Gisbourne, New Zealand
Gisbourne, New Zealand

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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.

Tsapras Y.,Las Cumbres Observatory Global Telescope Network | Tsapras Y.,Queen Mary, University of London | Choi J.-Y.,Chungbuk National University | Street R.A.,Las Cumbres Observatory Global Telescope Network | And 139 more authors.
Astrophysical Journal | Year: 2014

We present a detailed analysis of survey and follow-up observations of microlensing event OGLE-2012-BLG-0406 based on data obtained from 10 different observatories. Intensive coverage of the light curve, especially the perturbation part, allowed us to accurately measure the parallax effect and lens orbital motion. Combining our measurement of the lens parallax with the angular Einstein radius determined from finite-source effects, we estimate the physical parameters of the lens system. We find that the event was caused by a 2.73 ± 0.43 M J planet orbiting a 0.44 ± 0.07 M early M-type star. The distance to the lens is 4.97 ± 0.29 kpc and the projected separation between the host star and its planet at the time of the event is 3.45 ± 0.26 AU. We find that the additional coverage provided by follow-up observations, especially during the planetary perturbation, leads to a more accurate determination of the physical parameters of the lens. © 2014. The American Astronomical Society. All rights reserved..

Bachelet E.,French National Center for Scientific Research | Shin I.-G.,Chungbuk National University | Han C.,Chungbuk National University | Fouque P.,French National Center for Scientific Research | And 154 more authors.
Astrophysical Journal | Year: 2012

Microlensing detections of cool planets are important for the construction of an unbiased sample to estimate the frequency of planets beyond the snow line, which is where giant planets are thought to form according to the core accretion theory of planet formation. In this paper, we report the discovery of a giant planet detected from the analysis of the light curve of a high-magnification microlensing event MOA 2010-BLG-477. The measured planet-star mass ratio is q = (2.181 ± 0.004) × 10-3 and the projected separation is s = 1.1228 ± 0.0006 in units of the Einstein radius. The angular Einstein radius is unusually large θE = 1.38 ± 0.11 mas. Combining this measurement with constraints on the "microlens parallax" and the lens flux, we can only limit the host mass to the range 0.13 < M/M < 1.0. In this particular case, the strong degeneracy between microlensing parallax and planet orbital motion prevents us from measuring more accurate host and planet masses. However, we find that adding Bayesian priors from two effects (Galactic model and Keplerian orbit) each independently favors the upper end of this mass range, yielding star and planet masses of M * = 0.67+0.33 - 0.13 M and mp1.5+0.8 - 0.3 M JUP at a distance of D = 2.3 ± 0.6kpc, and with a semi-major axis of a = 2 +3 - 1AU. Finally, we show that the lens mass can be determined from future high-resolution near-IR adaptive optics observations independently from two effects, photometric and astrometric. © 2012. The American Astronomical Society. All rights reserved..

Furusawa K.,Nagoya University | Udalski A.,University of Warsaw | Sumi T.,Osaka University | Bennett D.P.,University of Notre Dame | And 136 more authors.
Astrophysical Journal | Year: 2013

We analyze the planetary microlensing event MOA-2010-BLG-328. The best fit yields host and planetary masses of Mh = 0.11 ± 0.01 M Ȯ and Mp = 9.2 ± 2.2 M ⊕, 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 ⊥ = 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. © 2013. The American Astronomical Society. All rights reserved.

Street R.A.,LCOGT | Choi J.-Y.,Chungbuk National University | Tsapras Y.,LCOGT | Tsapras Y.,Queen Mary, University of London | And 136 more authors.
Astrophysical Journal | Year: 2013

We present an analysis of the anomalous microlensing event, MOA-2010-BLG-073, announced by the Microlensing Observations in Astrophysics survey on 2010 March 18. This event was remarkable because the source was previously known to be photometrically variable. Analyzing the pre-event source light curve, we demonstrate that it is an irregular variable over timescales >200 days. Its dereddened color, (V-I)S, 0, is 1.221 ± 0.051 mag, and from our lens model we derive a source radius of 14.7 ± 1.3 R⊙, suggesting that it is a red giant star. We initially explored a number of purely microlensing models for the event but found a residual gradient in the data taken prior to and after the event. This is likely to be due to the variability of the source rather than part of the lensing event, so we incorporated a slope parameter in our model in order to derive the true parameters of the lensing system. We find that the lensing system has a mass ratio of q = 0.0654 ± 0.0006. The Einstein crossing time of the event, tE = 44.3 ± 0.1 days, was sufficiently long that the light curve exhibited parallax effects. In addition, the source trajectory relative to the large caustic structure allowed the orbital motion of the lens system to be detected. Combining the parallax with the Einstein radius, we were able to derive the distance to the lens, DL = 2.8 ± 0.4 kpc, and the masses of the lensing objects. The primary of the lens is an M-dwarf with ML,1 = 0.16 ± 0.03 M⊙, while the companion has ML,2 = 11.0 ± 2.0 MJ, putting it in the boundary zone between planets and brown dwarfs. © 2013 The American Astronomical Society. All rights reserved.

Han C.,Chungbuk National University | Udalski A.,University of Warsaw | Choi J.-Y.,Chungbuk National University | Yee J.C.,Ohio State University | And 34 more authors.
Astrophysical Journal Letters | Year: 2013

We report the discovery of a planetary system from observation of the high-magnification microlensing event OGLE-2012-BLG-0026. The lensing light curve exhibits a complex central perturbation with multiple features. We find that the perturbation was produced by two planets located near the Einstein ring of the planet host star. We identify four possible solutions resulting from the well-known close/wide degeneracy. By measuring both the lens parallax and the Einstein radius, we estimate the physical parameters of the planetary system. According to the best-fit model, the two planet masses are ∼0.11 M J and 0.68 MJ and they are orbiting a G-type main-sequence star with a mass ∼0.82 M⊙. The projected separations of the individual planets are beyond the snow line in all four solutions, being ∼3.8 AU and 4.6 AU in the best-fit solution. The deprojected separations are both individually larger and possibly reversed in order. This is the second multi-planet system with both planets beyond the snow line discovered by microlensing. This is the only such system (other than the solar system) with measured planet masses without sin i degeneracy. The planetary system is located at a distance 4.1 kpc from the Earth toward the Galactic center. It is very likely that extra light from stars other than the lensed star comes from the lens itself. If this is correct, it will be possible to obtain detailed information about the planet host star from follow-up observation. © 2013. The American Astronomical Society. All rights reserved.

Yee J.C.,Ohio State University | Hung L.-W.,Ohio State University | Hung L.-W.,University of California at Los Angeles | Bond I.A.,Massey University | And 144 more authors.
Astrophysical Journal | Year: 2013

We analyze MOA-2010-BLG-311, a high magnification (A max > 600) microlensing event with complete data coverage over the peak, making it very sensitive to planetary signals. We fit this event with both a point lens and a two-body lens model and find that the two-body lens model is a better fit but with only Δχ2 ∼ 80. The preferred mass ratio between the lens star and its companion is q = 10-3.7 ± 0.1, placing the candidate companion in the planetary regime. Despite the formal significance of the planet, we show that because of systematics in the data the evidence for a planetary companion to the lens is too tenuous to claim a secure detection. When combined with analyses of other high-magnification events, this event helps empirically define the threshold for reliable planet detection in high-magnification events, which remains an open question. © 2013. The American Astronomical Society. All rights reserved.

Skowron J.,Ohio State University | Udalski A.,University of Warsaw | Gould A.,Ohio State University | Dong S.,Institute for Advanced Study | And 104 more authors.
Astrophysical Journal | Year: 2011

We present the first example of binary microlensing for which the parameter measurements can be verified (or contradicted) by future Doppler observations. This test is made possible by a confluence of two relatively unusual circumstances. First, the binary lens is bright enough (I = 15.6) to permit Doppler measurements. Second, we measure not only the usual seven binary-lens parameters, but also the "microlens parallax" (which yields the binary mass) and two components of the instantaneous orbital velocity. Thus, we measure, effectively, six "Kepler+1" parameters (two instantaneous positions, two instantaneous velocities, the binary total mass, and the mass ratio). Since Doppler observations of the brighter binary component determine five Kepler parameters (period, velocity amplitude, eccentricity, phase, and position of periapsis), while the same spectroscopy yields the mass of the primary, the combined Doppler + microlensing observations would be overconstrained by 6 + (5 + 1) - (7 + 1) = 4 degrees of freedom. This makes possible an extremely strong test of the microlensing solution. We also introduce a uniform microlensing notation for single and binary lenses, define conventions, summarize all known microlensing degeneracies, and extend a set of parameters to describe full Keplerian motion of the binary lenses. © 2011. The American Astronomical Society. All rights reserved.

PubMed | University of Notre Dame, Korea Astronomy and Space Science Institute, Turitea Observatory, Tel Aviv University and 23 more.
Type: Journal Article | Journal: Science (New York, N.Y.) | Year: 2014

Using gravitational microlensing, we detected a cold terrestrial planet orbiting one member of a binary star system. The planet has low mass (twice Earths) and lies projected at ~0.8 astronomical units (AU) from its host star, about the distance between Earth and the Sun. However, the planets temperature is much lower, <60 Kelvin, because the host star is only 0.10 to 0.15 solar masses and therefore more than 400 times less luminous than the Sun. The host itself orbits a slightly more massive companion with projected separation of 10 to 15 AU. This detection is consistent with such systems being very common. Straightforward modification of current microlensing search strategies could increase sensitivity to planets in binary systems. With more detections, such binary-star planetary systems could constrain models of planet formation and evolution.

Gould A.,Ohio State University | Dong S.,Institute for Advanced Study | Gaudi B.S.,Ohio State University | Udalski A.,Optical Gravitational Lens Experiment OGLE | And 169 more authors.
Astrophysical Journal | Year: 2010

We present the first measurement of the planet frequency beyond the "snow line," for the planet-to-star mass-ratio interval-4.5 < logq <-2, corresponding to the range of ice giants to gas giants. We find d 2Np1/dlog q d log s= (0.36 ± 0.15) dex dlog q d logs at the mean mass ratio q = 5 × 10-4 with no discernible deviation from a flat (öpik's law) distribution in logprojected separation s. The determination is based on a sample of six planets detected from intensive follow-up observations of high-magnification (A > 200) microlensing events during 2005-2008. The sampled host stars have a typical mass Mhost ∼ 0.5 M⊙, and detection is sensitive to planets over a range of planet-star-projected separations (s max -1Re, s maxRE), where Re ∼ 3.5 AU(Mhost/M⊙) 1/2 is the Einstein radius and smax ∼ (q/10 -43)1/3. This corresponds to deprojected separations roughly three times the "snow line." We show that the observations of these events have the properties of a "controlled experiment," which is what permits measurement of absolute planet frequency. High-magnification events are rare, but the survey-plus-follow-up high-magnification channel is very efficient: half of all high-mag events were successfully monitored and half of these yielded planet detections. The extremely high sensitivity of high-mag events leads to a policy of monitoring them as intensively as possible, independent of whether they show evidence of planets. This is what allows us to construct an unbiased sample. The planet frequency derived from microlensing is a factor 8 larger than the one derived from Doppler studies at factor ∼25 smaller star-planet separations (i.e., periods 2-2000 days). However, this difference is basically consistent with the gradient derived from Doppler studies (when extrapolated well beyond the separations from which it is measured). This suggests a universal separation distribution across 2 dex in planet-star separation, 2 dex in mass ratio, and 0.3 dex in host mass. Finally, if all planetary systems were "analogs" of the solar system, our sample would have yielded 18.2 planets (11.4 "Jupiters," 6.4 "Saturns," 0.3 "Uranuses," 0.2 "Neptunes") including 6.1 systems with two or more planet detections. This compares to six planets including one twoplanet system in the actual sample, implying a first estimate of 1/6 for the frequency of solar-like systems.

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