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Shin I.-G.,Chungbuk National University | Choi J.-Y.,Chungbuk National University | Park S.-Y.,Chungbuk National University | Han C.,Chungbuk National University | And 162 more authors.
Astrophysical Journal | Year: 2012

Microlensing can provide a useful tool to probe binary distributions down to low-mass limits of binary companions. In this paper, we analyze the light curves of eight binary-lensing events detected through the channel of high-magnification events during the seasons from 2007 to 2010. The perturbations, which are confined near the peak of the light curves, can be easily distinguished from the central perturbations caused by planets. However, the degeneracy between close and wide binary solutions cannot be resolved with a 3σ confidence level for three events, implying that the degeneracy would be an important obstacle in studying binary distributions. The dependence of the degeneracy on the lensing parameters is consistent with a theoretical prediction that the degeneracy becomes severe as the binary separation and the mass ratio deviate from the values of resonant caustics. The measured mass ratio of the event OGLE-2008-BLG-510/MOA-2008-BLG-369 is q ∼ 0.1, making the companion of the lens a strong brown dwarf candidate. © 2012. The American Astronomical Society. All rights reserved.

Choi J.-Y.,Chungbuk National University | Shin I.-G.,Chungbuk National University | Park S.-Y.,Chungbuk National University | Han C.,Chungbuk National University | And 156 more authors.
Astrophysical Journal | Year: 2012

We present the analysis of the light curves of nine high-magnification single-lens gravitational microlensing events with lenses passing over source stars, including OGLE-2004-BLG-254, MOA-2007-BLG-176, MOA-2007-BLG-233/OGLE- 2007-BLG-302, MOA-2009-BLG-174, MOA-2010-BLG-436, MOA-2011-BLG-093, MOA-2011-BLG-274, OGLE-2011-BLG-0990/MOA-2011-BLG-300, and OGLE-2011-BLG-1101/ MOA-2011-BLG-325. For all of the events, we measure the linear limb-darkening coefficients of the surface brightness profile of source stars by measuring the deviation of the light curves near the peak affected by the finite-source effect. For seven events, we measure the Einstein radii and the lens-source relative proper motions. Among them, five events are found to have Einstein radii of less than 0.2 mas, making the lenses very low mass star or brown dwarf candidates. For MOA-2011-BLG-274, especially, the small Einstein radius of θE 0.08 mas combined with the short timescale of t E 2.7days suggests the possibility that the lens is a free-floating planet. For MOA-2009-BLG-174, we measure the lens parallax and thus uniquely determine the physical parameters of the lens. We also find that the measured lens mass of 0.84 M ⊙ is consistent with that of a star blended with the source, suggesting that the blend is likely to be the lens. Although we did not find planetary signals for any of the events, we provide exclusion diagrams showing the confidence levels excluding the existence of a planet as a function of the separation and mass ratio. © 2012. The American Astronomical Society. All rights reserved.

Bachelet E.,French National Center for Scientific Research | Fouque P.,French National Center for Scientific Research | Han C.,Microlensing Follow Up Network | Han C.,Chungbuk National University | And 160 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Caustic crossing is the clearest signature of binary lenses in microlensing. In the present context, this signature is diluted by the large source star but a detailed analysis has allowed the companion signal to be extracted. Aims. MOA 2009-BLG-411 was detected on August 5, 2009 by the MOA-Collaboration. Alerted as a high-magnification event, it was sensitive to planets. Suspected anomalies in the light curve were not confirmed by a real-time model, but further analysis revealed small deviations from a single lens extended source fit. Methods. Thanks to observations by all the collaborations, this event was well monitored. We first decided to characterize the source star properties by using a more refined method than the classical one: we measure the interstellar absorption along the line of sight in five different passbands (VIJHK). Secondly, we model the lightcurve by using the standard technique: make (s,q,α) grids to look for local minima and refine the results by using a downhill method (Markov chain Monte Carlo). Finally, we use a Galactic model to estimate the physical properties of the lens components. Results. We find that the source star is a giant G star with radius 9 R ·. The grid search gives two local minima, which correspond to the theoretical degeneracy s s-1. We find that the lens is composed of a brown dwarf secondary of mass MS = 0.05 M· orbiting a primary M-star of mass MP = 0.18 M·. We also reveal a new mass-ratio degeneracy for the central caustics of close binaries. Conclusions. As far as we are aware, this is the first detection using the microlensing technique of a binary system in our Galaxy composed of an M-star and a brown dwarf. © 2012 ESO.

Parsons S.G.,University of Warwick | Marsh T.R.,University of Warwick | Copperwheat C.M.,University of Warwick | Dhillon V.S.,University of Sheffield | And 9 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

We present high-speed ULTRACAM photometry of the eclipsing post-common-envelope binaries DE CVn, GK Vir, NN Ser, QS Vir, RR Cae, RX J2130.6+4710, SDSS 0110+1326 and SDSS 0303+0054 and use these data to measure precise mid-eclipse times in order to detect any period variations.We detect a large (~250 s) departure from linearity in the eclipse times of QS Vir which Applegate's mechanism fails to reproduce by an order of magnitude. The only mechanism able to drive this period change is a third body in a highly elliptical orbit. However, the planetary/sub-stellar companion previously suggested to exist in this system is ruled out by our data. Our eclipse times show that the period decrease detected in NN Ser is continuing, with magnetic braking or a third body the only mechanisms able to explain this change. The planetary/sub-stellar companion previously suggested to exist in NN Ser is also ruled out by our data. Our precise eclipse times also lead to improved ephemerides for DE CVn and GK Vir. The width of a primary eclipse is directly related to the size of the secondary star and variations in the size of this star could be an indication of Applegate's mechanism or Wilson (starspot) depressions which can cause jitter in the O-C curves. We measure the width of primary eclipses for the systems NN Ser and GK Vir over several years but find no definitive variations in the radii of the secondary stars. However, our data are precise enough (ΔRsec/Rsec < 10-5) to show the effects of Applegate's mechanism in the future. We find no evidence ofWilson depressions in either system.We also find tentative indications that flaring rates of the secondary stars depend on their mass rather than rotation rates. © 2010 The Authors. Journal compilation. © 2010 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.

Ryu Y.-H.,Chungbuk National University | Han C.,Chungbuk National University | Hwang K.-H.,Las Cumbres Observatory Global Telescope Network | Street R.,University of Warsaw | And 122 more authors.
Astrophysical Journal | Year: 2010

We report the result of the analysis of a dramatic repeating gravitational microlensing event OGLE-2009-BLG-092/MOA-2009-BLG-137, for which the light curve is characterized by two distinct peaks with perturbations near both peaks. We find that the event is produced by the passage of the source trajectory over the central perturbation regions associated with the individual components of a wide-separation binary. The event is special in the sense that the second perturbation, occurring ∼100 days after the first, was predicted by the real-time analysis conducted after the first peak, demonstrating that real-time modeling can be routinely done for binary and planetary events.With the data obtained from follow-up observations covering the second peak, we are able to uniquely determine the physical parameters of the lens system.We find that the event occurred on a bulge clump giant and itwas produced by a binary lens composed of a K- and M-type main-sequence stars. The estimated masses of the binary components are M1 = 0.69±0.11M⊙ and M2 = 0.36±0.06M⊙, respectively, and they are separated in projection by r⊥ = 10.9±1.3AU. The measured distance to the lens is DL = 5.6 ± 0.7 kpc. We also detect the orbital motion of the lens system. © 2010. The American Astronomical Society.

Fouque P.,French National Center for Scientific Research | Heyrovsky D.,Charles University | Dong S.,Microlensing Follow Up Network | Dong S.,Ohio State University | And 144 more authors.
Astronomy and Astrophysics | Year: 2010

Context. Not only is gravitational microlensing a successful tool for discovering distant exoplanets, but it also enables characterization of the lens and source stars involved in the lensing event. Aims. In high-magnification events, the lens caustic may cross over the source disk, which allows determination of the angular size of the source and measurement of its limb darkening. Methods. When such extended-source effects appear close to maximum magnification, the resulting light curve differs from the characteristic Paczyński point-source curve. The exact shape of the light curve close to the peak depends on the limb darkening of the source. Dense photometric coverage permits measurement of the respective limb-darkening coefficients. Results. In the case of the microlensing event OGLE 2008-BLG-290, the K giant source star reached a peak magnification at about 100. Thirteen different telescopes have covered this event in eight different photometric bands. Subsequent light-curve analysis yielded measurements of linear limb-darkening coefficients of the source in six photometric bands. The best-measured coefficients lead to an estimate of the source effective temperature of about 4700$^{+100}-{-200}$ K. However, the photometric estimate from colour-magnitude diagrams favours a cooler temperature of 4200 ± 100 K. Conclusions. Because the limb-darkening measurements, at least in the CTIO/SMARTS2 Vs- and Is-bands, are among the most accurate obtained, the above disagreement needs to be understood. A solution is proposed, which may apply to previous events where such a discrepancy also appeared. © 2010 ESO.

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