Korea Microlensing Telescope Network Research Articles

Systematic KMTNet Planetary Anomaly Search. XII. Complete Sample of 2017 Subprime Field Planets

We report the analysis of four unambiguous planets and one possible planet from the subprime fields (Γ ≤ 1 hr−1) of the 2017 Korea Microlensing Telescope Network (KMTNet) microlensing survey, to complete the KMTNet AnomalyFinder planetary sample for the 2017 subprime fields. They are KMT-2017-BLG-0849, KMT-2017-BLG-1057, OGLE-2017-BLG-0364, and KMT-2017-BLG-2331 (unambiguous), as well as KMT-2017-BLG-0958 (possible). For the four unambiguous planets, the mean planet–host mass ratios, q, are (1.0, 1.2, 4.6, 13) × 10−4, the median planetary masses are (6.4, 24, 76, 171) M ⊕, and the median host masses are (0.19, 0.57, 0.49, 0.40) M ⊙, respectively, found from a Bayesian analysis. We have completed the Anomaly Finder planetary sample from the first 4 yr of KMTNet data (2016–2019), with 112 unambiguous planets in total, which nearly tripled the microlensing planetary sample. The “sub-Saturn desert” ( logq=−3.6,−3.0 ) found in the 2018 and 2019 KMTNet samples is confirmed by the 2016 and 2017 KMTNet samples.

KMT-2023-BLG-1431Lb: A New q < 10−4 Microlensing Planet from a Subtle Signature

The current studies of microlensing planets are limited by small number statistics. Follow-up observations of high-magnification microlensing events can efficiently form a statistical planetary sample. Since 2020, the Korea Microlensing Telescope Network (KMTNet) and the Las Cumbres Observatory (LCO) global network have been conducting a follow-up program for high-magnification KMTNet events. Here, we report the detection and analysis of a microlensing planetary event, KMT-2023-BLG-1431, for which the subtle (0.05 mag) and short-lived (5 hr) planetary signature was characterized by the follow-up from KMTNet and LCO. A binary-lens single-source (2L1S) analysis reveals a planet/host mass ratio of q = (0.72 ± 0.07) × 10−4, and the single-lens binary-source (1L2S) model is excluded by Δχ 2 = 80. A Bayesian analysis using a Galactic model yields estimates of the host star mass of Mhost=0.57−0.29+0.33M⊙ , the planetary mass of Mplanet=13.5−6.8+8.1M⊕ , and the lens distance of DL=6.9−1.7+0.8 kpc. The projected planet-host separation of a⊥=2.3−0.5+0.5 au or a⊥=3.2−0.8+0.7 au, subject to the close/wide degeneracy. We also find that without the follow-up data, the survey-only data cannot break the degeneracy of central/resonant caustics and the degeneracy of 2L1S/1L2S models, showing the importance of follow-up observations for current microlensing surveys.

Helium-deficient ER UMa-type Dwarf Nova below the Period Minimum with a Hot Secondary

We present the discovery of a peculiar dwarf nova KSP-OT-201712a using high-cadence, multicolor observations made with the Korea Microlensing Telescope Network. KSP-OT-201712a exhibits a rare presence of outbursts during standstills, as well as strong Hα emission for a dwarf nova below the period minimum with an orbital period of 58.75 ± 0.02 minutes. The outburst cycles are ∼6.6 days within standstills but increase to ∼15 days outside of them. Both B − V and V − I colors become bluer and redder as the outburst luminosities increase and decrease, respectively, for the outburst within standstill, while they evolve in the opposite directions outside of the standstills. The presence of strong double-peaked Hα and weak He i emission lines with He/H flux ratio of 0.27, together with absorption lines of Mg b and Na D in the source, leads to the estimation T eff ≃ 4570 ± 40 K, [Fe/H] ≃ 0.06 ± 0.15 dex, and log g ≃ 4.5 ± 0.1 for its secondary. KSP-OT-201712a is the second He-deficient dwarf nova below the period minimum, while the temperature of the secondary is measured for the first time in such objects. We identify it to be an ER UMa-type dwarf nova, suggesting that the evolution of dwarf novae across the period minimum is accompanied by large mass transfers. The high temperature of the secondary indicates that the system started its mass transfer when the secondary was about 93% of its main-sequence age. The system will evolve to a helium cataclysmic variable or to AM CVn once its hydrogen envelope is exhausted before it explodes as a Type Ia supernova.

Improved Period Variations of 32 Contact Binaries with Rapidly Decreasing Periods in the Galactic Bulge

We present detailed analyses of updated eclipse timing diagrams for 32 contact binary merger candidates in the Galactic bulge. The photometric data was obtained from 2016 to 2021 using the Korea Microlensing Telescope Network with the 1.6 m telescopes located at three southern sites (CTIO, SAAO, and SSO). The times of minimum lights were determined by applying the binary-star model to full light curves created at half-year intervals from the observations. The orbital period variations of the binary systems were analyzed using the O − C diagrams from our new timings with the others published in the literature, which are based on the OGLE observations from 2001 to 2015. As results, the orbital periods and period-decreasing rates of 32 binary systems were located to be in the ranges of 0.370 to 1.238 days and from −3.0 to −13.1 × 10−6 day yr−1, respectively. Out of these stars, 24 systems show a combination effect of a parabola and a light travel time caused by a third body, and their outer orbital periods are in the range of 9.1–26.5 yr. We propose that all of our merger candidates need additional monitoring observations to study a luminous-red nova progenitor.

Photometric Selection of Unobscured QSOs at the Ecliptic Poles: KMTNet in the South Field and Pan-STARRS in the North Field

We search for quasi-stellar objects (QSOs) in a wide area of the south ecliptic pole (SEP) field, which has been and will continue to be intensively explored through various space missions. For this purpose, we obtain deep broadband optical images of the SEP field covering an area of ∼14.5 × 14.5 deg2 with the Korea Microlensing Telescope Network (KMTNet). The 5σ detection limits for point sources in the BVRI bands are estimated to be ∼22.59, 22.60, 22.98, and 21.85 mag, respectively. Utilizing data from the Wide-field Infrared Survey Explorer, unobscured QSO candidates are selected among the optically pointlike sources using mid-infrared (MIR) and optical–MIR colors. To refine our selection further and eliminate any contamination not adequately removed by the color-based selection, we perform spectral energy distribution fitting with archival photometric data ranging from optical to MIR. As a result, we identify a total of 2383 unobscured QSO candidates in the SEP field. We also apply a similar method to the north ecliptic pole field using Pan-STARRS data and obtain a similar result of identifying 2427 candidates. The differential number count per area of our QSO candidates is in good agreement with those measured from spectroscopically confirmed ones in other fields. Finally, we compare the results with the literature and discuss how this work will impact future studies, especially upcoming space missions.

Dwarf galaxy discoveries from the KMTNet supernova programme – III. The Milky-Way analogue NGC 2997 group

ABSTRACT We present the discovery of 48 new and the analysis of 55, including seven previously discovered dwarf galaxy candidates (DGCs) around the giant spiral galaxy NGC 2997 using deep BVI images from the Korea Microlensing Telescope Network Supernova Programme. Their V-band central surface brightness and total absolute magnitudes are in the range of 20.3–26.7 mag arcsec−2 and −(8.02–17.69) mag, respectively, while the I-band effective radii are 0.14–2.97 kpc. We obtain $\alpha \, \simeq$ −1.43 ± 0.02 for the faint-end slope of their luminosity function, comparable to previously measured values but shallower than theoretical predictions based on Λ cold dark matter models. The distance-independent distributions of their mass and colour suggest that the group could have recently accreted new massive members from the surrounding fields. The systematically bluer colours of the brighter members indicate younger stellar population and higher star formation activities in them, which appears to be consistent with similar findings from the SAGA or ELVES survey. We suggest that the massive and bluer dwarf galaxies in the group have experienced less environmental quenching due to their recent accretion, while environmental quenching has been more effective for the low-mass members. The interpretation of NGC 2997 being populationally young with recent accretion of massive members is also consistent with the overall morphological distribution of the dwarf galaxies showing a lack of morphologically evolved candidates but a plethora of irregularly shaped ones. Our detection rate of DGCs in the NGC 2997 group and their inferred star formation activities are comparable to those found in Milky Way analogue systems from recent surveys within the magnitude limit M$_{V}\, \lesssim$ −13 mag.

Taxonomic Classification of Asteroids Using the KMTNet Multiband Photometry Data Set

We report the multiband photometry of asteroids observed over 14 nights from 2015 December to 2017 December using the Korea Microlensing Telescope Network telescopes with the taxonomic classification of those objects. The data set contains the photometry of 6793 asteroids in the Sloan Digital Sky Survey griz bands. Following the method of DeMeo & Carry, we define classification criteria on the 2D color plane to assign nine taxonomic types (A, B, C, K, L&D, O, S, V, and X) for the observed objects. We also determine asteroid taxonomy in the newly defined 3D color space as suggested by Roh et al. with seven distinct types based on their novel semisupervised machine-learning model. Both methods distinguish between the S type and others but have difficulty separating the X and C types due to their weak and indistinguishable features and broad distribution in the color spaces. The heliocentric distribution of the observed asteroids with their taxonomic assignments confirms similar trends in the previous works; the number of S types decreases, while the fraction of C types increases with the heliocentric distance in the main belt. On the other hand, the D type dominates in the Jupiter Trojans.

Systematic KMTNet Planetary Anomaly Search. VII. Complete Sample of q < 10−4 Planets from the First 4 yr Survey

We present the analysis of seven microlensing planetary events with planet/host mass ratios q < 10−4: KMT-2017-BLG-1194, KMT-2017-BLG-0428, KMT-2019-BLG-1806, KMT-2017-BLG-1003, KMT-2019-BLG-1367, OGLE-2017-BLG-1806, and KMT-2016-BLG-1105. They were identified by applying the Korea Microlensing Telescope Network (KMTNet) AnomalyFinder algorithm to 2016–2019 KMTNet events. A Bayesian analysis indicates that all the lens systems consist of a cold super-Earth orbiting an M or K dwarf. Together with 17 previously published and three that will be published elsewhere, AnomalyFinder has found a total of 27 planets that have solutions with q < 10−4 from 2016–2019 KMTNet events, which lays the foundation for the first statistical analysis of the planetary mass-ratio function based on KMTNet data. By reviewing the 27 planets, we find that the missing planetary caustics problem in the KMTNet planetary sample has been solved by AnomalyFinder. We also find a desert of high-magnification planetary signals (A ≳ 65), and a follow-up project for KMTNet high-magnification events could detect at least two more q < 10−4 planets per year and form an independent statistical sample.

Mass Production of 2021 KMTNet Microlensing Planets II

We continue our program of publishing all planets (and possible planets) found by eye in 2021 Korea Microlensing Telescope Network (KMTNet) online data. We present four planets (KMT-2021-BLG-0712Lb, KMT-2021-BLG-0909Lb, KMT-2021-BLG-2478Lb, and KMT-2021-BLG-1105Lb), with planet-to-host mass ratios in the range . This brings the total of secure, by-eye, 2021 KMTNet planets to 16, including 8 in this series. The by-eye sample is an important check of the completeness of semiautomated detections, which are the basis for statistical analyses. One of the planets, KMT-2021-BLG-1105Lb, is blended with a relatively bright (I, V) ∼ (18.9, 21.6) star that may be the host. This could be verified immediately by high-resolution imaging. If so, the host is an early G dwarf, and the planet could be characterized by radial velocity observations on 30 m class telescopes.

KMTNet Nearby Galaxy Survey: Overview and Survey Description

Recently, there has been increasing demand for deep imaging surveys to investigate the history of the mass assembly of galaxies in detail by examining the remnants of mergers and accretions, both of which have very low surface brightness (LSB). In addition, the nature of star formation in LSB regions, such as galaxy outer disks, is also an intriguing topic in terms of understanding the physical mechanisms of disk evolution. To address these issues, this study conducted a survey project, called the Korea Microlensing Telescope Network Nearby Galaxy Survey, to construct a deep imaging data set of nearby galaxies in the southern hemisphere. It provides deep and wide-field images with a field of view of ∼12 deg2 for 13 nearby galaxies drawn from the Carnegie–Irvine Galaxy Survey catalog in optical broad bands (BRI) and an Hα narrow band. Through a dedicated data reduction, the surface brightness limit in 10″ × 10″ boxes was found to reach as deep as μ 1σ ∼ 29–31 mag arcsec−2 in the optical broad bands and f 1σ ∼ 1–2 × 10−18 erg s−1 cm−2 arcsec−2 in the Hα narrow band. To conclude the paper, several possible scientific applications for this data set are described.

Systematic KMTNet planetary anomaly search

We complete the analysis of all 2018 prime-field microlensing planets identified by the Korea Microlensing Telescope Network (KMTNet) Anomaly Finder. Among the ten previously unpublished events with clear planetary solutions, eight are either unambiguously planetary or are very likely to be planetary in nature: OGLE-2018-BLG-1126, KMT-2018-BLG-2004, OGLE-2018-BLG-1647, OGLE-2018-BLG-1367, OGLE-2018-BLG-1544, OGLE-2018-BLG-0932, OGLE-2018-BLG-1212, and KMT-2018-BLG-2718. Combined with the four previously published new Anomaly Finder events and 12 previously published (or in preparation) planets that were discovered by eye, this makes a total of 24 2018 prime-field planets discovered or recovered by Anomaly Finder. Together with a paper in preparation on 2018 subprime planets, this work lays the basis for the first statistical analysis of the planet mass-ratio function based on planets identified in KMTNet data. By systematically applying the heuristic analysis to each event, we identified the small modification in their formalism that is needed to unify the so-called close-wide and inner-outer degeneracies.

KMT-2021-BLG-0171Lb and KMT-2021-BLG-1689Lb: two microlensing planets in the KMTNet high-cadence fields with followup observations

ABSTRACT Follow-up observations of high-magnification gravitational microlensing events can fully exploit their intrinsic sensitivity to detect extrasolar planets, especially those with small mass ratios. To make followup observations more uniform and efficient, we develop a system, HighMagFinder, to automatically alert possible ongoing high-magnification events based on the real-time data from the Korea Microlensing Telescope Network (KMTNet). We started a new phase of follow-up observations with the help of HighMagFinder in 2021. Here we report the discovery of two planets in high-magnification microlensing events, KMT-2021-BLG-0171 and KMT-2021-BLG-1689, which were identified by the HighMagFinder. We find that both events suffer the ‘central-resonant’ caustic degeneracy. The planet-host mass-ratio is q ∼ 4.7 × 10−5 or q ∼ 2.2 × 10−5 for KMT-2021-BLG-0171, and q ∼ 2.5 × 10−4 or q ∼ 1.8 × 10−4 for KMT-2021-BLG-1689. Together with two other events, four cases that suffer such degeneracy have been discovered in the 2021 season alone, indicating that the degenerate solutions may have been missed in some previous studies. We also propose a quantitative factor to weight the probability of each solution from the phase space. The resonant interpretations for the two events are disfavoured under this consideration. This factor can be included in future statistical studies to weight degenerate solutions.

Systematic KMTNet planetary anomaly search. IV. Complete sample of 2019 prime-field

ABSTRACT We report the complete statistical planetary sample from the prime fields (Γ ≥ 2 h−1) of the 2019 Korea Microlensing Telescope Network (KMTNet) microlensing survey. We develop the optimized KMTNet AnomalyFinder algorithm and apply it to the 2019 KMTNet prime fields. We find a total of 13 homogeneously selected planets and report the analysis of three planetary events, KMT-2019-BLG-(1042,1552,2974). The planet–host mass ratios, q, for the three planetary events are 6.34 × 10−4, 4.89 × 10−3, and 6.18 × 10−4, respectively. A Bayesian analysis indicates the three planets are all cold giant planets beyond the snow line of their host stars. The 13 planets are basically uniform in log q over the range −5.0 &amp;lt; log q &amp;lt; −1.5. This result suggests that the planets below qbreak = 1.7 × 10−4 proposed by the MOA-II survey may be more common than previously believed. This work is an early component of a large project to determine the KMTNet mass-ratio function, and the whole sample of 2016–2019 KMTNet events should contain about 120 planets.

Discovery of a Short-period and Unusually Helium-deficient Dwarf Nova KSP-OT-201701a by the KMTNet Supernova Program

Abstract We present the first ever discovery of a short-period and unusually helium-deficient dwarf nova KSP-OT-201701a by the Korea Microlensing Telescope Network Supernova Program. The source shows three superoutbursts, each led by a precursor outburst, and several normal outbursts in BVI during the span of ∼2.6 yr with supercycle and normal cycle lengths of about 360 and 76 days, respectively. Spectroscopic observations near the end of a superoutburst reveal the presence of strong double-peaked H i emission lines together with weak He i emission lines. The helium-to-hydrogen intensity ratios measured by He i λ5876 and Hα lines are 0.10 ± 0.01 at a quiescent phase and 0.26 ± 0.04 at an outburst phase, similar to the ratios found in long-period dwarf novae, while significantly lower than those in helium cataclysmic variables (He CVs). Its orbital period of 51.91 ± 2.50 minutes, which is estimated based on time-series spectroscopy, is a bit shorter than the superhump period of 56.52 ± 0.19 minutes, as expected from the gravitational interaction between the eccentric disk and the secondary star. We measure its mass ratio to be 0.37 − 0.21 + 0.32 using the superhump period excess of 0.089 ± 0.053. The short orbital period, which is under the period minimum, the unusual helium deficiency, and the large mass ratio suggest that KSP-OT-201701a is a transition object evolving to an He CV from a long-period dwarf nova with an evolved secondary star.

Systematic Korea Microlensing Telescope Network planetary anomaly search – III. One wide-orbit planet and two stellar binaries

ABSTRACT Only a few wide-orbit planets around old stars have been detected, which limits our statistical understanding of this planet population. Following the systematic search for planetary anomalies in microlensing events found by the Korea Microlensing Telescope Network, we present the discovery and analysis of three events that were initially thought to contain wide-orbit planets. The anomalous feature in the light curve of OGLE-2018-BLG-0383 is caused by a planet with mass ratio q = 2.1 × 10−4 and a projected separation s = 2.45. This makes it the lowest mass-ratio microlensing planet at such wide orbits. The other two events, KMT-2018-BLG-0998 and OGLE-2018-BLG-0271, are shown to be stellar binaries (q &amp;gt; 0.1) with rather close (s &amp;lt; 1) separations. We briefly discuss the properties of known wide-orbit microlensing planets and show that the survey observations are crucial in discovering and further statistically constraining such a planet population.

Systematic KMTNet Planetary Anomaly Search. I. OGLE-2019-BLG-1053Lb, a Buried Terrestrial Planet

Abstract In order to exhume the buried signatures of “missing planetary caustics” in Korea Microlensing Telescope Network (KMTNet) data, we conducted a systematic anomaly search of the residuals from point-source point-lens fits, based on a modified version of the KMTNet EventFinder algorithm. This search revealed the lowest-mass-ratio planetary caustic to date in the microlensing event OGLE-2019-BLG-1053, for which the planetary signal had not been noticed before. The planetary system has a planet–host mass ratio of q = (1.25 ± 0.13) × 10−5. A Bayesian analysis yielded estimates of the mass of the host star, M host = 0.61 − 0.24 + 0.29 M ⊙ , the mass of its planet, M planet = 2.48 − 0.98 + 1.19 M ⊕ , the projected planet–host separation, a ⊥ = 3.4 − 0.5 + 0.5 au, and the lens distance, D L = 6.8 − 0.9 + 0.6 kpc. The discovery of this very-low-mass-ratio planet illustrates the utility of our method and opens a new window for a large and homogeneous sample to study the microlensing planet–host mass ratio function down to q ∼ 10−5.

KMTNet Nearby Galaxy Survey II. Searching for Dwarf Galaxies in Deep and Wide-field Images of the NGC 1291 System

Abstract We present newly discovered dwarf galaxy candidates in deep and wide-field images of NGC 1291 obtained with the Korea Microlensing Telescope Network. We identify 15 dwarf galaxy candidates by visual inspection. Using imaging simulations, we demonstrate that the completeness rate of our detection is greater than 70% for the central surface-brightness value of μ 0,R ≲ 26 mag arcsec−2 and for magnitudes M R ≲ −10 mag. The structural and photometric properties of the dwarf galaxy candidates appear to be broadly consistent with those of ordinary dwarf galaxies in nearby groups and clusters, with μ 0,R ∼ 22.5 to 26.5 mag arcsec−2 and effective radii of 200 pc to 1 kpc. The dwarf galaxy candidates show a concentration toward NGC 1291 and tend to be redder the closer they are to the center, possibly indicating that they are associated with NGC 1291. The dwarf candidates presented in this paper appear to be bluer than those in denser environments, revealing that the quenching of star formation in dwarf galaxies is susceptible to the environment, while the morphology shaping is not.

OGLE-2015-BLG-1771Lb: A Microlens Planet Orbiting an Ultracool Dwarf?

Abstract We report the discovery and the analysis of the short ( days) planetary microlensing event, OGLE-2015-BLG-1771. The event was discovered by the Optical Gravitational Lensing Experiment, and the planetary anomaly (at I ∼ 19) was captured by The Korea Microlensing Telescope Network. The event has three surviving planetary models that explain the observed light curves, with planet-host mass ratio q ∼ 5.4 × 10−3, 4.5 × 10−3 and 4.5 × 10−2, respectively. The first model is the best-fit model, while the second model is disfavored by Δχ 2 ∼ 3. The last model is strongly disfavored by Δχ 2 ∼ 15 but not ruled out. A Bayesian analysis using a Galactic model indicates that the first two models are probably composed of a Saturn-mass planet orbiting a late M dwarf, while the third one could consist of a super-Jovian planet and a mid-mass brown dwarf. The source-lens relative proper motion is μ rel ∼ 9 mas yr−1, so the source and lens could be resolved by current adaptive-optics instruments in 2020 if the lens is luminous.

KMT-2016-BLG-1836Lb: A Super-Jovian Planet from a High-cadence Microlensing Field

Abstract We report the discovery of a super-Jovian planet in the microlensing event KMT-2016-BLG-1836, which was found by the Korea Microlensing Telescope Network (KMTNet) high-cadence observations ( ). The planet–host mass ratio q ∼ 0.004. A Bayesian analysis indicates that the planetary system is composed of a super-Jovian planet orbiting an M or K dwarf, , at a distance of kpc. The projected planet–host separation is au, implying that the planet is located beyond the snow line of the host star. Future high-resolution images can potentially strongly constrain the lens brightness and thus the mass and distance of the planetary system. Without considering detailed detection efficiency, selection, or publication biases, we find a potential mass-ratio desert at −3.7 ≲ log q ≲ −3.0 for the 31 published KMTNet planets.

Dwarf Galaxy Discoveries from the KMTNet Supernova Program. II. The NGC 3585 Group and Its Dynamical State*

Abstract We present our discovery and analysis of dwarf galaxies in the NGC 3585 galaxy group by the Korea Microlensing Telescope Network (KMTNet) Supernova Program. Using deep stack images reaching ≃28 mag arcsec−2 in BVI, we discovered 46 dwarf galaxy candidates distributed in a field of 7 square degrees. The dwarf galaxy candidates exhibit a central surface brightness as faint as μ 0,V = 26.2 mag arcsec−2, with effective radii larger than 150 pc and total absolute magnitudes brighter than M V ≈ −10 mag, if at the distance of NGC 3585. The dwarf galaxy surface number density decreases with projected distance from NGC 3585. We estimate the background contamination to be about 20% based both on the number density profile and on diffuse galaxy counts in a control field. The dwarf galaxy colors and Sérsic structural parameters are consistent with those found for other dwarf galaxies. Unusually, there is no indication of a change in color or brightness in the dwarf galaxy candidates with projected distance from the group center. Approximately 20% of the candidates contain an unresolved nucleus. The nucleated fraction is larger for brighter (and redder) galaxies, but is independent of distance from the group center. We identify four ultra-diffuse galaxy candidates, all near the group center. We interpret these spatial properties as suggesting that the NGC 3585 group might be dynamically younger than the typical group. The galaxy luminosity function of the NGC 3585 group has a faint-end slope of α ≈ −1.39, which is roughly consistent with the slopes found for other nearby groups. The possible dependence of the slope on global group properties is still unclear and continues to motivate our homogeneous deep survey of dozens of nearby groups.