Journal of The Korean Astronomical Society (천문학회지)

The Korean Astronomical Society (한국천문학회)
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KMT-2016-BLG-0212: FIRST KMTNET-ONLY DISCOVERY OF A SUBSTELLAR COMPANION

  • Hwang, K.H. (Korea Astronomy and Space Science Institute) ;
  • Kim, H.W. (Korea Astronomy and Space Science Institute) ;
  • Kim, D.J. (Korea Astronomy and Space Science Institute) ;
  • Gould, A. (Korea Astronomy and Space Science Institute) ;
  • Albrow, M.D. (Department of Physics and Astronomy, University of Canterbury) ;
  • Chung, S.J. (Korea Astronomy and Space Science Institute) ;
  • Han, C. (Department of Physics, Chungbuk National University) ;
  • Jung, Y.K. (Korea Astronomy and Space Science Institute) ;
  • Ryu, Y.H. (Korea Astronomy and Space Science Institute) ;
  • Shin, I.G. (Harvard-Smithsonian CfA) ;
  • Shvartzvald, Y. (Jet Propulsion Laboratory, California Institute of Technology) ;
  • Yee, J.C. (Harvard-Smithsonian CfA) ;
  • Zang, W. (Physics Department and Tsinghua Centre for Astrophysics, Tsinghua University) ;
  • Zhu, W. (Canadian Institute for Theoretical Astrophysics, University of Toronto) ;
  • Cha, S.M. (Korea Astronomy and Space Science Institute) ;
  • Kim, S.L. (Korea Astronomy and Space Science Institute) ;
  • Lee, C.U. (Korea Astronomy and Space Science Institute) ;
  • Lee, D.J. (Korea Astronomy and Space Science Institute) ;
  • Lee, Y. (Korea Astronomy and Space Science Institute) ;
  • Park, B.G. (Korea Astronomy and Space Science Institute) ;
  • Pogge, R.W. (Department of Astronomy, Ohio State University)
  • Received : 2018.04.17
  • Accepted : 2018.12.06
  • Published : 2018.12.31
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Abstract

We present the analysis of KMT-2016-BLG-0212, a low flux-variation ($I_{flux-var}{\sim}20mag$) microlensing event, which is in a high-cadence (${\Gamma}=4hr^{-1}$) field of the three-telescope Korea Microlensing Telescope Network (KMTNet) survey. The event shows a short anomaly that is incompletely covered due to the brief visibility intervals that characterize the early microlensing season when the anomaly occurred. We show that the data are consistent with two classes of solutions, characterized respectively by low-mass brown-dwarf (q = 0.037) and sub-Neptune (q < $10^{-4}$) companions. Future high-resolution imaging should easily distinguish between these solutions.

Keywords

CMHHBA_2018_v51n6_197_f0001.tif 이미지

Figure 1. Lightcurve with single lens (1L1S) model for KMTNet observations of KMT-2016-BLG-0212. The top panel shows the caustic crossing, which is excluded from the fit, while the middle panel shows surrounding regions. The magnitude of the flux variation, Iflux−var ≡ −2.5 log(10−0.4Ipeak− 10−0.4Ibase) ∼ 20 is quite low by the standards of published microlensing events. Here Ipeak = 18.8 and Ibase = 19.2 are respectively the peak and baseline of the underlying Paczy´nski (1986) event.

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Figure 2. Results of the grid search over s, q and α. While s and q are fixed in the grid search, α is allowed to vary from its initial seed position. The colors indicate Δχ2 ≤ (nσ)2 where n = 3 and σ =[1 (black), 2 (yellow), 3 (green), 4 (blue), 5 (purple)] relative to the best grid point (e.g., green corresponds to 36 ≤ Δχ2 < 81). The X’s indicate the locations of the nine refined solutions. All of the solutions are topologically isolated except for Wide (2a,2b,3) in the lower-right of the (s, q) panel. See Figure 3. Thus, theirX’s sometimes overlap.

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Figure 3. Caustic-chirality diagram of three solutions: Wide (2a, 2b, 3). The abscissa is the (signed) impact parameter of the source trajectory relatively to the center of the caustic. Like the other six solutions shown in Figure 2, Wide 3 is topologically isolated. However, Wide 2a and Wide 2b are weak local minima separated by a “barrier” whose height is only Δχ2 ∼ 5 within a long valley in mass ratio q. Δχ2 < (1, 4, 9, 16, 25) is marked in (red, yellow, green, blue, and purple, respectively.

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Figure 4. Source trajectory and caustic geometries for nine solutions, representing seven different topologies.

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Figure 5. Zoom of fits for nine different model geometries of KMT-2016-BLG-0212 over the anomaly. Solutions “close 4” and “wide 4” have poor fits and are excluded.

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Figure 6. Comparison of nine models (without data) that are broadly compatible with the data. The upper panel is a zoom of the region near the caustic.

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Figure 7. Residuals of the data relative to the nine models shown in Figure 6. The quite poor match of model “close 2” during the 1.5 days centered on HJD′ ∼ 7473.5 explains the high χ2 of this model. The origin of the relatively high χ2 of “close 3” model is the systematic deviation of the model in KMTA data near HJD′ ∼ 7471.2. The mismatch of models “close 4” and “wide 4” are noticeable here but are more apparent in Figure 5.

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Figure 8. Color-magnitude diagram (CMD) from py-DIA reductions of KMTNet data, calibrated to OGLE-III (Szymański et al. 2011). The positions of the clump and of the source for two of the solutions (“close 1” and “wide 2a”) are shown. The source positions for the other two “Class II” solutions (“wide 2b” and “wide 3”) are nearly identical to “wide 2a”.

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Figure 9. Bayesian estimates, based on the Han & Gould (1995) Galactic model for the host mass and system distance of KMT-2016-BLG-0212 for the “close 1” (BD-class) solution. The distributions are quite broad.

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Figure 10. Bayesian estimates, based on the Han & Gould (1995) Galactic model, for the host mass and system distance of KMT-2016-BLG-0212 for the “wide 2a” (sub-Neptune-class) solution. The distributions are qualitatively similar to those of the “close 1” solution (Figure 9). They are also extremely similar to the distributions for the “wide 2b” and “wide 3” solutions, for which reason these latter are not shown.

Table 1 Parameters for scaling data errorbars

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Table 2 Lensing parameters of close models

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Table 3 Lensing parameters of wide models

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Table 4 Physical properties

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References

  1. Albrow, M. D., Horne, K., Bramich, D. M., et al. 2009, Difference Imaging Photometry of Blended Gravitational Microlensing Events with a Numerical Kernel, MNRAS, 397, 2099 https://doi.org/10.1111/j.1365-2966.2009.15098.x
  2. Beaulieu, J.-P. Bennett, D. P., Fouque, P., et al. 2006, Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing, Nature, 439, 437
  3. Bensby, T., Yee, J. C., Feltzing, S., et al. 2013, 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, A&A, 549, 147 https://doi.org/10.1051/0004-6361/201220678
  4. Bessell, M. S., & Brett, J. M. 1988, JHKLM Photometry - Standard Systems, Passbands, and Intrinsic Colors, PASP, 100, 1134 https://doi.org/10.1086/132281
  5. Chabrier, G. 2003, Galactic Stellar and Substellar Initial Mass Function, PASP, 115, 763 https://doi.org/10.1086/376392
  6. Gould, A., & Horne, K. 2013, Kepler-Like Multi-Plexing for Mass Production of Microlens Parallaxes, ApJ, 779, L28 https://doi.org/10.1088/2041-8205/779/2/L28
  7. Gould, A., & Loeb, A. 1992, Discovering Planetary Systems Through Gravitational Microlenses, ApJ, 396, 104 https://doi.org/10.1086/171700
  8. Gould, A., Udalski, A., An, D., et al. 2006, Microlens OGLE-2005-BLG-169 Implies Cool Neptune-Like Planets are Common, ApJ, 644, L37 https://doi.org/10.1086/505421
  9. Gould, A., Dong, S., Gaudi, B. S., et al. 2010, Frequency of Solar-Like Systems and of Ice and Gas Giants Beyond the Snow Line from High-Magnification Microlensing Events in 2005-2008, ApJ, 720, 1073 https://doi.org/10.1088/0004-637X/720/2/1073
  10. Griest, K., & Safizadeh, N. 1998, The Use of High- Magnification Microlensing Events in Discovering Extrasolar Planets, ApJ, 500, 37
  11. Han, C. 2006, Properties of Planetary Caustics in Gravitational Microlensing, ApJ, 638, 1080 https://doi.org/10.1086/498937
  12. Han, C., & Gould, A. 1995, The Mass Spectrum of MACHOs From Parallax Measurements, ApJ, 447, 53 https://doi.org/10.1086/175856
  13. Han, C., Udalski, A., & Gould, A. 2017, OGLE-2016-BLG-0613LABb: A Microlensing Planet in a Binary System, AJ, 154, 223 https://doi.org/10.3847/1538-3881/aa9179
  14. Henderson, C. B., Gaudi, B. S., Han, C., et al. 2014, Optimal Survey Strategies and Predicted Planet Yields for the Korean Microlensing Telescope Network, ApJ, 794, 52 https://doi.org/10.1088/0004-637X/794/1/52
  15. Henderson, C. B., Poleski, R., Penny, M., et al. 2016, Campaign 9 of the K2 Mission: Observational Parameters, Scientific Drivers, and Community Involvement for a Simultaneous Space- and Ground-Based Microlensing Survey, PASP, 128, 124401 https://doi.org/10.1088/1538-3873/128/970/124401
  16. Hwang, K.-H., Udalski, A., Shvartzvald, Y., et al. 2018, OGLE-2017-BLG-0173Lb: Low Mass-Ratio Planet in a "Hollywood" Microlensing Event, AJ, 155, 20
  17. Jung, Y. K., Udalski, A., Sumi, T., et al. 2015, OGLE-2013-BLG-0102LA,B: Microlensing Binary with Components at Star/Brown Dwarf and Brown Dwarf/Planet Boundaries, ApJ, 798, 123 https://doi.org/10.1088/0004-637X/798/2/123
  18. Kervella, P., Thevenin, F., Di Folco, E., & Segransan, D. 2004, The Angular Sizes of Dwarf Stars and Subgiants. Surface Brightness Relations Calibrated by Interferometry, A&A, 426, 297 https://doi.org/10.1051/0004-6361:20035930
  19. Kim, S.-L., Lee, C.-U., Park, B.-G., et al. 2016, KMTNet: A Network of 1.6m Wide-Field Optical Telescopes Installed at Three Southern Observatories, JKAS, 49, 37
  20. Kim, D.-J., Kim, H.-W., Hwang, K.-H., et al. 2018a, Korea Microlensing Telescope Network Microlensing Events from 2015: Event-Finding Algorithm, Vetting, and Photometry, AJ, 155, 76
  21. Kim, H.-W., Hwang, K.-H., Kim, D.-J., et al. 2018b, The KMTNet/K2-C9 (Kepler) Data Release, AJ, 155, 186 https://doi.org/10.3847/1538-3881/aab76c
  22. Nataf, D.M., Gould, A., Fouque, P., et al. 2013, Reddening and Extinction Toward the Galactic Bulge from OGLEIII: The Inner Milky Way's Rv - 2.5 Extinction Curve, ApJ, 769, 88 https://doi.org/10.1088/0004-637X/769/2/88
  23. Paczynski, B. 1986, Gravitational Microlensing by the Galactic Halo, ApJ, 304, 1 https://doi.org/10.1086/164140
  24. Poleski, R., Udalski, A., Dong, S., et al. 2014, Super-Massive Planets around Late-Type Stars-the Case of OGLE-2012-BLG-0406Lb, ApJ, 782, 47 https://doi.org/10.1088/0004-637X/782/1/47
  25. Schechter, P. L., Mateo, M., & Saha, A. 1993, DOPHOT, a CCD Photometry Program: Description and Tests, PASP, 105, 1342 https://doi.org/10.1086/133316
  26. Shvartzvald, Y., Maoz, D., Udalski, A., et al. 2016, The Frequency of Snowline-Region Planets from Four Years of OGLE-MOA-Wise Second-Generation Microlensing, MNRAS, 457, 408
  27. Skowron, J., Ryu, Y.-H., Hwang, K.-H., et al. 2018, OGLE- 2017-BLG-0373Lb: A Jovian Mass-Ratio Planet Exposes A New Accidental Microlensing Degeneracy, Acta Astronomica, 68, 43
  28. Sumi, T., Bennett, D. P., Bond, I. A., et al. 2010, A Cold Neptune-Mass Planet OGLE-2007-BLG-368Lb: Cold Neptunes Are Common, ApJ, 710, 1641 https://doi.org/10.1088/0004-637X/710/2/1641
  29. Suzuki, D., Bennett, D. P., Sumi, T., et al. 2016, The Exoplanet Mass-Ratio Function from the MOA-II Survey: Discovery of a Break and Likely Peak at a Neptune Mass, ApJ, 833, 145 https://doi.org/10.3847/1538-4357/833/2/145
  30. Szymanski, M. K., Udalski, A., Soszynski, I., et al. 2011, The Optical Gravitational Lensing Experiment. OGLEIII Photometric Maps of the Galactic Bulge Fields, Acta Astron., 61, 83
  31. Udalski, A.,Szymanski, M., Kaluzny, J., et al. 1994, The Optical Gravitational Lensing Experiment. The Early Warning System: Real Time Microlensing, Acta Astron., 44, 227
  32. Udalski, A., Jaroszynski, M., Paczynski, B., et al. 2005, A Jovian-Mass Planet in Microlensing Event OGLE-2005-BLG-071, ApJ, 628, L109 https://doi.org/10.1086/432795
  33. Udalski, A., Ryu, Y.-H., Sajadian, S., et al. 2018, OGLE- 2017-BLG-1434Lb: Eighth q < 1 $\times$ 10-4 Mass-Ratio Microlens Planet Confirms Turnover in Planet Mass-Ratio Function, Acta Astron., 68, 1
  34. Wozniak, P. R. 2000, Difference Image Analysis of the OGLE-II Bulge Data. I. The Method, Acta Astron., 50, 421