OGLE-2018-BLG-0567Lb and OGLE-2018-BLG-0962Lb: Two Microlensing Planets through Planetary-Caustic Channel
Youn Kil Jung, Cheongho Han, Andrzej Udalski, Andrew Gould, Jennifer C. Yee, Michael D. Albrow, Sun-Ju Chung, Kyu-Ha Hwang, Yoon-Hyun Ryu, In-Gu Shin, Yossi Shvartzvald, Wei Zhu, Weicheng Zang, Sang-Mok Cha, Dong-Jin Kim, Hyoun-Woo Kim, Seung-Lee Kim, Chung-Uk Lee, Dong-Joo Lee, Yongseok Lee, Byeong-Gon Park, Richard W. Pogge Przemek Mróz, Micha? K. Szyma?ski, Jan Skowron, Radek Poleski, Igor Soszy?ski, Pawe? Pietrukowicz, Szymon Koz?owski, Krzystof Ulaczyk, Krzysztof A. Rybicki, Patryk Iwanek, Marcin Wrona
aa r X i v : . [ a s t r o - ph . E P ] F e b OGLE-2018-BLG-0567Lb and OGLE-2018-BLG-0962Lb: TwoMicrolensing Planets through Planetary-Caustic Channel
Youn Kil Jung , , , Cheongho Han , , Andrzej Udalski , , Andrew Gould , , , ,Jennifer C. Yee , ,andMichael D. Albrow , Sun-Ju Chung , , Kyu-Ha Hwang , Yoon-Hyun Ryu , In-Gu Shin ,Yossi Shvartzvald , Wei Zhu , Weicheng Zang , Sang-Mok Cha , , Dong-Jin Kim ,Hyoun-Woo Kim , Seung-Lee Kim , , Chung-Uk Lee , , Dong-Joo Lee , Yongseok Lee , ,Byeong-Gon Park , , Richard W. Pogge (The KMTNet Collaboration)Przemek Mr´oz , , Micha l K. Szyma´nski , Jan Skowron , Radek Poleski , , Igor Soszy´nski ,Pawe l Pietrukowicz , Szymon Koz lowski , Krzystof Ulaczyk , Krzysztof A. Rybicki ,Patryk Iwanek , Marcin Wrona (The OGLE Collaboration) Korea Astronomy and Space Science Institute, Daejon 34055, Republic of Korea University of Science and Technology, Korea, 217 Gajeong-ro Yuseong-gu, Daejeon 34113,Korea Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH43210, USA Max-Planck-Institute for Astronomy, K ¨o nigstuhl 17, 69117 Heidelberg, Germany Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138,USA University of Canterbury, Department of Physics and Astronomy, Private Bag 4800,Christchurch 8020, New Zealand Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot76100, Israel The KMTNet Collaboration. The OGLE Collaboration. Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St GeorgeStreet, Toronto, ON M5S 3H8, Canada Department of Astronomy, Tsinghua University, Beijing 100084, China School of Space Research, Kyung Hee University, Yongin 17104, Republic of Korea Division of Physics, Mathematics, and Astronomy, California Institute of Technology,Pasadena, CA 91125, USA Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL,UK
ABSTRACT
We present the analyses of two microlensing events, OGLE-2018-BLG-0567and OGLE-2018-BLG-0962. In both events, the short-lasting anomalies weredensely and continuously covered by two high-cadence surveys. The light-curvemodeling indicates that the anomalies are generated by source crossings over theplanetary caustics induced by planetary companions to the hosts. The estimatedplanet/host separation (scaled to the angular Einstein radius θ E ) and mass ra-tio are ( s, q ) = (1 . , . × − ) and ( s, q ) = (1 . , . × − ), respectively.From Bayesian analyses, we estimate the host and planet masses as ( M h , M p ) =(0 . +0 . − . M ⊙ , . +0 . − . M J ) and ( M h , M p ) = (0 . +0 . − . M ⊙ , . +0 . − . M J ), respec-tively. These planetary systems are located at a distance of 7 . +0 . − . kpc forOGLE-2018-BLG-0567 and 6 . +1 . − . kpc for OGLE-2018-BLG-0962, suggestingthat they are likely to be near the Galactic bulge. The two events prove the capa-bility of current high-cadence surveys for finding planets through the planetary-caustic channel. We find that most published planetary-caustic planets are foundin Hollywood events in which the source size strongly contributes to the anomalycross section relative to the size of the caustic. Subject headings: gravitational lensing: micro – planetary systems
1. Introduction
The signature of a microlening planet is almost always a short-lasting anomaly in thesmooth and symmetric lensing light curve produced by the host of the planet. In principle, 3 –the signature can appear at any position of the lensing light curve (Gaudi 2012). In real-ity, however, the signatures of planets detected in the earlier phase of lensing experimentsappeared mainly near the peak of lensing light curves.The bias toward central anomalies is mostly attributed to the limitation of early lensingsurveys. With a roughly 1 day cadence of the first-generation survey experiments, e.g.,the MACHO (Alcock et al. 1995), OGLE-I (Udalski et al. 1992), MOA-I (Bond et al. 2001)surveys, it was difficult to detect planetary signals lasting of order 1 day or less by the surveyexperiments. To meet the cadence requirement for planet detections, Gould & Loeb (1992)proposed an observational mode, in which wide-field surveys with a low cadence monitor alarge area of sky mainly to detect lensing events, and followup experiments conduct high-cadence observations for a small number of lensing events detected by the surveys using anetwork of multiple narrow-field telescopes. However, this mode of observations had thedrawback that only a handful of lensing events could be monitored by followup observations.Combined with the low probability of planetary perturbations, this implied a low planetdetection rate for this phase of the experiments. In fact, for the first several years, therewere no securely detected planets using this mode, although there was one tentative detectionin the event MACHO 98-BLG-35 (Rhie et al. 2000).The first three microlensing detections were found using the survey+followup strategy.In the first event, OGLE 2003-BLG-235/MOA 2003-BLG-53 (Bond et al. 2004), the planetwas found by the surveys, but the MOA survey carried out additional followup observa-tions in response to the planetary anomaly. The next two planets, OGLE-2005-BLG-071Lb(Udalski et al. 2005) and OGLE-2005-BLG-390Lb (Beaulieu et al. 2006), were both foundthrough extensive followup observations of known microlensing events that were initiatedbefore the planetary anomaly began. The discovery of OGLE-2005-BLG-071Lb provideda practical lesson in the value of high-magnification events (Griest & Safizadeh 1998) fordetecting planets through followup observations.In the following years, the planet detection rate using the survey+followup mode wassubstantially increased by focusing on events with very high magnifications. Several factorscontributed to the increase of the detection rate. First, the planet detection efficiency forhigh-magnification events is high. This is because a planet located in the lensing zone of itshost always induces a small central caustic near the position of the host, and the trajectory ofa high-magnification event passes close to the central caustic. This yields a high probabilitythat a planet will produce a perturbation and also confine that perturbation to a shortduration of time while the event is highly magnified, not throughout the whole event. As aresult, the time of the planetary signal, i.e., the peak of the light curve, can be predicted inadvance and enable one to efficiently use resources for followup observations. By contrast, 4 –predicting the time of a planetary signal through other channels is difficult. Finally, highlymagnified source stars are bright enough to be observed with small-aperture telescopes, downto sub-meter amateur-class telescopes, and this enables one to maximize available telescopesfor followup observations, e.g., OGLE-2005-BLG-071 (Udalski et al. 2005; Dong et al. 2009).Thus, the planets detected from the survey+followup experiments were detected mainlythrough the high-magnification channel, and this led to the bias toward central causticperturbations.The current planetary lensing experiments are in the second phase, in which lensingevents are observed by high-cadence surveys. The observational cadence of the lensingsurveys in this phase has greatly increased with the employment of large-format camerasyielding very wide fields of view. The MOA experiment entered a new phase (MOA-II) byupgrading its instrument with a new wide-field camera composed of ten 2k ×
4k chips yieldinga 2 . field of view (Sumi et al. 2013). The OGLE survey is in its fourth phase (OGLE-IV) using 1 . camera composed of 32 2k ×
4k chips (Udalski et al. 2015). The KMTNetsurvey, which commenced its full operation in 2016 (Kim et al. 2016), utilizes three globallydistributed telescopes, each of which has a camera with a 4 . field of view. Being ableto cover a large area of sky from a single exposure, the observational cadence of the currentsurvey experiments now reaches Γ ∼ − toward the dense bulge fields. This enablesplanet detections without additional followup observations.With the operation of the high-cadence surveys, the detection rate of planets is rapidlyincreasing. One important reason for the rapid increase of the detection rate is that planetscan be detected not only through the central-caustic channel but also through the additionalplanetary-caustic channel. Planets are detected through the planetary-caustic channel asanomalies produced by the source’s approach close to the “planetary caustic”, which de-notes one of the two sets of planet-induced caustics lying away from the host. The planetarycaustic lies at a position with a separation from the host of s − /s , and thus planetarysignals produced by this caustic can appear at any part of the lensing light curve depend-ing on the planet-host separation s (normalized to the angular Einstein radius θ E ). Theplanetary caustic is substantially larger than the central caustic, and thus the probabilityof a planetary perturbation is higher. Another importance of detecting planets throughthe planetary-caustic channel is that interpreting the planetary signal is usually not sub-ject to the close-wide degeneracy (Griest & Safizadeh 1998; Dominik 1999), which causesambiguity in estimating the planet-host separations for most planets detected through thecentral-caustic channel.In this paper, we present the analysis of two planetary microlensing events OGLE-2018-BLG-0567 and OGLE-2018-BLG-0962, for which planets are both detected through a 5 –planetary-caustic channel. For both events, the signatures of the planets were densely andcontinuously covered by two high-cadence lensing surveys, and this leads us to unambiguouslyinterpret the planetary signals.
2. Observation
The two planetary events were observed by the two lensing surveys conducted by theOGLE and KMTNet groups. The OGLE survey uses the 1 . . I -band, and a fraction of the images were taken in the V -band todetermine the color of the microlensed source stars.OGLE-2018-BLG-0567, (RA , Dec)
J2000 =(17:56:04.42, − l, b ) = (1 ◦ . , − ◦ . − for OGLE and Γ = 2 hr − for KMTNet.OGLE-2018-BLG-0962 was discovered by the OGLE EWS on 2018 June 2. It islocated at (RA , Dec)
J2000 =(17:52:41.95, − l, b ) =( − ◦ . , − ◦ . − for KMTC and Γ = 2 .
25 hr − for KMTS and KMTA.For both events, the data sets were reduced based on the image subtraction method-ology (Tomaney & Crotts 1996; Alard & Lupton 1998), specifically Albrow et al. (2009) forKMTNet and Wo´zniak (2000) for OGLE. The photometric error bars were then readjustedfollowing the prescription presented in Yee et al. (2012). We note that for the source colormeasurement, we additionally carried out pyDIA (Albrow 2017) reductions for a subset ofthe KMTNet data, which simultaneously returns the light curve and field-star photometryon the same system. 6 –
3. Light Curve Analysis
Figures 1 and 2 show the light curves of OGLE-2018-BLG-0567 and OGLE-2018-BLG-0962, respectively. It is found that the two events share various characteristics in common.First, the apparent peak magnifications of the baseline single-lens single-source (1L1S) lightcurves are not high: A peak ∼ . A peak ∼ . M and M .The standard 2L1S modeling requires one to include seven fitting parameters to describean observed light curve. The first three are the Paczy´nski (1986) parameters ( t , u , t E ),respectively the time of closest source approach to the lens, the impact parameter (scaledto θ E ), and the event timescale. The next three ( s, q, α ) describe the binary-lens geometry:the projected binary separation (scaled to θ E ), the binary mass ratio ( q = M /M ), and theorientation angle of the binary axis (relative to the source trajectory), respectively. The lastdescribes the source radius ρ = θ ∗ /θ E , where θ ∗ is the angular source radius.The modeling is conducted following the procedure described in Jung et al. (2015). Inthe first step, we carry out grid searches for the binary parameters ( s, q, α ). At each grid, wefix ( s, q ) and find the remaining parameters using Markov Chain Monte Carlo (MCMC) χ minimization. In this modeling, the initial values of the parameters ( t , u , t E ) are given asthe values estimated from a 1L1S fit for the data excluding the anomaly. The initial valueof the normalized source radius is estimated from the caustic-crossing timescale, which isrelated to the event timescale by t ∗ = ρt E . Here we use inverse ray shooting (Kayser et al.1986; Schneider & Weiss 1987) to compute the finite-source lensing magnifications. The fluxvalues from the source, f S ,i , and blend, f B ,i , for the data set obtained from the i th observatoryis estimated by f i ( t ) = f S ,i A ( t ) + f B ,i , where f i is the observed flux. Once local solutions arefound from the first-round modeling, we refine the individual locals by releasing all fittingparameters and allowing them to be free parameters in an MCMC.From the modeling, it is found that the observed lensing light curves of both events arewell described by unique 2L1S models, in which the mass ratios between M and M are inthe planetary regime. The estimated binary parameters are ( s, q ) = (1 . , . × − ) forOGLE-2018-BLG-0567, and ( s, q ) = (1 . , . × − ) for OGLE-2018-BLG-0962. The fulllensing parameters and their uncertainties are presented in Table 1. The model curves of 7 –the solutions are drawn over the data points in Figures 1 and 2 for OGLE-2018-BLG-0567and OGLE-2018-BLG-BLG-0962, respectively.In Figures 3 and 4, we present the lens-system configurations of the individual events,showing the source trajectory with respect to the lens components and resulting caustics.From the configurations, it is found that the anomalies of both events are produced by thesource crossing over the planetary caustic of the lens system. For OGLE-2018-BLG-0567, thesource size is comparable to the caustic size, and thus the detailed caustic-crossing features,two caustic spikes and a U-shape trough region between the spikes, was smeared out by finite-source effects. For OGLE-2018-BLG-0962, on the other hand, the caustic is much bigger thanthe source size, and thus detailed caustic-crossing feature of the anomaly is well delineated. Itis found the first part of the anomaly, centered at HJD ′ (= HJD − , ,
000 days) ∼ . ′ ∼ . × ( t , u , ρ ) for the two sources, I -band flux ratio q F,I ,and a shared timescale t E . The results are listed in Table 2. We find that the 1L2S solutionis disfavored by ∆ χ >
4. Physical Parameters
The results in Table 1 show that for both events, the normalized source radii are preciselymeasured. This enables us to estimate θ E = θ ∗ /ρ provided that θ ∗ is measured. Then, wecan use the estimated θ E to constrain the lens total mass M and distance D L as given by θ ≡ κM π rel ; π rel = au (cid:18) D L − D S (cid:19) , (1)where κ ≡ G/ ( c au) ≃ .
14 mas /M ⊙ , π rel is the lens-source relative parallax, and D S is thesource distance. Hence, we evaluate θ ∗ using the method of Yoo et al. (2004). 8 –We first estimate the intrinsic source color ( V − I ) , S and magnitude I , S . We dothis in the following ways. First, we calibrate the KMTNet pyDIA photometry to thestandard Johnson-Cousins system using the OGLE-III catalog (Szyma´nski et al. 2011). Wenext construct a ( V − I, I ) color-magnitude diagram (CMD) with field stars around thesource. We next find the source location ( V − I, I ) S in the CMD from the best-fit model.We also find the position of the red clump centroid (RCC), i.e., ( V − I, I ) RCC . Figure 6 showsthe source and RCC positions in the CMDs for the individual events. We then measure theoffset ∆( V − I, I ) = ( V − I, I ) S − ( V − I, I ) RCC . Finally, we find the intrinsic source positionas ( V − I, I ) , S = ∆( V − I, I ) + ( V − I, I ) , RCC , (2)where ( V − I, I ) , RCC is the intrinsic RCC position measured from independent observations(Bensby et al. 2013; Nataf et al. 2013). Here we assume that the source star experiences thesame amount of extinction as the RCC. In Table 3, we list our estimated values of ( V − I, I ) S ,( V − I, I ) RCC , ( V − I, I ) , RCC , and ( V − I, I ) , S .We now derive θ ∗ based on the estimated intrinsic source position. For this, we apply( V − I ) , S to the V IK relation (Bessell & Brett 1988) to find ( V − K ) , S . We then obtain θ ∗ from the ( V − K ) , S − θ ∗ relations, specifically Kervella et al. (2004a) for OGLE-2018-BLG-0567 and Kervella et al. (2004b) for OGLE-2018-BLG-0962. We note that we add 5%error to θ ∗ to consider the uncertainty of the intrinsic RCC position and the color/surfacebrightness conversion. From the measured t E and ρ , we then find θ E and the lens-sourcerelative proper motion, µ rel = θ E /t E . The estimated values of θ ∗ , θ E , and µ rel are also listedin Table 3.For both events, we are unable to constrain the microlens parallax vector π E . Thisimplies that we cannot directly derive M and D L by M = θ E κπ E ; D L = au π E θ E + π S (3)from the microlensing data (Gould 2000). Here π S = au /D S . Hence, we estimate the lensproperties based on a Bayesian analysis with Galactic model priors.The Galactic model is constructed based on a mass function (MF), a density pro-file (DP), and a velocity distribution (VD). For the MF, we use the models presented inJung et al. (2018). For the DP, we use the Han & Gould (2003) model for the bulge and theRobin et al. (2003) model for the disk, respectively. We note that the former is normalizedbased on the star counts results of Holtzman et al. (1998), while the latter is normalized bythe local column density Σ( D L = 0) = Σ disk , = 36 M ⊙ pc − from Han & Gould (2003).The bulge VD is modeled based on stars in the Gaia catalog (Gaia Collaboration et al.2016, 2018). That is, we first find red giant stars in the catalog within 2 arcmin centered on 9 –the event location. We then derive their mean velocity and its dispersion in GalactocentricCartesian coordinates ( x, y, z ) as defined in Han & Gould (1995).For the disk VD, we adopt Gaussian forms of f ( v y , v z ) = f ( v y ) f ( v z ) from Han & Gould(1995), which we then modify to consider the change in the matter distribution. We dothis in two ways. First, we introduce an asymmetric drift v ad to the y -direction velocity v y . That is, v ad ( D L ) = (0 . σ xyz, / ¯ v y )[Σ( D L ) / Σ disk , ] / , where ¯ v y = 220 km s − is the mean y -direction velocity, σ xyz, = σ x, + σ y, + σ z, , and ( σ x, , σ y, , σ z, ) = (34 , ,
20) km s − are the mean velocity dispersions along the ( x, y, z ) directions (in the solar neighborhood),respectively. The y -direction velocity at a given line-of-sight distance D L is then calculatedby v y ( D L ) = ¯ v y − v ad ( D L ). Second, we subsequently modify the velocity dispersion as σ y ( D L ) = σ y, [Σ( D L ) / Σ disk , ] / for the y -direction and σ z ( D L ) = σ z, [Σ( D L ) / Σ disk , ] / forthe z -direction.We now carry out the Bayesian analysis with the constraints ( t E , θ E ). For this, we followthe procedure of Jung et al. (2018). The estimated lens properties for the individual eventsare listed in Table 4. The corresponding posterior probabilities for M and D L are shownin Figure 7. We find that the host mass is M = 0 . +0 . − . M ⊙ for OGLE-2018-BLG-0567and M = 0 . +0 . − . M ⊙ for OGLE-2018-BLG-0962. The planet masses ( M = qM ) andthe projected planet-host separations ( a ⊥ = sD L θ E ) of the individual events are then esti-mated to ( M , a ⊥ ) = (0 . +0 . − . M J , . +0 . − . au) and ( M , a ⊥ ) = (1 . +0 . − . M J , . +0 . − . au),respectively. These estimates suggest that the two planetary systems are likely composedof M-type dwarfs and giant planets lying beyond the snow line (Kennedy & Kenyon 2008).Here, the snow line is the location in the proto-planetary disk where icy material can con-dense and where giant planets are thought to be formed (Ida & Lin 2004). The distanceto the planet is D L = 7 . +0 . − . kpc for OGLE-2018-BLG-0567 and D L = 6 . +1 . − . kpc forOGLE-2018-BLG-0962, indicating that they are likely to be in or near the Galactic bulge.
5. Microlensing Planets in the (log s, log q ) plane Our two survey-only microlensing planets are detected from the perturbations causedby the planetary caustics (see Figures 3 and 4). In particular, the planetary perturbation ofOGLE-2018-BLG-0567 was generated by a “Hollywood” geometry (Gould 1997), in whichthe source size contributes strongly to, or dominates, the anomaly cross section relative tothe size of the caustic. These detections prove the capability of the high-cadence surveys fordetecting planets through the planetary-caustic channel.Figure 8 illustrates the positions of OGLE-2018-BLG-0567Lb and OGLE-2018-BLG- 10 –0962Lb in the (log s, log q ) plane along with other microlensing planets . This figure showsthat OGLE-2018-BLG-0567Lb is located in a previously underpopulated region. The twogreen solid lines indicate the boundary between resonant and non-resonant caustics (Schneider & Weiss1986; Dominik 1999). For planets just outside of the resonance region, the excess magnifica-tion pattern (from the underlying 1L1S magnification) extends out all the way between thecentral and planetary caustics. This implies that the effective cross-section of these causticsfor a planet-induced perturbation and their lensing behavior are similar to those of resonantcaustics. Recently, Yee et al. (2021) classified such kinds of caustics as the “near-resonant”,specifically a set of caustics that has a excess magnification contour that connects the centraland planetary caustics and has at least 10% excess magnification along the entire causticridges. The two green dashed lines are the boundary of near-resonant caustics. Yee et al.(2021) empirically estimated that at a fixed q , the maximum size of a 10% deviation cor-responds to ∼ s r , c for s < ∼ . s r , w for s > s r , c and s r , w are the boundary values of s between resonant and non-resonant caustics (Dominik1999). Planets (except for our two planets) with a single and multiple solutions are codedby black and red (with connected lines) colors, respectively. OGLE-2018-BLG-0567Lb andOGLE-2018-BLG-0962Lb are marked by yellow and blue colors, respectively. The shape ofthe symbols represent the type of caustics that yield the planetary perturbation: circles forresonant/near-resonant, squares for central, and triangles for planetary caustics. The filledtriangles are the planets found in the Hollywood events.Figure 8 shows that the majority of planets are located inside the near-resonant bound-ary rather than being due to planetary caustics. The bias toward resonant caustics mainlycomes from the relatively large size of resonant caustics (scaled as q / ) compared to thatof planetary caustics (scaled as q / ) (Dominik 1999; Han 2006; Yee et al. 2021). There isalso a bias against planetary caustics due to the observational strategy of earlier microlens-ing experiments that was focused on high-magnification events (Griest & Safizadeh 1998;Gould et al. 2010).Only 24 planets are placed outside the near-resonant boundary and 18 planets amongthem are detected from the perturbations produced by clearly isolated planetary caustics . https://exoplanetarchive.ipac.caltech.edu The corresponding planetary-caustic events are OGLE-2005-BLG-390 (Beaulieu et al. 2006), MOA-bin-1 (Bennett et al. 2012), OGLE-2006-BLG-109 (Gaudi et al. 2008; Bennett et al. 2010), OGLE-2008-BLG-092 (Poleski et al. 2014), MOA-2010-BLG-353 (Rattenbury et al. 2015), MOA-2011-BLG-028(Skowron et al. 2016), MOA-2012-BLG-006 (Poleski et al. 2017), OGLE-2012-BLG-0838 (Poleski et al.2020), OGLE-2013-BLG-0341 (Gould et al. 2014), MOA-2013-BLG-605 (Sumi et al. 2016), OGLE-2014-BLG-1722 (Suzuki et al. 2018), OGLE-2016-BLG-0263 (Han et al. 2017), OGLE-2016-BLG-1227 (Han et al.
11 –We find that most of these planetary-caustic planets (12 planets) are found in the Hollywoodevents and they are located in high-cadence observational fields of the lensing surveys. Thisproves the capability of the Hollywood strategy of following big stars to find planets (Gould1997). The majority of the Hollywood planets are located in the region s >
1. This ismainly due to the difference in the size of planetary caustics. For s >
1, there is one four-sided planetary caustic. For s <
1, on the other hand, there are two triangular planetarycaustics and each of which size is much smaller than that of s >
1. In addition, the planetarysignals from the these smaller planetary caustics tends to be more significantly diminished bythe finite-source effects (Gould & Gaucherel 1997). As a result, the wide-planetary caustichas a larger effective cross section and therefore higher sensitivity for finding planets.
6. Summary and Conclusions
We present the discovery of two cold giant planets orbiting M-dwarfs in two eventsOGLE-2018-BLG-0567 and OGLE-2018-BLG-0962. Both events clearly showed deviationsfrom the 1L1S model, caused by the presence of a companion to the lens host with preciselymeasured planet/host mass ratios of (1 . ± . × − and (2 . ± . × − , respectively.In both events, the finite-source effects are clearly detected, but the microlens parallax effectsare not meaningfully constrained. Hence, we constrain the lens properties using the Bayesiananalysis. From this, we estimate planet masses of 0 . +0 . − . M J for OGLE-2018-BLG-0567Lband 1 . +0 . − . M J for OGLE-2018-BLG-0962Lb, and their physical projected separations of2 . +0 . − . au and 3 . +0 . − . au, respectively. These planets likely belong to a class of giantplanets orbiting M-dwarfs outside the snow line. The detection rate of microlensing planetshas rapidly increased with the advent of high-cadence lensing surveys, and thus, planetspresented here and future detections will expand our understanding of the planet populationaround M-dwarfs.The planet hosts can be precisely constrained by future high-resolution imaging withadaptive optics (AO) mounted on “30 m” class telescopes. That is, they have only a smallprobability of being non-luminous (see Figure 7). The Bayesian estimates suggest that thedereddened H -band magnitude of the host is H = 21 . +1 . − . for OGLE-2018-BLG-0567and H = 19 . +2 . − . for OGLE-2018-BLG-0962 (Pecaut & Mamajek 2013). In addition,both events have µ rel > . − . Therefore, in 2030, the hosts will be separated from
12 –the microlensed source by ∆ θ &
40 mas.This research has made use of the KMTNet system operated by the Korea Astron-omy and Space Science Institute (KASI) and the data were obtained at three host sitesof CTIO in Chile, SAAO in South Africa, and SSO in Australia. Work by CH was sup-ported by the grants of National Research Foundation of Korea (2017R1A4A1015178 and2019R1A2C2085965). The OGLE has received funding from the National Science Centre,Poland, grant MAESTRO 2014/14/A/ST9/00121 to A.U.
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This preprint was prepared with the AAS L A TEX macros v5.2.
16 –Table 1. Lensing ParametersParameters OGLE-2018-BLG-0567 OGLE-2018-BLG-0962 χ /dof 8677.3/9252 6892.1/6833 t (HJD ′ ) 8244.845 ± ± u ± ± t E (days) 24.641 ± ± s ± ± q (10 − ) 1.240 ± ± α (rad) 0.623 ± ± ρ (10 − ) 17.675 ± ± f S ± ± f B ± ± χ /dof 9223.0/9251 t , (HJD ′ ) 8244.323 ± u , ± t , (HJD ′ ) 8670.134 ± u , (10 − ) 2.603 ± t E (days) 24.088 ± ρ – ρ (10 − ) 6.295 ± q F,I (10 − ) 7.520 ± f s ± f b ± V − I, I ) S (3.59 ± ± ± ± V − I, I ) RCC (3.67 ± ± ± ± V − I, I ) , RCC (1.06, 14.37) (1.06, 14.56)( V − I, I ) , S (0.98 ± ± ± ± θ ∗ ( µ as) 3.769 ± ± θ E (mas) 0.213 ± ± µ rel (mas yr − ) 3.161 ± ± M ( M ⊙ ) 0 . +0 . − . . +0 . − . M ( M J ) 0 . +0 . − . . +0 . − . a ⊥ (au) 2 . +0 . − . . +0 . − . D L (kpc) 7 . +0 . − . . +1 . − .
18 –Fig. 1.— Light curve of OGLE-2018-BLG-0567. The black solid curve on the data is thebest-fit 2L1S solution. The upper panel shows the enlarged view of the planet-inducedanomaly centered on HJD ′ ∼ ′ ∼ . ′ ∼ . ρ ) on the trajectory are the source positions at the times of observations. The twoorange circles are the positions of binary-lens masses ( M and M ). In each panel, the cuspyclosed curve drawn in black color represents the caustic. The upper panel shows the enlargedview of the planetary caustic. Lengths are scaled to the angular Einstein radius of the lenssystem. 21 –Fig. 4.— Caustic geometry of OGLE-2018-BLG-0962. Notations are identical to those ofFigure 3. 22 –Fig. 5.— Light curve of the 1L2S model for OGLE-2018-BLG-0567. The dashed gray andsolid black lines are the best-fit models from the 1L2S and 2L1S interpretations, respectively.The lower two panels show the residuals from the two models. 23 –Fig. 6.— Color-magnitude diagrams of OGLE-2018-BLG-0567 (upper panel) and OGLE-2018-BLG-0962 (lower panel). In each panel, the CMD is constructed using stars in the2 ′ × ′ field centered on the event location based on KMTNet pyDIA photometry calibratedto the OGLE-III catalog (Szyma´nski et al. 2011). The blue and red circles are the positionsof source and red clump centroid, respectively. 24 –Fig. 7.— Posterior distributions of M (left panels) and D L (right panels) for the individualevents. In each panel, the red and blue distributions are, respectively, the contributions bythe bulge and disk lens populations. The black distribution is the total contribution of thetwo lens populations. The median value and its 68% confidence interval are represented bythe vertical solid and two dotted lines, respectively. 25 –Fig. 8.— Microlensing planets in the (log s, log qq