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Kumeu, New Zealand

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.


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.


Gould A.,A+ Network | Gould A.,Ohio State University | Dong S.,A+ Network | Dong S.,Institute for Advanced Study | And 176 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.


Yee J.C.,Ohio State University | Shvartzvald Y.,Tel Aviv University | Gal-Yam A.,Weizmann Institute of Science | Bond I.A.,Massey University | And 75 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..


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..

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