Korea Microlensing Telescope Network Research Articles

KMT-2018-BLG-1990Lb: A Nearby Jovian Planet From A Low-cadence Microlensing Field

Abstract We report the discovery of KMT-2018-BLG-1990Lb, a Jovian planet orbiting a late M dwarf at a distance of , and projected at 2.6 ± 0.6 times the snow line distance, i.e., . This is the second Jovian planet discovered by the Korea Microlensing Telescope Network (KMTNet) in its low-cadence (0.4 hr−1) fields, demonstrating that this population can be probed based on relatively low-cadence survey-only microlensing data. However, while many Jovian-planet microlensing events will not have caustic crossings (e.g., the previous case, KMT-2016-BLG-1397) and some that do will be well characterized by low-cadence survey observations (e.g., the current case), in general higher-cadence follow-up observations of the caustic crossings are preferable. Such follow-up observations require microlensing alerts, which KMTNet now provides.

KSP-OT-201611a: A Distant Population II Dwarf Nova Candidate Discovered by the KMTNet Supernova Program

Abstract We present a multicolor, high-cadence photometric study of a distant dwarf nova KSP-OT-201611a discovered by the Korea Microlensing Telescope Network Supernova Program. From 2016 October to 2017 May, two outbursts, which comprise a super/long outburst followed by a normal/short outburst separated by ∼91 days, were detected in the BVI bands. The shapes and amplitudes of the outbursts reveal the nature of KSP-OT-201611a to be an SU UMa- or U Gem-type dwarf nova. Color variations of periodic humps in the super/long outburst possibly indicate that KSP-OT-201611a is an SU UMa-type dwarf nova. The super and normal outbursts show distinctively different color evolutions during the outbursts due most likely to the difference in time when the cooling wave is formed in the accretion disk. The outburst peak magnitudes and the orbital period of the dwarf nova indicate that it is at a large Galactocentric distance (∼13.8 kpc) and height (∼1.7 kpc) from the Galactic plane. KSP-OT-201611a, therefore, may provide a rare opportunity to study the accretion disk process of Population II dwarf novae.

Candidates for RR Lyrae in binary systems from the OGLE Galactic bulge survey

ABSTRACT We present 20 newly discovered candidates for binary systems with an RR Lyrae companion. Using the photometric data from the Optical Gravitational Lensing Experiment (OGLE) and Korea Microlensing Telescope Network (KMTNet) for the Galactic bulge, we searched for binary candidates among non-modulated fundamental-mode RR Lyrae variables. We constructed and investigated over 9000 O − C diagrams for individual pulsators, in order to find signs of the light-travel time effect. We found 20 variables exhibiting cyclic variation in the O − C (time of the observed minus calculated brightness maximum) diagram, which can be associated with a second component in the system, but for confirmation of binarity, a long-term radial velocity study is necessary. The modelling of the O − C diagrams yields orbital parameters, estimates of the semi-amplitude of the radial velocity curve, and the mass function. The orbital periods of our candidates range from 3 to 15 yr. Two of the binary candidates display a minimum mass higher than the assumed mass of the RR Lyrae component, which points towards an evolved companion that can under some circumstances contribute significantly to the total light of the system.

RR Lyrae Stars in the Field of Sagittarius II

Abstract We present the detection of RR Lyrae variable stars in the field of the Sagittarius II (Sgr II) ultra-faint dwarf (UFD) galaxy. Using B, V time-series photometry obtained with the Korea Microlensing Telescope Network (KMTNet) 1.6 m telescope at CTIO and G-band data from Gaia Data Release 2 (DR2), we identified and characterized two ab-type and four c-type RR Lyrae variables. Five out of the six stars are clustered within three half-light radii (∼4.′8) of the galaxy indicating their association with Sgr II, while the RRab star V4 is located ∼22′ from the galaxy center. By excluding V4, the high c-type fraction (0.8) and the period of the only RRab star V3 (P V3 ≃ 0.666 days) suggest an Oosterhoff II (Oo II) classification for Sgr II. Located close to the locus of Oo II clusters in the period–amplitude diagram, V3 is similar to RRab stars in other UFDs having Oosterhoff-intermediate and Oo II properties. Sgr II is, however, more compact than usual UFDs, placed in between star clusters and dwarf galaxies in the size–luminosity plane, and therefore spectroscopic studies are eventually required to ascertain the true nature of this stellar system. We derive the metallicity ([Fe/H]RRab ≃ −2.1 ± 0.3) and heliocentric distance (∼64 ± 3 kpc) of Sgr II from the RR Lyrae stars, and estimate its age (∼12 Gyr) based on our stellar population models. The Oosterhoff properties of UFDs can be explained with the evolution effect of RR Lyrae stars in the instability strip.

Two Jupiter-mass Planets Discovered by the KMTNet Survey in 2017

Abstract We report two microlensing events, KMT-2017-BLG-1038 and KMT-2017-BLG-1146, that are caused by planetary systems. These events were discovered by Korea Microlensing Telescope Network survey observations from the 2017 bulge season. The discovered systems consist of a planet and host star with mass ratios of and , respectively. Based on a Bayesian analysis assuming a Galactic model without stellar remnant hosts, we find that the planet KMT-2017-BLG-1038Lb is a super-Jupiter-mass planet ( ) orbiting a mid-M dwarf host ( ) that is located at kpc toward the Galactic bulge. The other planet, KMT-2017-BLG-1146Lb, is a sub-Jupiter-mass planet ( ) orbiting a mid-M dwarf host ( ) at a distance of kpc toward the Galactic bulge. Both are potentially gaseous planets that are beyond their hosts’ snow lines. These typical microlensing planets will be routinely discovered by second-generation microlensing surveys, rapidly increasing the number of detections.

KMTNet Nearby Galaxy Survey. I. Optimal Strategy for Low Surface Brightness Imaging with KMTNet

Abstract In hierarchical galaxy formation models, galaxies evolve through mergers and accretions. Tidally disrupted debris from these processes can remain as diffuse, faint structures, which can provide useful insight into the assembly history of galaxies. To investigate the properties of the faint structures in outskirts of nearby galaxies, we conduct deep and wide-field imaging survey with Korea Microlensing Telescope Network (KMTNet). We present our observing strategy and optimal data reduction process to recover the faint extended features in the imaging data of NGC 1291 taken with KMTNet. Through the dark sky flat-fielding and optimal sky subtraction, we can effectively remove inhomogeneous patterns. In the combined images, the peak-to-peak global sky gradients were reduced to less than ∼0.5% and ∼0.3% of the original B- and R-band sky levels, respectively. However, we find local spatial fluctuations in the background sky that can affect the precise measurement of the sky value. Consequently, we can reach the surface brightness of and mag arcsec−2 in azimuthally averaged one-dimensional surface brightness profiles, which is mainly limited by the uncertainty in the sky determination. These results suggest that the deep imaging data produced by KMTNet are suitable to study the faint features of nearby galaxies such as outer disks and dwarf companions, but unideal (not impossible) to detect stellar halos. The one-dimensional profile revealed that NGC 1291 appeared to have a Type I disk out to R ∼30 kpc with no obvious color gradient, and excess light due to a stellar halo was undetected.

KMT-2016-BLG-1397b: KMTNET-only Discovery of a Microlens Giant Planet

Abstract We report the discovery of a giant planet in the KMT-2016-BLG-1397 microlensing event, which was found by The Korea Microlensing Telescope Network alone. The timescale of this event is t E = 40.0 ± 0.5 days, and the mass ratio between the lens star and its companion is q = 0.016 ± 0.002. The planetary perturbation in the light curve is a smooth bump, resulting in the classical binary-lens/binary-source (2L1S/1L2S) degeneracy. We measure the V − I color of the (putative) two sources in the 1L2S model, and then effectively rule out the binary-source solution. The finite-source effect is marginally detected. Combined with the limits on the blend flux and the probability distribution of the source size normalized by the Einstein radius ρ, a Bayesian analysis yields the lens mass , at distance of kpc. Thus, the companion is a super-Jupiter of a mass , at a projected separation au, indicating that the planet is well beyond the snow line of the host star.

KMT-2016-BLG-1820 and KMT-2016-BLG-2142: Two Microlensing Binaries Composed of Planetary-mass Companions and Very-low-mass Primaries

Abstract We present the analyses of two short-timescale microlensing events, KMT-2016-BLG-1820 and KMT-2016-BLG-2142. In both light curves, the brief anomalies were clearly captured and densely covered by the Korea Microlensing Telescope Network survey. From these analyses, we find that both events have small Einstein radii of , suggesting that the binary-lens systems are composed of very-low-mass components and/or are located much closer to the lensed stars than to Earth. From Bayesian analyses, we find that these binaries have total system masses of and , implying that they are well within the very-low-mass regime. The estimated lens-component masses indicate that the binary lenses consist of a giant-planet/brown-dwarf pair (KMT-2016-BLG-1820) and a dark/faint object pair (KMT-2016-BLG-2140) that are located near the deuterium-burning and hydrogen-burning mass limits, respectively. Both lens systems are likely to be in the Galactic disk with estimated distances of about 6 and 7 kpc. The projected lens-components separations are 1.1 and 0.8 au, and the mass ratios are 0.11 and 0.20. These prove that the microlensing method is effective to identify these closely separated very-low-mass binaries having low mass-ratios.

Taxonomy and Light-curve Data of 1000 Serendipitously Observed Main-belt Asteroids

Abstract We present VRI spectrophotometry of 1003 main-belt asteroids (MBAs) observed with the Sutherland, South Africa node of the Korea Microlensing Telescope Network (KMTNet). All of the observed MBAs were serendipitously captured in KMTNet’s large 2° × 2° field of view during a separate targeted near-Earth Asteroid study. Our broadband spectrophotometry is reliable enough to distinguish among four asteroid taxonomies and we confidently categorize 836 of the 1003 observed targets as either a S-, C-, X-, or D-type asteroid by means of a machine learning algorithm approach. Our data show that the ratio between S-type MBAs and (C+X+D)-type MBAs, with H magnitudes between 12 and 18 (12 km ≳ diameter ≳ 0.75 km), is almost exactly 1:1. Additionally, we report 0.5–3 hr (median: 1.3 hr) light-curve data for each MBA and we resolve the complete rotation periods and amplitudes for 59 targets. Of the 59 targets, 2 have rotation periods potentially below the theoretical zero-cohesion boundary limit of 2.2 hr. We report lower limits for the rotation periods and amplitudes for the remaining targets. Using the resolved and unresolved light curves we determine the shape distribution for this population using a Monte Carlo simulation. Our model suggests a population with an average elongation b/a = 0.74 ± 0.07 and also shows that this is independent of asteroid size and taxonomy.

RR Lyrae Variables in the Crater II Dwarf Galaxy

Abstract We report the detection of RR Lyrae variable stars in Crater II, a recently discovered large and diffuse satellite dwarf galaxy of the Milky Way (MW). Based on B, V time-series photometry obtained with the Korea Microlensing Telescope Network 1.6 m telescope at Cerro Tololo Inter-American Observatory, we identified 83 ab-type and 13 c-type pulsators by fitting template light curves. The detected RR Lyrae stars are centrally concentrated, which ensures that most of them are members of Crater II. In terms of the distribution of RRab stars in the period-amplitude diagram, Crater II is clearly different from ultra-faint dwarf (UFD) galaxies, but very similar to the two classical MW dwarf spheroidal (dSph) galaxies Draco and Carina with Oosterhoff-intermediate (Oo-int) properties. Combined with the mean period of ab-type variables ( ) and the c-type fraction (∼0.14) in Crater II, this suggests an Oo-int classification for Crater II and implies that its nature is more like a dSph rather than a UFD. We also estimated the mean metallicity, reddening, and distance of Crater II, from the photometric and pulsation properties of the RR Lyrae stars. The stellar population model we have constructed indicates that Crater II is dominated by an old population, but is relatively younger than the oldest globular clusters in the MW. With a lack of high-amplitude short-period RRab stars, Crater II, like most of the other less massive dSphs, is probably not a surviving counterpart of the major building blocks of the MW halo.

Difference in Dwarf Galaxy Surface Brightness Profiles as a Function of Environment*

Abstract We investigate surface brightness profiles (SBPs) of dwarf galaxies in field, group, and cluster environments. With deep BV I images from the Korea Microlensing Telescope Network Supernova Program, SBPs of 38 dwarfs in the NGC 2784 group are fitted by a single-exponential or double-exponential model. We find that 53% of the dwarfs are fitted with single-exponential profiles (“Type I”), while 47% of the dwarfs show double-exponential profiles; 37% of all dwarfs have smaller sizes for the outer part than the inner part (“Type II”), while 10% have a larger outer than inner part (“Type III”). We compare these results with those in the field and in the Virgo cluster, where the SBP types of 102 field dwarfs are compiled from a previous study and the SBP types of 375 cluster dwarfs are measured using SDSS r-band images. As a result, the distributions of SBP types are different in the three environments. Common SBP types for the field, the NGC 2784 group, and the Virgo cluster are Type II, Type I and II, and Type I and III profiles, respectively. After comparing the sizes of dwarfs in different environments, we suggest that since the sizes of some dwarfs are changed due to environmental effects, SBP types are capable of being transformed and the distributions of SBP types in the three environments are different. We discuss possible environmental mechanisms for the transformation of SBP types.

The KMTNet/K2-C9 (Kepler) Data Release

Abstract We present Korea Microlensing Telescope Network (KMTNet) light curves for microlensing-event candidates in the Kepler K2 C9 field having peaks within three effective timescales of the Kepler observations. These include 181 “clear microlensing” and 84 “possible microlensing” events found by the KMTNet event finder, plus 56 other events found by OGLE and/or MOA that were not found by KMTNet. All data for the first two classes are immediately available for public use without restriction.

KMTNet Time-series Photometry of the Doubly Eclipsing Binary Stars Located in the Large Magellanic Cloud

We report the results of photometric observations for doubly eclipsing binaries OGLE-LMC-ECL-15674 and OGLE-LMC-ECL-22159, both of which are composed of two pairs (designated A&B) of a detached eclipsing binary located in the Large Magellanic Cloud. The light curves were obtained by high-cadence time-series photometry using the Korea Microlensing Telescope Network 1.6 m telescopes located at three southern sites (CTIO, SAAO, and SSO) between 2016 September and 2017 January. The orbital periods were determined to be 1.433 and 1.387 days for components A and B of OGLE-LMC-ECL-15674, respectively, and 2.988 and 3.408 days for OGLE-LMC-ECL-22159A and B, respectively. Our light curve solutions indicate that the significant changes in the eclipse depths of OGLE-LMC-ECL-15674A and B were caused by variations in their inclination angles. The eclipse timing diagrams of the A and B components of OGLE-LMC-ECL-15674 and OGLE-LMC-ECL-22159 were analyzed using 28, 44, 28, and 26 new times of minimum light, respectively. The apsidal motion period of OGLE-LMC-ECL-15674B was estimated by detailed analysis of eclipse timings for the first time. The detached eclipsing binary OGLE-LMC-ECL-15674B shows a fast apsidal period of 21.5 ± 0.1 years.

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 grid of point-lens microlensing models. This approach is rendered computationally efficient by restricting u 0 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.

Characterization of Near-Earth Asteroids Using KMTNET-SAAO

Abstract We present here VRI spectrophotometry of 39 near-Earth asteroids (NEAs) observed with the Sutherland, South Africa, node of the Korea Microlensing Telescope Network (KMTNet). Of the 39 NEAs, 19 were targeted, but because of KMTNet’s large 2° × 2° field of view, 20 serendipitous NEAs were also captured in the observing fields. Targeted observations were performed within 44 days (median: 16 days, min: 4 days) of each NEA’s discovery date. Our broadband spectrophotometry is reliable enough to distinguish among four asteroid taxonomies and we were able to confidently categorize 31 of the 39 observed targets as either an S-, C-, X-, or D-type asteroid by means of a Machine Learning algorithm approach. Our data suggest that the ratio between “stony” S-type NEAs and “not-stony” (C+X+D)-type NEAs, with H magnitudes between 15 and 25, is roughly 1:1. Additionally, we report ∼1 hr light curve data for each NEA, and of the 39 targets, we were able to resolve the complete rotation period and amplitude for six targets and report lower limits for the remaining targets.

PHOTOMETRIC STUDY OF NPA ROTATOR (5247) KRYLOV

We conduct BVRI and R band photometric observations of asteroid (5247) Krylov from January 2016 to April 2016 for 51 nights using the Korea Microlensing Telescope Network (KMTNet). The color indices of (5247) Krylov at the light curve maxima are determined as <TEX>$B-V=0.841{\pm}0.035$</TEX>, <TEX>$V-R=0.418{\pm}0.031$</TEX>, and <TEX>$V-I=0.871{\pm}0.031$</TEX> where the phase angle is <TEX>$14.1^{\circ}$</TEX>. They are acquired after the standardization of BVRI instrumental measurements using the ensemble normalization technique. Based on the color indices, (5247) Krylov is classified as a S-type asteroid. Double periods, that is, a primary period <TEX>$P_1=82.188{\pm}0.013h$</TEX> and a secondary period <TEX>$P_2=67.13{\pm}0.20h$</TEX> are identified from period searches of its R band light curve. The light curve phases with <TEX>$P_1$</TEX> and this indicate that it is a typical Non-Principal Axis (NPA) asteroid. We discuss the possible causes of its NPA rotation.

An Earth-mass Planet in a 1 au Orbit around an Ultracool Dwarf

Abstract We combine Spitzer and ground-based Korea Microlensing Telescope Network microlensing observations to identify and precisely measure an Earth-mass ( ) planet OGLE-2016-BLG-1195Lb at orbiting a ultracool dwarf. This is the lowest-mass microlensing planet to date. At kpc, it is the third consecutive case among the Spitzer “Galactic distribution” planets toward the Galactic bulge that lies in the Galactic disk as opposed to the bulge itself, hinting at a skewed distribution of planets. Together with previous microlensing discoveries, the seven Earth-size planets orbiting the ultracool dwarf TRAPPIST-1, and the detection of disks around young brown dwarfs, OGLE-2016-BLG-1195Lb suggests that such planets might be common around ultracool dwarfs. It therefore sheds light on the formation of both ultracool dwarfs and planetary systems at the limit of low-mass protoplanetary disks.

Preliminary results of variable star search using KMTNet data

The KMTNet (Korea Microlensing Telescope Network) was completed on October 2nd in 2015. There are three 1.6 m telescopes in Chile (CTIO), Australia (SSO) and South Africa (SAAO). The field of view of the CCD camera is 2 degree × 2 degree. We are monitoring and observing the bulge region for 24-hour continuous observations using the KMTNet telescopes. I present the characteristics of CCD camera and telescope, and the status of observation programs. We are still developing CCD cameras to accurate photometry. From the KMTNet data, we can obtain accurate 24-hour continuous time-series data for many variable stars.

TIME-SERIES PHOTOMETRY OF VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 288

We present the results of BV time-series photometry of the globular cluster NGC 288. Ob-servations were carried out to search for variable stars using the Korea Microlensing Telescope Network (KMTNet) 1.6-m telescopes and a 4k pre-science CCD camera during a test observation from August to December, 2014. We found a new SX Phe star and confirmed twelve previously known variable stars in NGC 288. For the semi-regular variable star V1, we newly determined a period of 37.3 days from light curves spanning 137 days. The light-curve solution of the eclipsing binary V10 indicates that the system is probably a detached system. The pulsation properties of nine SX Phe stars were examined by applying multiple frequency analysis to their light curves. We derived a new Period-Luminosity (P-L) relation, = −2.476(±0.300) logP −0.354(±0.385), from six SX Phe stars showing the fundamental mode. Additionally, the period ratios of three SX Phe stars that probably have a double-radial mode were investigated; P FO /P F = 0.779 for V5, P TO /P FO = 0.685 for V9, P SO /P FO = 0.811 for V11. This paper is the first contribution in a series assessing the detections and properties of variable stars in six southern globular clusters with the KMTNet system.

THREE-SITE PHOTOMETRIC MONITORING OF THE δ SCT-TYPE PULSATING STAR V1162 ORIONIS : PERIOD CHANGE AND ITS IMPLICATIONS FOR PRE-MAIN SEQUENCE EVOLUTION

We present photometric results of the δ Sct star V1162 Ori, which is extensively monitored for a total of 49 nights from mid-December 2014 to early-March 2015. The observations are made with three KMTNet (Korea Microlensing Telescope Network) 1.6 m telescopes installed in Chile, South Africa, and Australia. Multiple frequency analysis is applied to the data and resulted in clear detection of seven frequencies without an alias problem: five known frequencies and two new ones with small amplitudes of 1.2-1.7 mmag. The amplitudes of all but one frequency are significantly different from previous results, confirming the existence of long-term amplitude changes. We examine the variations in pulsation timings of V1162 Ori for about 30 years by using the times of maximum light obtained from our data and collected from the literatures. The O − C (Observed minus Calculated) timing diagram shows a combination of a downward parabolic variation with a period decreasing rate of (1/P)dP/dt = −4.22 × 10⁻⁶ year⁻¹ and a cyclic change with a period of about 2780 days. The most probable explanation for this cyclic variation is the light-travel-time effect caused by an unknown binary companion, which has a minimum mass of 0.69 M_⊙. V1162 Ori is the first δ Sct-type pulsating star of which the observed fast period decrease can be interpreted as an evolutionary effect of a pre-main sequence star, considering its membership of the Orion OB 1c association.