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OGLE-2018-BLG-0799Lb: a Sub-Saturn-Mass Planet Orbiting a Very Low Mass Dwarf

Zang Weicheng, Shvartzvald Yossi, Udalski Andrzej, Yee Jennifer C., Lee Chung-Uk, Sumi Takahiro, Zhang Xiangyu, Yang Hongjing, Mao Shude, Novati Sebastiano Calchi
17 Oct 2020 astro-ph.EP astro-ph.GA arxiv.org/abs/2010.08732

We report the discovery and analysis of a sub-Saturn-mass planet in the microlensing event OGLE-2018-BLG-0799. The planetary signal was observed by several ground-based telescopes, and the planet-host mass ratio is $q = (2.65 \pm 0.16) \times 10^{-3}$. The ground-based observations yield a constraint on the angular Einstein radius $\theta_{\rm E}$, and the microlens parallax $\pi_{\rm E}$ is measured from the joint analysis of the Spitzer and ground-based observations, which suggests that the host star is most likely to be a very low-mass dwarf. A full Bayesian analysis using a Galactic model indicates that the planetary system is composed of an $M_{\rm planet} = 0.22_{-0.06}^{+0.19}~M_{J}$ planet orbiting an $M_{\rm host} = 0.080_{-0.020}^{+0.080}~M_\odot$, at a distance of $D_{\rm L} = 4.42_{-1.23}^{+1.73}$ kpc. The projected planet-host separation is $r_\perp = 1.27_{-0.29}^{+0.45}$ AU, implying that the planet is located beyond the snowline of the host star. However, because of systematics in the Spitzer photometry, there is ambiguity in the parallax measurement, so the system could be more massive and farther away.

KMT-2017-BLG-2820 and the Nature of the Free-Floating Planet Population

Ryu Yoon-Hyun, , Gould Andrew, Hwang Kyu-Ha, Kim Hyoun-Woo, Yee Jennifer C., Albrow Michael D., Chung Sun-Ju, Jung Youn Kil, Shin In-Gu
15 Oct 2020 astro-ph.EP astro-ph.GA arxiv.org/abs/2010.07527

We report a new free-floating planet (FFP) candidate, KMT-2017-BLG-2820, with Einstein radius $\theta_\e\simeq 6\,\muas$, lens-source relative proper motion $\mu_\rel \simeq 8\,\masyr$, and Einstein timescale $t_\e=6.5\,$hr. It is the third FFP candidate found in an ongoing study of giant-source finite-source point-lens (FSPL) events in the KMTNet data base, and the sixth FSPL FFP candidate overall. We find no significant evidence for a host. Based on their timescale distributions and detection rates, we argue that five of these six FSPL FFP candidates are drawn from the same population as the six point-source point-lens (PSPL) FFP candidates found by \citet{mroz17} in the OGLE-IV data base. The $\theta_\e$ distribution of the FSPL FFPs implies that they are either sub-Jovian planets in the bulge or super-Earths in the disk. However, the apparent "Einstein Desert" ($10\la\theta_\e/\muas\la 30$) would argue for the latter. Whether each of the 12 (6 FSPL and 6 PSPL) FFP candidates is truly an FFP, or simply a very wide-separation planet, can be determined at first adaptive optics (AO) light on 30m telescopes, and earlier for some. If the latter, a second epoch of AO observations could measure the projected planet-host separation with a precision ${\cal O}(10\,\au)$. At the present time, the balance of evidence favors the unbound-planet hypothesis.

Four Microlensing Planets with Faint-source Stars Identified in the 2016 and 2017 Season Data

Han Cheongho, Udalski Andrzej, Kim Doeon, Jung Youn Kil, Ryu Yoon-Hyun, Albrow Michael D., Chung Sun-Ju, Gould Andrew, Hwang Kyu-Ha, Lee Chung-Uk
20 Aug 2020 astro-ph.EP astro-ph.GA arxiv.org/abs/2008.09258

Microlensing planets occurring on faint source stars can escape detection due to their weak signals. Occasionally, detections of such planets are not reported due to the difficulty of extracting high-profile scientific issues on the detected planets. For the solid demographic census of microlensing planetary systems based on a complete sample, we investigate the microlensing data obtained in the 2016 and 2017 seasons to search for planetary signals in faint-source lensing events. From this investigation, we find four unpublished microlensing planets including KMT-2016-BLG-2364Lb, KMT-2016-BLG-2397Lb, OGLE-2017-BLG-0604Lb, and OGLE-2017-BLG-1375Lb. We analyze the observed lensing light curves and determine their lensing parameters. From Bayesian analyses conducted with the constraints from the measured parameters, it is found that the masses of the hosts and planets are in the ranges $0.50\lesssim M_{\rm host}/M_\odot\lesssim 0.85$ and $0.5 \lesssim M_{\rm p}/M_{\rm J}\lesssim 13.2$, respectively, indicating that all planets are giant planets around host stars with subsolar masses. The lenses are located in the distance range of $3.8 \lesssim \dl/{\rm kpc}\lesssim 6.4$. It is found that the lenses of OGLE-2017-BLG-0604 and OGLE-2017-BLG-1375 are likely to be in the Galactic disk.

KMT-2019-BLG-2073: Fourth Free-Floating-Planet Candidate with $\theta_\rm E < 10 \rm\mu as$

Kim Hyoun-Woo, Hwang Kyu-Ha, Gould Andrew, Yee Jennifer C., Ryu Yoon-Hyun, Albrow Michael D., Chung Sun-Ju, Han Cheongho, Jung Youn Kil, Lee Chung-Uk
14 Jul 2020 astro-ph.EP astro-ph.GA astro-ph.IM astro-ph.SR arxiv.org/abs/2007.06870

We analyze the very short Einstein timescale ($t_\rm E \simeq 7\,{\rm hr}$) event KMT-2019-BLG-2073. Making use of the pronounced finite-source effects generated by the clump-giant source, we measure the Einstein radius $\theta_\rm E \simeq 4.8\,\rm \mu as$, and so infer a mass $M = 59\,M_\oplus (\pi_\rm{rel}/16 \,\rm \mu as)^{-1}$, where $\pi_\rm{rel}$ is the lens-source relative parallax. We find no significant evidence for a host of this planetary mass object, though one could be present at sufficiently wide separation. If so, it would be detectable after about 10 years. This is the fourth isolated microlens with a measured Einstein radius $\theta_\rm{E}<10\,\rm \mu as$, which we argue is a useful threshold for a "likely free-floating planet (FFP)" candidate. We outline a new approach to constructing a homogeneous sample of giant-star finite-source/point-lens (FSPL) events, within which the subsample of FFP candidates can be statistically analyzed. We illustrate this approach using 2019 KMTNet data and show that there is a large $\theta_\rm{E}$ gap between the two FFP candidates and the 11 other FSPL events. We argue that such sharp features are more identifiable in a sample selected on $\theta_\rm{E}$ compared to the traditional approach of identifying candidates based on short $t_\rm{E}$.

KMT-2018-BLG-0748Lb: Sub-Saturn Microlensing Planet Orbiting an Ultracool Host

Han Cheongho, Shin In-Gu, Jung Youn Kil, Kim Doeon, Yee Jennifer C., Albrow Michael D., Chung Sun-Ju, Gould Andrew, Hwang Kyu-Ha, Lee Chung-Uk
10 Jul 2020 astro-ph.EP astro-ph.SR arxiv.org/abs/2007.05204

We announce the discovery of a microlensing planetary system, in which a sub-Saturn planet is orbiting an ultracool dwarf host. We detect the planetary system by analyzing the short-timescale ($t_{\rm E}\sim 4.4$~days) lensing event KMT-2018-BLG-0748. The central part of the light curve exhibits asymmetry due to the negative deviations in the rising part and the positive deviations in the falling part. We find that the deviations are explained by a binary-lens model with a mass ratio between the lens components of $q\sim 2\times 10^{-3}$. The short event timescale together with the small angular Einstein radius, $\theta_{\rm E}\sim 0.11$~mas, indicate that the mass of the planet host is very small. The Bayesian analysis conducted under the assumption that the planet frequency is independent of the host mass indicates that the mass of the planet is $M_{\rm p}=0.18^{+0.29}{-0.10}~M{\rm J}$, and the mass of the host, $M_{\rm h}= 0.087^{+0.138}{-0.047}~M\odot$, is near the star/brown dwarf boundary, but the estimated host mass is sensitive to the assumption about the planet hosting probability. High-resolution follow-up observations would lead to revealing the nature of the planet host.

OGLE-2018-BLG-1269Lb: A Jovian Planet With A Bright, $I=16$ Host

Jung Youn Kil, Gould Andrew, Udalski Andrzej, Sumi Takahiro, Yee Jennifer C., Han Cheongho, Albrow Michael D., Chung Sun-Ju, Hwang Kyu-Ha, Ryu Yoon-Hyun
28 Jun 2020 astro-ph.SR astro-ph.EP astro-ph.GA arxiv.org/abs/2006.15774

We report the discovery of a planet in the microlensing event OGLE-2018-BLG-1269, with planet-host mass ratio $q \sim 6\times10^{-4}$, i.e., $0.6$ times smaller than the Jupiter/Sun mass ratio. Combined with the $Gaia$ parallax and proper motion, a strong one-dimensional constraint on the microlens parallax vector allows us to significantly reduce the uncertainties of lens physical parameters. A Bayesian analysis that ignores any information about light from the host yields that the planet is a cold giant $(M_{2} = 0.69_{-0.22}^{+0.44}\,M_{\rm J})$ orbiting a Sun-like star $(M_{1} = 1.13_{-0.35}^{+0.72}\,M_{\odot})$ at a distance of $D_{\rm L} = 2.56_{-0.62}^{+0.92}\,{\rm kpc}$. The projected planet-host separation is $a_{\perp} = 4.61_{-1.17}^{+1.70}\,{\rm au}$. Using {\it Gaia} astrometry, we show that the blended light lies $\lesssim 12\,$mas from the host and therefore must be either the host star or a stellar companion to the host. An isochrone analysis favors the former possibility at $>99.6\%$. The host is therefore a subgiant. For host metallicities in the range of $0.0 \leq {\rm [Fe/H]} \leq +0.3$, the host and planet masses are then in the range of $1.16 \leq M_{1}/M_{\odot} \leq 1.38$ and $0.74 \leq M_{2}/M_{\rm J} \leq 0.89$, respectively. Low host metallicities are excluded. The brightness and proximity of the lens make the event a strong candidate for spectroscopic followup both to test the microlensing solution and to further characterize the system.

OGLE-2017-BLG-0406: ${\it Spitzer}$ Microlens Parallax Reveals Saturn-mass Planet orbiting M-dwarf Host in the Inner Galactic Disk

Hirao Yuki, Bennett David P., Ryu Yoon-Hyun, Koshimoto Naoki, Udalski Andrzej, Yee Jennifer C., Sumi Takahiro, Bond Ian A., Shvartzvald Yossi, Abe Fumio
20 Apr 2020 astro-ph.EP astro-ph.GA astro-ph.SR arxiv.org/abs/2004.09067

We report the discovery and analysis of the planetary microlensing event OGLE-2017-BLG-0406, which was observed both from the ground and by the ${\it Spitzer}$ satellite in a solar orbit. At high magnification, the anomaly in the light curve was densely observed by ground-based-survey and follow-up groups, and it was found to be explained by a planetary lens with a planet/host mass ratio of $q=7.0 \times 10^{-4}$ from the light-curve modeling. The ground-only and ${\it Spitzer}$-"only" data each provide very strong one-dimensional (1-D) constraints on the 2-D microlens parallax vector $\bf{\pi_{\rm E}}$. When combined, these yield a precise measurement of $\bf{\pi_{\rm E}}$, and so of the masses of the host $M_{\rm host}=0.56\pm0.07\,M_\odot$ and planet $M_{\rm planet} = 0.41 \pm 0.05\,M_{\rm Jup}$. The system lies at a distance $D_{\rm L}=5.2 \pm 0.5 \ {\rm kpc}$ from the Sun toward the Galactic bulge, and the host is more likely to be a disk population star according to the kinematics of the lens. The projected separation of the planet from the host is $a_{\perp} = 3.5 \pm 0.3 \ {\rm au}$, i.e., just over twice the snow line. The Galactic-disk kinematics are established in part from a precise measurement of the source proper motion based on OGLE-IV data. By contrast, the ${\it Gaia}$ proper-motion measurement of the source suffers from a catastrophic $10\,\sigma$ error.

KMT-2019-BLG-1339L: an M Dwarf with a Giant Planet or a Companion Near the Planet/Brown Dwarf Boundary

Han Cheongho, Kim Doeon, Udalski Andrzej, Gould Andrew, Albrow Michael D., Chung Sun-Ju, Hwang Kyu-Ha, Jung Youn Kil, Lee Chung-Uk, Ryu Yoon-Hyun
04 Mar 2020 astro-ph.EP astro-ph.GA astro-ph.SR arxiv.org/abs/2003.02375

We analyze KMT-2019-BLG-1339, a microlensing event with an obvious but incompletely resolved brief anomaly feature around the peak of the light curve. Although the origin of the anomaly is identified to be a companion to the lens with a low mass ratio $q$, the interpretation is subject to two different degeneracy types. The first type is the ambiguity in $\rho$, representing the angular source radius scaled to the angular radius of the Einstein ring, $\theta_{\rm E}$, and the other is the $s\leftrightarrow s^{-1}$ degeneracy. The former type, `finite-source degeneracy', causes ambiguities in both $s$ and $q$, while the latter induces an ambiguity only in $s$. Here $s$ denotes the separation (in units of $\theta_{\rm E}$) in projection between the lens components. We estimate that the lens components have masses $(M_1, M_2)\sim (0.27^{+0.36}{-0.15}~M\odot, 11^{+16}{-7}~M{\rm J})$ and $\sim (0.48^{+0.40}{-0.28}~M\odot, 1.3^{+1.1}{-0.7}~M{\rm J})$ according to the two solutions subject to the finite-source degeneracy, indicating that the lens comprises an M dwarf and a companion with a mass around the planet/brown dwarf boundary or a Jovian-mass planet. It is possible to lift the finite-source degeneracy by conducting future observations utilizing a high resolution instrument because the relative lens-source proper motion predicted by the solutions are widely different.

One Planet or Two Planets? The Ultra-sensitive Extreme-magnification Microlensing Event KMT-2019-BLG-1953

Han Cheongho, Kim Doeon, Jung Youn Kil, Gould Andrew, Bond Ian A., Albrow Michael D., Chung Sun-Ju, Hwang Kyu-Ha, Lee Chung-Uk, Ryu Yoon-Hyun
12 Feb 2020 astro-ph.EP astro-ph.GA astro-ph.SR arxiv.org/abs/2002.05310

We present the analysis of a very high-magnification ($A\sim 900$) microlensing event KMT-2019-BLG-1953. A single-lens single-source (1L1S) model appears to approximately delineate the observed light curve, but the residuals from the model exhibit small but obvious deviations in the peak region. A binary lens (2L1S) model with a mass ratio $q\sim 2\times 10^{-3}$ improves the fits by $\Delta\chi^2=181.8$, indicating that the lens possesses a planetary companion. From additional modeling by introducing an extra planetary lens component (3L1S model) and an extra source companion (2L2S model), it is found that the residuals from the 2L1S model further diminish, but claiming these interpretations is difficult due to the weak signals with $\Delta\chi^2=16.0$ and $13.5$ for the 3L1S and 2L2L models, respectively. From a Bayesian analysis, we estimate that the host of the planets has a mass of $M_{\rm host}=0.31^{+0.37}{-0.17}~M\odot$ and that the planetary system is located at a distance of $D_{\rm L}=7.04^{+1.10}{-1.33}~{\rm kpc}$ toward the Galactic center. The mass of the securely detected planet is $M{\rm p}=0.64^{+0.76}{-0.35}~M{\rm J}$. The signal of the potential second planet could have been confirmed if the peak of the light curve had been more densely observed by followup observations, and thus the event illustrates the need for intensive followup observations for very high-magnification events even in the current generation of high-cadence surveys.

KMT-2019-BLG-0842Lb: A Cold Planet Below the Uranus/Sun Mass Ratio

Jung Youn Kil, Udalski Andrzej, Zang Weicheng, Bond Ian A., Yee Jennifer C., Han Cheongho, Albrow Michael D., Chung Sun-Ju, Gould Andrew, Hwang Kyu-Ha
08 Dec 2019 astro-ph.EP astro-ph.GA astro-ph.SR arxiv.org/abs/1912.03822

We report the discovery of a cold planet with a very low planet/host mass ratio of $q=(4.09\pm0.27) \times 10^{-5}$, which is similar to the ratio of Uranus/Sun ($q=4.37 \times 10^{-5}$) in the Solar system. The Bayesian estimates for the host mass, planet mass, system distance, and planet-host projected separation are $M_{\rm host}=0.76\pm 0.40 M_\odot$, $M_{\rm planet}=10.3\pm 5.5 M_\oplus$, $D_{\rm L} = 3.3\pm1.3\,{\rm kpc}$, and $a_\perp = 3.3\pm 1.4\,{\rm au}$, respectively. The consistency of the color and brightness expected from the estimated lens mass and distance with those of the blend suggests the possibility that the most blended light comes from the planet host, and this hypothesis can be established if high resolution images are taken during the next (2020) bulge season. We discuss the importance of conducting optimized photometry and aggressive follow-up observations for moderately or very high magnification events to maximize the detection rate of planets with very low mass ratios.