K. -H. Hwang

Publications (12)6.02 Total impact

• Source

  Available from: Jeremy Tregloan-Reed

  Article: Microlensing Discovery of a Population of Very Tight, Very Low-mass Binary Brown Dwarfs

  J. -Y. Choi, C. Han, A. Udalski, T. Sumi, B. S. Gaudi, A. Gould, D. P. Bennett, M. Dominik, J. -P. Beaulieu, Y. Tsapras, [......], J. Greenhill, D. Kubas, J. -B. Marquette, J. W. Menzies, K. C. Sahu, M. Zub, D. M. Bramich, K. Horne, I. A. Steele, R. A. Street
  [show abstract] [hide abstract]
  ABSTRACT: Although many models have been proposed, the physical mechanisms responsible for the formation of low-mass brown dwarfs are poorly understood. The multiplicity properties and minimum mass of the brown-dwarf mass function provide critical empirical diagnostics of these mechanisms. We present the discovery via gravitational microlensing of two very low-mass, very tight binary systems. These binaries have directly and precisely measured total system masses of 0.025 Msun and 0.034 Msun, and projected separations of 0.31 AU and 0.19 AU, making them the lowest-mass and tightest field brown-dwarf binaries known. The discovery of a population of such binaries indicates that brown dwarf binaries can robustly form at least down to masses of ~0.02 Msun. Future microlensing surveys will measure a mass-selected sample of brown-dwarf binary systems, which can then be directly compared to similar samples of stellar binaries.
  02/2013;
• Article: MOA-2010-BLG-073L: An M-Dwarf with a Substellar Companion at the Planet/Brown Dwarf Boundary

  R. A. Street, J. -Y. Choi, Y. Tsapras, C. Han, K. Furusawa, M. Hundertmark, A. Gould, T. Sumi, I. A. Bond, D. Wouters, [......], M. Mathiasen, M. T. Penny, S. Proft, S. Rahvar, D. Ricci, G. Scarpetta, S. Schafer, F. Schonebeck, J. Southworth, J. Surdej
  [show abstract] [hide abstract]
  ABSTRACT: We present an analysis of the anomalous microlensing event, MOA-2010-BLG-073, announced by the Microlensing Observations in Astrophysics survey on 2010-03-18. This event was remarkable because the source was previously known to be photometrically variable. Analyzing the pre-event source lightcurve, we demonstrate that it is an irregular variable over time scales >200d. Its dereddened color, $(V-I)_{S,0}$, is 1.221$\pm$0.051mag and from our lens model we derive a source radius of 14.7$\pm$1.3 $R_{\odot}$, 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$\pm$0.0006. The Einstein crossing time of the event, $T_{\rm{E}}=44.3$\pm$0.1d, was sufficiently long that the lightcurve 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, $D_L$=2.8$\pm$0.4kpc, and the masses of the lensing objects. The primary of the lens is an M-dwarf with $M_{L,p}$=0.16$\pm0.03M_{\odot}$ while the companion has $M_{L,s}$=11.0$\pm2.0M_{\rm{J}}$ putting it in the boundary zone between planets and brown dwarfs.
  11/2012;
• Article: The second multiple-planet system discovered by microlensing: OGLE-2012-BLG-0026Lb, c, a pair of jovian planets beyond the snow line

  C. Han, A. Udalski, J. -Y. Choi, J. C. Yee, A. Gould, G. Christie, T. -G. Tan, M. K. Szymański, M. Kubiak, I. Soszyński, [......], C. -U. Lee, J. -R. Koo, J. McCormick, L. A. G. Monard, T. Natusch, H. Ngan, H. Park, R. W. Pogge, Ian Porritt, I. -G. Shin
  [show abstract] [hide abstract]
  ABSTRACT: 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 4 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_Jupiter and 0.68 M_Jupiter and they are orbiting a G-type main sequence star with a mass ~0.82 M_Sun. 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 a 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.
  10/2012;
• Source

  Available from: Kouji Ohnishi

  Article: OGLE-2009-BLG-023/MOA-2009-BLG-028: Characterization of a Binary Microlensing Event Based on Survey Data

  K. -H. Hwang, C Han, A. Udalski, T Sumi, A. Gould, M. Jaroszynski, M. Kubiak, M. K. Szymanski, G. Pietrzynski, I. Soszynski, [......], Y. Muraki, K Ohnishi, Y. C. Perrott, N. J. Rattenbury, To. Saito, T Sako, D. J. Sullivan, W. L. Sweatman, P. J. Tristram, P. C. M. Yock
  [show abstract] [hide abstract]
  ABSTRACT: 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 $(0.50\pm 0.07) \ M_\odot$ and $(0.15\pm 0.02)\ M_\odot$ located in the Galactic disk with a distance of $\sim 1.8$ kpc 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.
  04/2011;
• Article: OGLE-2009-BLG-092/MOA-2009-BLG-137: A Dramatic Repeating Event with the Second Perturbation Predicted by Real-time Analysis

  Y -H Ryu, C Han, K -H Hwang, R Street, A Udalski, T Sumi, A Fukui, J -P Beaulieu, A Gould, M Dominik, [......], M Mathiasen, S Rahvar, D Ricci, G Scarpetta, J Skottfelt, J Surdej, J Southworth, J Wambsganss, F Zimmer, MiNDSTEp Collaboration
  apj. 11/2010; 723:81-88.
• Article: OGLE-2009-BLG-092/MOA-2009-BLG-137: A Dramatic Repeating Event with the Second Perturbation Predicted by Real-time Analysis

  Y -H Ryu, C Han, K -H Hwang, R Street, A Udalski, T Sumi, A Fukui, J -P Beaulieu, A Gould, M Dominik, [......], M Mathiasen, S Rahvar, D Ricci, G Scarpetta, J Skottfelt, J Surdej, J Southworth, J Wambsganss, F Zimmer, MiNDSTEp Collaboration
  apj. 11/2010; 723:81-88.
• Article: OGLE-2005-BLG-153: Microlensing Discovery and Characterization of a Very Low Mass Binary

  K.-H. Hwang, A. Udalski, C. Han, Y.-H. Ryu, I. A. Bond, J.-P. Beaulieu, M. Dominik, K. Horne, A. Gould, B. S. Gaudi, [......], J. Wambsganss, A. Williams, K. Woller, M. Burgdorf, C. Snodgrass, M. Bode, D. L. Depoy, C.-U. Lee, B.-G. Park, R. W. Pogge
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  ABSTRACT: 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.
  The Astrophysical Journal 10/2010; 723(1):797. · 6.02 Impact Factor
• Source

  Available from: Stephane Brillant

  Article: OGLE-2005-BLG-153: Microlensing Discovery and Characterization of A Very Low Mass Binary

  K. -H. Hwang, A. Udalski, C. Han, Y. -H. Ryu, I. A. Bond, J. -P. Beaulieu, M. Dominik, K. Horne, A. Gould, B. S. Gaudi, [......], J. Wambsganss, A. Williams, K. Woller, M. Burgdorf, C. Snodgrass, M. Bode, D. L. Depoy, C. -U. Lee, B. -G. Park, R. W. Pogge
  [show abstract] [hide abstract]
  ABSTRACT: 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 $0.10\pm 0.01\ M_\odot$ and $0.09\pm 0.01\ M_\odot$. 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.
  09/2010;
• Source

  Available from: Stephane Brillant

  Article: OGLE-2009-BLG-092/MOA-2009-BLG-137: A Dramatic Repeating Event With the Second Perturbation Predicted by Real-Time Analysis

  Y. -H. Ryu, C Han, K. -H. Hwang, R. Street, A. Udalski, T Sumi, A Fukui, J. P. Beaulieu, A. Gould, M. Dominik, [......], L. Mancini, M. Mathiasen, S. Rahvar, D. Ricci, G. Scarpetta, J. Skottfelt, J. Surdej, J. Southworth, J. Wambsganss, F. Zimmer
  [show abstract] [hide abstract]
  ABSTRACT: 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 $\sim 100$ 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 it was produced by a binary lens composed of a K and M-type main-sequence stars. The estimated masses of the binary components are $M_1=0.69 \pm 0.11\ M_\odot$ and $M_2=0.36\pm 0.06\ M_\odot$, respectively, and they are separated in projection by $r_\perp=10.9\pm 1.3\ {\rm AU}$. The measured distance to the lens is $D_{\rm L}=5.6 \pm 0.7\ {\rm kpc}$. We also detect the orbital motion of the lens system. Comment: 18 pages, 5 figures, 1 table
  09/2010;
• Source

  Available from: Kouji Ohnishi

  Article: Determining the Physical Lens Parameters of the Binary Gravitational Microlensing Event MOA-2009-BLG-016

  K. -H. Hwang, C Han, I. A. Bond, N Miyake, F Abe, D. P. Bennett, C. S. Botzler, A Fukui, K. Furusawa, F Hayashi, [......], D Suzuki, W. L. Sweatman, J. Tristram, K Wada, P. C. M. Yock D. L, Depoy, B. S. Gaudi, A. Gould, C. -U. Lee, R. W. Pogge
  [show abstract] [hide abstract]
  ABSTRACT: 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_sun, implying a brown-dwarf companion. However, there exists a solution that is worse only by \Delta\chi^2 ~ 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) and thus the lens will remain blended with the source for the next several decades. Comment: 7 pages, 2 tables, and 5 figures
  06/2010;
• Source

  Available from: Stephane Brillant

  Article: Interpretation of Strong Short-Term Central Perturbations in the Light Curves of Moderate-Magnification Microlensing Events

  C Han, K. -H. Hwang, D Kim, A. Udalski, F Abe, L. A. B. Monard, J McCormick, M. K. Szymanski, M. Kubiak, G. Pietrzynski, [......], J. Menzies, K. R. Pollard, K. C. Sahu, C. Snodgrass, I. Steele, R. Street, Y. Tsapras, J. Wambsganss, A Williams, M. Zub
  [show abstract] [hide abstract]
  ABSTRACT: To improve the planet detection efficiency, current planetary microlensing experiments are focused on high-magnification events searching for planetary signals near the peak of lensing light curves. However, it is known that central perturbations can also be produced by binary companions and thus it is important to distinguish planetary signals from those induced by binary companions. In this paper, we analyze the light curves of microlensing events OGLE-2007-BLG-137/MOA-2007-BLG-091, OGLE-2007-BLG-355/MOA-2007-BLG-278, and MOA-2007-BLG-199/OGLE-2007-BLG-419, for all of which exhibit short-term perturbations near the peaks of the light curves. From detailed modeling of the light curves, we find that the perturbations of the events are caused by binary companions rather than planets. From close examination of the light curves combined with the underlying physical geometry of the lens system obtained from modeling, we find that the short time-scale caustic-crossing feature occurring at a low or a moderate base magnification with an additional secondary perturbation is a typical feature of binary-lens events and thus can be used for the discrimination between the binary and planetary interpretations. Comment: 17 pages, 4 figures, 1 table
  11/2009;
• Source

  Available from: Stephane Brillant

  Article: OGLE-2005-BLG-153: Microlensing Discovery and Characterization of a Very Low Mass Binary

  K. -H. Hwang, A. Udalski, C Han, Y. -H. Ryu, I. A. Bond, J. P. Beaulieu, M. Dominik, K. Horne, A. Gould, B. S. Gaudi, [......], K. Woller, M. Burgdorf, C. Snodgrass, M. Bode, D. L. DePoy, The Planet/RoboNet Collaboration, C. -U. Lee, B. -G. Park, R. W. Pogge, The μFUN Collaboration
  Astrophysical Journal, v.723, 797-802 (2010).