P.,University of Notre Dame | And 20 more authors.
Astrophysical Journal | Year: 2016

Two cold gas giant planets orbiting a G-type main-sequence star in the galactic disk were previously discovered in the high-magnification microlensing event OGLE-2012-BLG-0026. Here, we present revised host star flux measurements and a refined model for the two-planet system using additional light curve data. We performed high angular resolution adaptive optics imaging with the Keck and Subaru telescopes at two epochs while the source star was still amplified. We detected the lens flux, H = 16.39 ± 0.08. The lens, a disk star, is brighter than predicted from the modeling in the original study. We revisited the light curve modeling using additional photometric data from the B&C telescope in New Zealand and CTIO 1.3 m H-band light curve. We then include the Keck and Subaru adaptive optic observation constraints. The system is composed of a ∼4-9 Gyr lens star of M lens = 1.06 ± 0.05 M o at a distance of D lens = 4.0 ± 0.3 kpc, orbited by two giant planets of 0.145 ± 0.008 M Jup and 0.86 ± 0.06 M Jup, with projected separations of 4.0 ± 0.5 au and 4.8 ± 0.7 au, respectively. Because the lens is brighter than the source star by 16 ± 8% in H, with no other blend within one arcsec, it will be possible to estimate its metallicity using subsequent IR spectroscopy with 8-10 m class telescopes. By adding a constraint on the metallicity it will be possible to refine the age of the system. © 2016. The American Astronomical Society. All rights reserved.

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