Chung-Uk Lee

Bio: Chung-Uk Lee is an academic researcher from Max Planck Society. The author has contributed to research in topics: Gravitational microlensing & Light curve. The author has an hindex of 14, co-authored 34 publications receiving 571 citations. Previous affiliations of Chung-Uk Lee include Korea Astronomy and Space Science Institute & University of Toronto.

Korea Microlensing Telescope Network Microlensing Events from 2015: Event-finding Algorithm, Vetting, and Photometry

Abstract: We present microlensing events in the 2015 Korea Microlensing Telescope Network (KMTNet) data and our procedure for identifying these events. In particular, candidates were detected with a novel "completed event" microlensing event-finder algorithm. The algorithm works by making linear fits to a (t0,teff,u0) grid of point-lens microlensing models. This approach is rendered computationally efficient by restricting u0 to just two values (0 and 1), which we show is quite adequate. The implementation presented here is specifically tailored to the commission-year character of the 2015 data, but the algorithm is quite general and has already been applied to a completely different (non-KMTNet) data set. We outline expected improvements for 2016 and future KMTNet data. The light curves of the 660 "clear microlensing" and 182 "possible microlensing" events that were found in 2015 are presented along with our policy for their public release.

A Neptune-mass Free-floating Planet Candidate Discovered by Microlensing Surveys

Abstract: Current microlensing surveys are sensitive to free-floating planets down to Earth-mass objects. All published microlensing events attributed to unbound planets were identified based on their short timescale (below two days), but lacked an angular Einstein radius measurement (and hence lacked a significant constraint on the lens mass). Here, we present the discovery of a Neptune-mass free-floating planet candidate in the ultrashort ($t_{\\rm E}=0.320\\pm0.003$ days) microlensing event OGLE-2016-BLG-1540. The event exhibited strong finite-source effects, which allowed us to measure its angular Einstein radius of $\\theta_{\\rm E}=9.2\\pm0.5\\,\\mu$as. There remains, however, a degeneracy between the lens mass and distance. The combination of the source proper motion and source-lens relative proper motion measurements favors a Neptune-mass lens located in the Galactic disk. However, we cannot rule out that the lens is a Saturn-mass object belonging to the bulge population. We exclude stellar companions up to 15 au.

Two new free-floating or wide-orbit planets from microlensing

Abstract: Planet formation theories predict the existence of free-floating planets that have been ejected from their parent systems. Although they emit little or no light, they can be detected during gravitational microlensing events. Microlensing events caused by rogue planets are characterized by very short timescales t E (typically below two days) and small angular Einstein radii θ E (up to several μ as). Here we present the discovery and characterization of two ultra-short microlensing events identified in data from the Optical Gravitational Lensing Experiment (OGLE) survey, which may have been caused by free-floating or wide-orbit planets. OGLE-2012-BLG-1323 is one of the shortest events discovered thus far (t E = 0.155 ± 0.005 d, θ E = 2.37 ± 0.10μ as) and was caused by an Earth-mass object in the Galactic disk or a Neptune-mass planet in the Galactic bulge. OGLE-2017-BLG-0560 (t E = 0.905 ± 0.005 d, θ E = 38.7 ± 1.6μ as) was caused by a Jupiter-mass planet in the Galactic disk or a brown dwarf in the bulge. We rule out stellar companions up to a distance of 6.0 and 3.9 au, respectively. We suggest that the lensing objects, whether located on very wide orbits or free-floating, may originate from the same physical mechanism. Although the sample of ultrashort microlensing events is small, these detections are consistent with low-mass wide-orbit or unbound planets being more common than stars in the Milky Way.

THE ECLIPSING SYSTEM V404 LYR: LIGHT-TRAVEL TIMES AND γ DORADUS PULSATIONS

Abstract: We present the physical properties of V404 Lyr exhibiting eclipse timing variations and multiperiodic pulsations from all historical data including the Kepler and SuperWASP observations. Detailed analyses of 2922 minimum epochs showed that the orbital period has varied through a combination of an upward-opening parabola and two sinusoidal variations, with periods of P {sub 3} = 649 days and P {sub 4} = 2154 days and semi-amplitudes of K {sub 3} = 193 s and K {sub 4} = 49 s, respectively. The secular period increase at a rate of +1.41 × 10{sup –7} days yr{sup –1} could be interpreted as a combination of the secondary to primary mass transfer and angular momentum loss. The most reasonable explanation for both sinusoids is a pair of light-travel-time effects due to two circumbinary objects with projected masses of M {sub 3} = 0.47 M {sub ☉} and M {sub 4} = 0.047 M {sub ☉}. The third-body parameters are consistent with those calculated using the Wilson-Devinney binary code. For the orbital inclinations i {sub 4} ≳ 43°, the fourth component has a mass within the hydrogen-burning limit of ∼0.07 M {sub ☉}, which implies that it is a brown dwarf. A satisfactory more » model for the Kepler light curves was obtained by applying a cool spot to the secondary component. The results demonstrate that the close eclipsing pair is in a semi-detached, but near-contact, configuration; the primary fills approximately 93% of its limiting lobe and is larger than the lobe-filling secondary. Multiple frequency analyses were applied to the light residuals after subtracting the synthetic eclipsing curve from the Kepler data. This revealed that the primary component of V404 Lyr is a γ Dor type pulsating star, exhibiting seven pulsation frequencies in the range of 1.85-2.11 day{sup –1} with amplitudes of 1.38-5.72 mmag and pulsation constants of 0.24-0.27 days. The seven frequencies were clearly identified as high-order low-degree gravity-mode oscillations which might be excited through tidal interaction. Only eight eclipsing binaries have been known to contain γ Dor pulsating components and, therefore, V404 Lyr will be an important test bed for investigating these rare and interesting objects. « less

KMT-2017-BLG-0165Lb: A Super-Neptune-mass Planet Orbiting a Sun-like Host Star

Abstract: We report the discovery of a low-mass-ratio planet (q = 1.3 × 10^(−4)), i.e., 2.5 times higher than the Neptune/Sun ratio. The planetary system was discovered from the analysis of the KMT-2017-BLG-0165 microlensing event, which has an obvious short-term deviation from the underlying light curve produced by the host of the planet. Although the fit improvement with the microlens parallax effect is relatively low, one component of the parallax vector is strongly constrained from the light curve, making it possible to narrow down the uncertainties of the lens physical properties. A Bayesian analysis yields that the planet has a super-Neptune mass (M_2 = 34^(+15)_(-12) M⊕) orbiting a Sun-like star (M_1 = 0.76^(+0.34)_(-0.27) M⊙) located at 4.5 kpc. The blended light is consistent with these host properties. The projected planet-host separation is a⊥ = 3.45^(+0.98)_(-0.95) au, implying that the planet is located outside the snow line of the host, i.e., a_(sl) ~ 2.1 au. KMT-2017-BLG-0165Lb is the sixteenth microlensing planet with mass ratio q 3.3, is quite severe. Alternatively, the distribution is also suggestive of a pileup of planets at Neptune-like mass ratios, below which there is a dramatic drop in frequency.

Constraints on Earth-mass primordial black holes from OGLE 5-year microlensing events

Abstract: We constrain the abundance of primordial black holes (PBH) using 2622 microlensing events obtained from 5-years observations of stars in the Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE). The majority of microlensing events display a single or at least continuous population that has a peak around the light curve timescale ${t}_{\mathrm{E}}\ensuremath{\simeq}20\text{ }\text{ }\mathrm{days}$ and a wide distribution over the range ${t}_{\mathrm{E}}\ensuremath{\simeq}[1,300]\text{ }\text{ }\mathrm{days}$, while the data also indicates a second population of 6 ultrashort-timescale events in ${t}_{\mathrm{E}}\ensuremath{\simeq}[0.1,0.3]\text{ }\text{ }\mathrm{days}$, which are advocated to be due to free-floating planets. We confirm that the main population of OGLE events can be well modeled by microlensing due to brown dwarfs, main sequence stars and stellar remnants (white dwarfs and neutron stars) in the standard Galactic bulge and disk models for their spatial and velocity distributions. Using the dark matter (DM) model for the Milky Way (MW) halo relative to the Galactic bulge/disk models, we obtain the tightest upper bound on the PBH abundance in the mass range ${M}_{\mathrm{PBH}}\ensuremath{\simeq}[{10}^{\ensuremath{-}6},{10}^{\ensuremath{-}3}]\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ (Earth-Jupiter mass range), if we employ the ``null hypothesis'' that the OGLE data does not contain any PBH microlensing event. More interestingly, we also show that Earth-mass PBHs can well reproduce the 6 ultrashort-timescale events, without the need of free-floating planets, if the mass fraction of PBH to DM is at a per cent level, which is consistent with other constraints such as the microlensing search for Andromeda galaxy (M31) and the longer timescale OGLE events. Our result gives a hint of PBH existence, and can be confirmed or falsified by microlensing search for stars in M31, because M31 is towards the MW halo direction and should therefore contain a much less number of free-floating planets, even if exist, than the direction to the MW center.

Detecting Earth-Mass Planets with Gravitational Microlensing

Abstract: We show that Earth mass planets orbiting stars in the Galactic disk and bulge can be detected by monitoring microlensed stars in the Galactic bulge. The star and its planet act as a binary lens which generates a lightcurve which can differ substantially from the lightcurve due only to the star itself. We show that the planetary signal remains detectable for planetary masses as small as an Earth mass when realistic source star sizes are included in the lightcurve calculation. These planets are detectable if they reside in the ``lensing zone" which is centered between 1 and 4 AU from the lensing star and spans about a factor of 2 in distance. If we require a minimum deviation of 4\% from the standard point-lens microlensing lightcurve, then we find that more than 2\% of all $\mearth$ planets and 10\% of all $10\mearth$ in the lensing zone can be detected. If a third of all lenses have no planets, a third have $1\mearth$ planets and the remaining third have $10\mearth$ planets then we estimate that an aggressive ground based microlensing planet search program could find one earth mass planet and half a dozen $10\mearth$ planets per year.

A comprehensive study of the Kepler triples via eclipse timing

Abstract: We produce and analyze eclipse time variation (ETV) curves for some 2600 Kepler binaries. We find good to excellent evidence for a third body in 222 systems via either the light-travel-time (LTTE) or dynamical effect delays. Approximately half of these systems have been discussed in previous work, while the rest are newly reported here. Via detailed analysis of the ETV curves using high-level analytic approximations, we are able to extract system masses and information about the three-dimensional characteristics of the triple for 62 systems which exhibit both LTTE and dynamical delays; for the remaining 160 systems we give improved LTTE solutions. New techniques of preprocessing the flux time series are applied to eliminate false positive triples and to enhance the ETV curves. The set of triples with outer orbital periods shorter than ~2000 days is now sufficiently numerous for meaningful statistical analysis. We find that (i) as predicted, there is a peak near i_m~40 deg in the distribution of the triple vs. inner binary mutual inclination angles that provides strong confirmation of the operation of Kozai-Lidov cycles with tidal friction; (ii) the median eccentricity of the third-body orbits is e_2=0.35; (iii) there is a deficit of triple systems with binary periods <1 day and outer periods between ~50 and 200 days which might help guide the refinement of theories of the formation and evolution of close binaries; and (iv) the substantial fraction of Kepler binaries which have third-body companions is consistent with a very large fraction of all binaries being part of triples.

Catalogue and Properties of δ Scuti Stars in Binaries

Abstract: The catalogue contains 199 confirmed cases of binary systems containing at least one pulsating component of {\delta} Scuti type The sample is divided into subgroups in order to describe the properties and characteristics of the {\delta} Sct type stars in binaries according to their Roche geometry Demographics describing quantitatively our knowledge for these systems as well as the distributions of their pulsating components in the Mass-Radius, Colour-Magnitude and Evolutionary Status-Temperature diagrams are presented and discussed It is shown that a threshold of ~13 days of the orbital period regarding the influence of binarity on the pulsations is established Finally, the correlations between the pulsation periods and the orbital periods, evolutionary status, and companion's gravity influence are updated based on the largest sample to date

Planetary System Disruption by Galactic Perturbations to Wide Binary Stars

Abstract: Numerical simulations of a widely separated binary star system demonstrate that planetary systems around one star may often be strongly perturbed by the other star, triggering planetary ejections and increasing the orbital eccentricities of surviving planets. Most stars are found in binary systems, so an understanding of how stellar companions affect planetary dynamics and formation is important for exoplanet research. Previous studies have assumed that stellar companions of more than 1,000 AU ('wide binaries') are inconsequential. This study demonstrates the opposite: external galactic perturbations that are insignificant in tighter binaries drive wide binaries to systematically disrupt planetary systems. The results suggest that although wide binaries eventually truncate their planetary systems, most isolated giant exoplanet systems harbour additional distant, still undetected planets. Nearly half the exoplanets found within binary star systems reside1 in very wide binaries with average stellar separations greater than 1,000 astronomical units (one astronomical unit (au) being the Earth–Sun distance), yet the influence of such distant binary companions on planetary evolution remains largely unstudied. Unlike their tighter counterparts, the stellar orbits of wide binaries continually change under the influence of the Milky Way’s tidal field and impulses from other passing stars. Here we report numerical simulations demonstrating that the variable nature of wide binary star orbits dramatically reshapes the planetary systems they host, typically billions of years after formation. Contrary to previous understanding2, wide binary companions may often strongly perturb planetary systems, triggering planetary ejections and increasing the orbital eccentricities of surviving planets. Although hitherto not recognized, orbits of giant exoplanets within wide binaries are statistically more eccentric than those around isolated stars. Both eccentricity distributions are well reproduced when we assume that isolated stars and wide binaries host similar planetary systems whose outermost giant planets are scattered beyond about 10 au from their parent stars by early internal instabilities. Consequently, our results suggest that although wide binaries eventually remove the most distant planets from many planetary systems, most isolated giant exoplanet systems harbour additional distant, still undetected planets.