T Sumi

Publications (107)202.97 Total impact

• Article: The Microlensing Event Rate and Optical Depth Toward the Galactic Bulge from MOA-II

  T. Sumi, D. P. Bennett, I. A. Bond, F. Abe, C. S. Botzler, A. Fukui, K. Furusawa, Y. Itow, C. H. Ling, K. Masuda, [......], Y. Muraki, K. Ohnishi, N. Rattenbury, To. Saito, D. J. Sullivan, D. Suzuki, W. L. Sweatman, J. Tristram, K. Wada, P. C. M. Yock
  [show abstract] [hide abstract]
  ABSTRACT: We present measurements of the microlensing optical depth and event rate toward the Galactic Bulge based on two years of the MOA-II survey. This sample contains ~1000 microlensing events, with an Einstein Radius crossing time of t_E < 200 days between -5 <l< 10 degree and -7 <b< -1 degree. Our event rate and optical depth analysis uses 474 events with well defined microlensing parameters. In the central fields with |l|< 5 degree, we find an event rates of \Gamma = [2.39+/-1.1]e^{[0.60\pm0.05](3-|b|)}x 10^{-5}/star/yr and an optical depth of \tau_{200} = [2.35+/-0.18]e^{[0.51+/-0.07](3-|b|)}x 10^{-6} for the 427 events using all sources brighter than I_s = 20 mag centered at (l,b)=(0.38, -3.72). We find that the event rate is maximized at low latitudes and a longitude of $l~1 degree. For the 111 events in 3.2 deg^2 of the central Galactic Bulge at |b| < 3.0 degree and 0.0 < l < 2.0, centered at (l,b)=(0.97, -2.26), we find \Gamma = 4.57_{-0.46}^{+0.51} x 10^{-5}/star/yr and \tau_{200} = 3.64_{-0.45}^{+ 0.51} x 10^{-6}. We also consider a Red Clump Giant (RCG) star sample with I_s<17.5 mag. Our results are consistent with previous optical depth measurements. We find that the previously observed difference in optical depth measurements between all-source and RCG samples may be largely due to statistical fluctuations. These event rate measurements towards the central galactic bulge are necessary to predict the microlensing event rate and to optimize the survey fields in the future space mission such as WFIRST.
  05/2013;
• Source

  Available from: Jeremy Tregloan-Reed

  Article: A Giant Planet beyond the Snow Line in Microlensing Event OGLE-2011-BLG-0251

  N. Kains, R. Street, J. -Y. Choi, C. Han, A. Udalski, L. A. Almeida, F. Jablonski, P. Tristram, U. G. Jorgensen, M. K. Szymanski, [......], S. R. Kane, D. Kubas, J. -B. Marquette, R. Martin, P. Meintjes, J. Menzies, K. R. Pollard, A. Williams, D. Wouters, M. Zub
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  ABSTRACT: We present the analysis of the gravitational microlensing event OGLE-2011-BLG-0251. This anomalous event was observed by several survey and follow-up collaborations conducting microlensing observations towards the Galactic Bulge. Based on detailed modelling of the observed light curve, we find that the lens is composed of two masses with a mass ratio q=1.9 x 10^-3. Thanks to our detection of higher-order effects on the light curve due to the Earth's orbital motion and the finite size of source, we are able to measure the mass and distance to the lens unambiguously. We find that the lens is made up of a planet of mass 0.53 +- 0.21,M_Jup orbiting an M dwarf host star with a mass of 0.26 +- 0.11 M_Sun. The planetary system is located at a distance of 2.57 +- 0.61 kpc towards the Galactic Centre. The projected separation of the planet from its host star is d=1.408 +- 0.019, in units of the Einstein radius, which corresponds to 2.72 +- 0.75 AU in physical units. We also identified a competitive model with similar planet and host star masses, but with a smaller orbital radius of 1.50 +- 0.50 AU. The planet is therefore located beyond the snow line of its host star, which we estimate to be around 1-1.5 AU.
  03/2013;
• 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
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  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-523: "Failed Planet" = RS CVn Star

  A Gould, J ~C Yee, I ~A Bond, A Udalski, C Han, U ~G Jørgensen, J Greenhill, Y Tsapras, M ~H Pinsonneault, T Bensby, [......], C ~B Henderson, D Kubas, J -B Marquette, R Martin, J ~W Menzies, B Shappee, A Williams, J Saders, M Zub, The PLANET Collaboration
  apj. 02/2013; 763:141.
• Article: Microlensing Binaries with Candidate Brown Dwarf Companions

  I -G Shin, C Han, A Gould, A Udalski, T Sumi, M Dominik, J -P Beaulieu, Y Tsapras, V Bozza, M ~K Szyma'nski, [......], J -B Marquette, R Martin, P Meintjes, J Menzies, K ~R Pollard, K ~C Sahu, J Wambsganss, A Williams, C Vinter, M Zub
  The Astrophysical Journal 12/2012; 760:116. · 6.02 Impact Factor
• Article: Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. V. Evidence for a wide age distribution and a complex MDF

  T. Bensby, J. C. Yee, S. Feltzing, J. A. Johnson, A. Gould, J. G. Cohen, M. Asplund, J. Meléndez, S. Lucatello, C. Han, [......], D. P. Bennett, I. A. Bond, W. Kohei, T. Sumi, D. Suzuki, K. Suzuki, S. Takino, P. Tristram, N. Yamai, A. Yonehara
  [show abstract] [hide abstract]
  ABSTRACT: Based on high-resolution spectra obtained during gravitational microlensing events we present a detailed elemental abundance analysis of 32 dwarf and subgiant stars in the Galactic bulge. [ABRIDGED], we now have 58 microlensed bulge dwarfs and subgiants that have been homogeneously analysed. The main characteristics of the sample and the findings that can be drawn are: (i) The metallicity distribution (MDF) is wide and spans all metallicities between [Fe/H]=-1.9 to +0.6; (ii) The dip in the MDF around solar metallicity that was apparent in our previous analysis of a smaller sample (26 microlensed stars) is no longer evident; instead it has a complex structure and indications of multiple components are starting to emerge. [ABRIDGED]; (iii) The stars with [Fe/H]<-0.1 are old with ages between 10 and 12 Gyr; (iv) The metal-rich stars with [Fe/H]>-0.1 show a wide variety of ages, ranging from 2 to 12 Gyr with a distribution that has a dominant peak around 4-5 Gyr and a tail towards higher ages; (v) There are indications in the [alpha/Fe] - [Fe/H] that the "knee" occurs around [Fe/H] = -0.3 to -0.2, which is a slightly higher metallicity as compared to the "knee" for the local thick disk. This suggests that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk. The results from the microlensed bulge dwarf stars in combination with other findings in the literature, in particular the evidence that the bulge has cylindrical rotation, indicate that the Milky Way could be an almost pure disk galaxy. The bulge would then just be a conglomerate of the other Galactic stellar populations (thin disk, thick disk, halo, and ...?), residing together in the central parts of the Galaxy, influenced by the Galactic bar.
  11/2012;
• 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
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  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: A brown dwarf orbiting an M-dwarf: MOA 2009-BLG-411L

  E Bachelet, P Fouqué, C Han, A Gould, M ~D Albrow, J -P Beaulieu, E Bertin, I ~A Bond, G ~W Christie, D Heyrovský, [......], G Maier, L Mancini, S Rahvar, D Ricci, G Scarpetta, J Skottfelt, J Southworth, J Surdej, F Zimmer, Mindstep Consortium
  Astronomy and Astrophysics 11/2012; 547:A55. · 4.59 Impact Factor
• Article: MOA-2010-BLG-311: A planetary candidate below the threshold of reliable detection

  J. C. Yee, L. -W. Hung, I. A. Bond, W. Allen, L. A. G. Monard, M. D. Albrow, P. Fouque, M. Dominik, Y. Tsapras, A. Udalski, [......], S. Rahvar, D. Ricci, K. C. Sahu, G. Scarpetta, S. Schafer, F. Schonebeck, C. Snodgrass, J. Southworth, J. Surdej, J. Wambsgans
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  ABSTRACT: 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 2-body lens model and find that the 2-body lens model is a better fit but with only Delta chi^2~140. 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.
  10/2012;
• Article: MOA-2010-BLG-523: "Failed Planet" = RS CVn Star

  A. Gould, J. C. Yee, I. A. Bond, A. Udalski, C. Han, U. G. Jorgensen, J. Greenhill, Y. Tsapras, M. H. Pinsonneault, T. Bensby, [......], P. Fouque, C. B. Henderson, D. Kubas, J. -B Marquette, R. Martin, J. W. Menzies, B. Shappee, A. Williams, J. van Saders, M. Zub
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  ABSTRACT: The Galactic bulge source MOA-2010-BLG-523S exhibited short-term deviations from a standard microlensing lightcurve near the peak of an Amax ~ 265 high-magnification microlensing event. The deviations originally seemed consistent with expectations for a planetary companion to the principal lens. We combine long-term photometric monitoring with a previously published high-resolution spectrum taken near peak to demonstrate that this is an RS CVn variable, so that planetary microlensing is not required to explain the lightcurve deviations. This is the first spectroscopically confirmed RS CVn star discovered in the Galactic bulge.
  10/2012;
• Article: Using Orbital Effects to Break the Close/wide Degeneracy in Binary-lens Microlensing Events

  I. -G. Shin, T. Sumi, A. Udalski, J. -Y. Choi, C. Han, A. Gould, F. Abe, D. P. Bennett, I. A. Bond, C. S. Botzler, [......], M. K. Szymański, M. Kubiak, I. Soszyński, G. Pietrzyński, R. Poleski, K. Ulaczyk, P. Pietrukowicz, S. Kozłowski, J. Skowron, Ł. Wyrzykowski
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  ABSTRACT: Microlensing can provide an important tool to study binaries, especially those composed of faint or dark objects. However, accurate analysis of binary-lens light curves is often hampered by the well-known degeneracy between close (s<1) and wide (s>1) binaries, which can be very severe due to an intrinsic symmetry in the lens equation. Here s is the normalized projected binary separation. In this paper, we propose a method that can resolve the close/wide degeneracy using the effect of a lens orbital motion on lensing light curves. The method is based on the fact that the orbital effect tends to be important for close binaries while it is negligible for wide binaries. We demonstrate the usefulness of the method by applying it to an actually observed binary-lens event MOA-2011-BLG-040/OGLE-2011-BLG-0001, which suffers from severe close/wide degeneracy. From this, we are able to uniquely specify that the lens is composed of K and M-type dwarfs located at ~3.5 kpc from the Earth.
  10/2012;
• Article: MOA-2010-BLG-311: A planetary candidate below the threshold of reliable detection

  J ~C Yee, L -W Hung, I ~A Bond, W Allen, L ~A ~G Monard, M ~D Albrow, P Fouque, M Dominik, Y Tsapras, A Udalski, [......], S Rahvar, D Ricci, K ~C Sahu, G Scarpetta, S Schafer, F Schonebeck, C Snodgrass, J Southworth, J Surdej, J Wambsgans
  ArXiv e-prints. 10/2012;
• Article: MOA-2010-BLG-311: A planetary candidate below the threshold of reliable detection

  J ~C Yee, L -W Hung, I ~A Bond, W Allen, L ~A ~G Monard, M ~D Albrow, P Fouque, M Dominik, Y Tsapras, A Udalski, [......], S Rahvar, D Ricci, K ~C Sahu, G Scarpetta, S Schafer, F Schonebeck, C Snodgrass, J Southworth, J Surdej, J Wambsgans
  ArXiv e-prints. 10/2012;
• Article: Microlensig Binaries with Candidate Brown Dwarf Companions

  I. -G. Shin, C. Han, A. Gould, A. Udalski, T. Sumi, M. Dominik, J. -P. Beaulieu, Y. Tsapras, V. Bozza, M. K. Szymański, [......], J. -B. Marquette, R. Martin, P. Meintjes, J. Menzies, K. R. Pollard, K. C. Sahu, J. Wambsganss, A. Williams, C. Vinter, M. Zub
  [show abstract] [hide abstract]
  ABSTRACT: Brown dwarfs are important objects because they may provide a missing link between stars and planets, two populations that have dramatically different formation history. In this paper, we present the candidate binaries with brown dwarf companions that are found by analyzing binary microlensing events discovered during 2004 - 2011 observation seasons. Based on the low mass ratio criterion of q < 0.2, we found 7 candidate events, including OGLE-2004-BLG-035, OGLE-2004-BLG-039, OGLE-2007-BLG-006, OGLE-2007-BLG-399/MOA-2007-BLG-334, MOA-2011-BLG-104/OGLE-2011-BLG-0172, MOA-2011-BLG-149, and MOA-201-BLG-278/OGLE-2011-BLG-012N. Among them, we are able to confirm that the companions of the lenses of MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149 are brown dwarfs by determining the mass of the lens based on the simultaneous measurement of the Einstein radius and the lens parallax. The measured mass of the brown dwarf companions are (0.02 +/- 0.01) M_Sun and (0.019 +/- 0.002) M_Sun for MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149, respectively, and both companions are orbiting low mass M dwarf host stars. More microlensing brown dwarfs are expected to be detected as the number of lensing events with well covered light curves increases with new generation searches.
  08/2012;
• Article: A possible binary system of a stellar remnant in the high magnification gravitational microlensing event OGLE-2007-BLG-514

  N. Miyake, A. Udalski, T. Sumi, D. P. Bennett, S. Dong, R. A. Street, J. Greenhill, I. A. Bond, A. Gould, M. Kubiak, [......], S. Kane, D. Kubas, J. B. Marquette, R. Martin, J. Menzies, K. R. Pollard, K. C. Sahu, J. Wambsganss, A. Williams, M. Zub
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  ABSTRACT: We report the extremely high magnification (A > 1000) binary microlensing event OGLE-2007-BLG-514. We obtained good coverage around the double peak structure in the light curve via follow-up observations from different observatories. The binary lens model that includes the effects of parallax (known orbital motion of the Earth) and orbital motion of the lens yields a binary lens mass ratio of q = 0.321 +/- 0.007 and a projected separation of s = 0.072 +/- 0.001$ in units of the Einstein radius. The parallax parameters allow us to determine the lens distance D_L = 3.11 +/- 0.39 kpc and total mass M_L=1.40 +/- 0.18 M_sun; this leads to the primary and secondary components having masses of M_1 = 1.06 +/- 0.13 M_sun and M_2 = 0.34 +/- 0.04 M_sun, respectively. The parallax model indicates that the binary lens system is likely constructed by the main sequence stars. On the other hand, we used a Bayesian analysis to estimate probability distributions by the model that includes the effects of xallarap (possible orbital motion of the source around a companion) and parallax (q = 0.270 +/- 0.005, s = 0.083 +/- 0.001). The primary component of the binary lens is relatively massive with M_1 = 0.9_{-0.3}^{+4.6} M_sun and it is at a distance of D_L = 2.6_{-0.9}^{+3.8} kpc. Given the secure mass ratio measurement, the companion mass is therefore M_2 = 0.2_{-0.1}^{+1.2} M_sun. The xallarap model implies that the primary lens is likely a stellar remnant, such as a white dwarf, a neutron star or a black hole.
  07/2012;
• Article: MOA-2010-BLG-477Lb: constraining the mass of a microlensing planet from microlensing parallax, orbital motion and detection of blended light

  E. Bachelet, I. -G. Shin, C. Han, P. Fouqué, A. Gould, J. W. Menzies, J. -P. Beaulieu, D. P. Bennett, I. A. Bond, Subo Dong, [......], M. T. Penny, S. Proft, S. Rahvar, D. Ricci, G. Scarpetta, S. Schäfer, F. Schönebeck, J. Southworth, J. Surdej, J. Wambsganss
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  ABSTRACT: 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 $q=(2.181\pm0.004)\times 10^{-3}$ and the projected separation is $s=1.1228\pm0.0006$ in units of the Einstein radius. The angular Einstein radius is unusually large $\theta_{\rm E}=1.38\pm 0.11$ mas. Combining this measurement with constraints on the "microlens parallax" and the lens flux, we can only limit the host mass to the range $0.13<M/M_\odot<1.0$. 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 $M_*=0.67^{+0.33}_{-0.13}\ M_\odot$ and $m_p=1.5^{+0.8}_{-0.3}\ M_{\rm JUP}$ at a distance of $D=2.3\pm0.6$ kpc, and with a semi-major axis of $a=2^{+3}_{-1}$ AU. 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.
  05/2012;
• Article: A New Type of Ambiguity in the Planet and Binary Interpretations of Central Perturbations of High-Magnification Gravitational Microlensing Events

  J. -Y. Choi, I. -G. Shin, C. Han, A. Udalski, T. Sumi, A. Gould, V. Bozza, M. Dominik, P. Fouqué, K. Horne, [......], C. Coutures, S. Dieters, D. Dominis Prester, J. Donatowicz, J. Greenhill, D. Kubas, J. -B. Marquette, J. W. Menzies, K. C. Sahu, M. Zub
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  ABSTRACT: High-magnification microlensing events provide an important channel to detect planets. Perturbations near the peak of a high-magnification event can be produced either by a planet or a binary companion. It is known that central perturbations induced by both types of companions can be generally distinguished due to the basically different magnification pattern around caustics. In this paper, we present a case of central perturbations for which it is difficult to distinguish the planetary and binary interpretations. The peak of a lensing light curve affected by this perturbation appears to be blunt and flat. For a planetary case, this perturbation occurs when the source trajectory passes the negative perturbation region behind the back end of an arrowhead-shaped central caustic. For a binary case, a similar perturbation occurs for a source trajectory passing through the negative perturbation region between two cusps of an astroid-shaped caustic. We demonstrate the degeneracy for 2 high-magnification events of OGLE-2011-BLG-0526 and OGLE-2011-BLG-0950/MOA-2011-BLG-336. For OGLE-2011-BLG-0526, the $\chi^2$ difference between the planetary and binary model is $\sim$ 3, implying that the degeneracy is very severe. For OGLE-2011-BLG-0950/MOA-2011-BLG-336, the stellar binary model is formally excluded with $\Delta \chi^2 \sim$ 105 and the planetary model is preferred. However, it is difficult to claim a planet discovery because systematic residuals of data from the planetary model are larger than the difference between the planetary and binary models. Considering that 2 events observed during a single season suffer from such a degeneracy, it is expected that central perturbations experiencing this type of degeneracy is common.
  04/2012;
• Article: Characterizing Low-Mass Binaries From Observation of Long Time-scale Caustic-crossing Gravitational Microlensing Events

  I. -G. Shin, C. Han, J. -Y. Choi, A. Udalski, T. Sumi, A. Gould, V. Bozza, M. Dominik, P. Fouqué, K. Horne, [......], C. Coutures, S. Dieters, D. Dominis Prester, J. Donatowicz, J. Greenhill, D. Kubas, J. -B. Marquette, J. W. Menzies, K. C. Sahu, M. Zub
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  ABSTRACT: Despite astrophysical importance of binary star systems, detections are limited to those located in small ranges of separations, distances, and masses and thus it is necessary to use a variety of observational techniques for a complete view of stellar multiplicity across a broad range of physical parameters. In this paper, we report the detections and measurements of 2 binaries discovered from observations of microlensing events MOA-2011-BLG-090 and OGLE-2011-BLG-0417. Determinations of the binary masses are possible by simultaneously measuring the Einstein radius and the lens parallax. The measured masses of the binary components are 0.43 $M_{\odot}$ and 0.39 $M_{\odot}$ for MOA-2011-BLG-090 and 0.57 $M_{\odot}$ and 0.17 $M_{\odot}$ for OGLE-2011-BLG-0417 and thus both lens components of MOA-2011-BLG-090 and one component of OGLE-2011-BLG-0417 are M dwarfs, demonstrating the usefulness of microlensing in detecting binaries composed of low-mass components. From modeling of the light curves considering full Keplerian motion of the lens, we also measure the orbital parameters of the binaries. The blended light of OGLE-2011-BLG-0417 comes very likely from the lens itself, making it possible to check the microlensing orbital solution by follow-up radial-velocity observation. For both events, the caustic-crossing parts of the light curves, which are critical for determining the physical lens parameters, were resolved by high-cadence survey observations and thus it is expected that the number of microlensing binaries with measured physical parameters will increase in the future.
  04/2012;
• Article: Planetary and Other Short Binary Microlensing Events from the MOA Short Event Analysis

  D. P. Bennett, T. Sumi, I. A. Bond, K. Kamiya, F. Abe, C. S. Botzler, A. Fukui, K. Furusawa, Y. Itow, A. V. Korpela, [......], Y. Muraki, K. Ohnishi, N. J. Rattenbury, To. Saito, D. J. Sullivan, D. Suzuki, W. L. Sweatman, P. J. Tristram, K. Wada, P. C. M. Yock
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  ABSTRACT: We present the analysis of four candidate short duration binary microlensing events from the 2006-2007 MOA Project short event analysis. These events were discovered as a byproduct of an analysis designed to find short timescale single lens events that may be due to free-floating planets. Three of these events are determined to be microlensing events, while the fourth is most likely caused by stellar variability. For each of the three microlensing events, the signal is almost entirely due to a brief caustic feature with little or no lensing attributable mainly to the lens primary. One of these events, MOA-bin-1, is due to a planet, and it is the first example of a planetary event in which stellar host is only detected through binary microlensing effects. The mass ratio and separation are q = 4.9 +- 1.4 x 10^{-3} and s = 2.10 +- 0.05, respectively. A Bayesian analysis based on a standard Galactic model indicates that the planet, MOA-bin-1Lb, has a mass of m_p = 3.7 +- 2.1 M_{Jup}, and orbits a star of M_* = 0.75{+0.33 -0.41} M_solar at a semi-major axis of a = 8.3 {+4.5 -2.7} AU. This is one of the most massive and widest separation planets found by microlensing. The scarcity of such wide separation planets also has implications for interpretation of the isolated planetary mass objects found by this analysis. If we assume that we have been able to detect wide separation planets with a efficiency at least as high as that for isolated planets, then we can set limits on the distribution on planets in wide orbits. In particular, if the entire isolated planet sample found by Sumi et al. (2011) consists of planets bound in wide orbits around stars, we find that it is likely that the median orbital semi-major axis is > 30 AU.
  03/2012;
• Article: OGLE-2008-BLG-510: first automated real-time detection of a weak microlensing anomaly - brown dwarf or stellar binary?

  V. Bozza, M. Dominik, N. J. Rattenbury, U. G. Joergensen, Y. Tsapras, D. M. Bramich, A. Udalski, I. A. Bond, C. Liebig, A. Cassan, [......], T. Sumi, D. Suzuki, W. L. Sweatman, P. J. Tristram, K. Wada, P. C. M. Yock, A. Gulbis, Y. Hashimoto, A. Kniazev, P. Vaisanen
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  ABSTRACT: The microlensing event OGLE-2008-BLG-510 is characterised by an evident asymmetric shape of the peak, promptly detected by the ARTEMiS system in real time. The skewness of the light curve appears to be compatible both with binary-lens and binary-source models, including the possibility that the lens system consists of an M dwarf orbited by a brown dwarf. The detection of this microlensing anomaly and our analysis demonstrates that: 1) automated real-time detection of weak microlensing anomalies with immediate feedback is feasible, efficient, and sensitive, 2) rather common weak features intrinsically come with ambiguities that are not easily resolved from photometric light curves, 3) a modelling approach that finds all features of parameter space rather than just the `favourite model' is required, and 4) the data quality is most crucial, where systematics can be confused with real features, in particular small higher-order effects such as orbital motion signatures. It moreover becomes apparent that events with weak signatures are a silver mine for statistical studies, although not easy to exploit. Clues about the apparent paucity of both brown-dwarf companions and binary-source microlensing events might hide here.
  03/2012;