In-Gu Shin

Bio: In-Gu Shin is an academic researcher from Korea Astronomy and Space Science Institute. The author has contributed to research in topics: Gravitational microlensing & Light curve. The author has an hindex of 19, co-authored 107 publications receiving 1288 citations. Previous affiliations of In-Gu Shin include Chungbuk National University & Tsinghua University.

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

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

Microlensing discovery of a tight, low-mass-ratio planetary-mass object around an old field brown dwarf

Abstract: Observations of accretion disks around young brown dwarfs (BDs) have led to the speculation that they may form planetary systems similar to normal stars. While there have been several detections of planetary-mass objects around BDs (2MASS 1207-3932 and 2MASS 0441-2301), these companions have relatively large mass ratios and projected separations, suggesting that they formed in a manner analogous to stellar binaries. We present the discovery of a planetary-mass object orbiting a field BD via gravitational microlensing, OGLE-2012-BLG-0358Lb. The system is a low secondary/primary mass ratio (0.080 ± 0.001), relatively tightly separated (~0.87 AU) binary composed of a planetary-mass object with 1.9 ± 0.2 Jupiter masses orbiting a BD with a mass 0.022 M ☉. The relatively small mass ratio and separation suggest that the companion may have formed in a protoplanetary disk around the BD host in a manner analogous to planets.

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

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

OGLE-2017-BLG-1522: A giant planet around a brown dwarf located in the Galactic bulge

Abstract: We report the discovery of a giant planet in the OGLE-2017-BLG-1522 microlensing event. The planetary perturbations were clearly identified by high-cadence survey experiments despite the relatively short event timescale of $t_{\\rm E} \\sim 7.5$ days. The Einstein radius is unusually small, $\\theta_{\\rm E} = 0.065\\,$mas, implying that the lens system either has very low mass or lies much closer to the microlensed source than the Sun, or both. A Bayesian analysis yields component masses $(M_{\\rm host}, M_{\\rm planet})=(46_{-25}^{+79}, 0.75_{-0.40}^{+1.26})~M_{\\rm J}$ and source-lens distance $D_{\\rm LS} = 0.99_{-0.54}^{+0.91}~{\\rm kpc}$, implying that this is a brown-dwarf/Jupiter system that probably lies in the Galactic bulge, a location that is also consistent with the relatively low lens-source relative proper motion $\\mu = 3.2 \\pm 0.5~{\\rm mas}~{\\rm yr^{-1}}$. The projected companion-host separation is $0.59_{-0.11}^{+0.12}~{\\rm AU}$, indicating that the planet is placed beyond the snow line of the host, i.e., $a_{sl} \\sim 0.12~{\\rm AU}$. Planet formation scenarios combined with the small companion-host mass ratio $q \\sim 0.016$ and separation suggest that the companion could be the first discovery of a giant planet that formed in a protoplanetary disk around a brown dwarf host.

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

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

Water loss from terrestrial planets orbiting ultracool dwarfs: implications for the planets of TRAPPIST-1

Abstract: Ultracool dwarfs (UCDs) encompass the population of extremely low mass stars (later than M6-type) and brown dwarfs.Because UCDs cool monotonically, their habitable zone (HZ) sweeps inward in time.Assuming they possess water, planets found in the HZ of UCDs have experienced a runaway greenhouse phase too hot for liquid water prior to entering the HZ.It has been proposed that such planets are desiccated by this hot early phase and enter the HZ as dry, inhospitable worlds.Here we model the water loss during this pre-HZ hot phase taking into account recent upper limits on the XUV emission of UCDs and using 1D radiation-hydrodynamic simulations.We address the whole range of UCDs but also focus on the planets b, c and d recently found around the 0.08 M⊙ dwarf TRAPPIST-1.Despite assumptions maximizing the FUV-photolysis of water and the XUV-driven escape of hydrogen, we find that planets can retain significant amounts of water in the HZ of UCDs, with a sweet spot in the 0.04-0.06 M⊙ range.We also studied the TRAPPIST-1 system using observed constraints on the XUV-flux.We found that TRAPPIST-1b and c can lose as much as 15 Earth Ocean and planet d -- which may be inside the HZ depending on its actual period -- may have lost less than 1 Earth Ocean.Depending on its initial content, they could have enough water to remain habitable.TRAPPIST-1 planets are key targets for atmospheric characterization and could provide strong constraints on the water erosion around UCDs.

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

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

How to Constrain Your M dwarf II: the mass-luminosity-metallicity relation from 0.075 to 0.70$M_\odot$

Abstract: The mass-luminosity relation for late-type stars has long been a critical tool for estimating stellar masses. However, there is growing need for both a higher-precision relation and a better understanding of systematic effects (e.g., metallicity). Here we present an empirical relationship between Mks and mass spanning $0.075M_\odot

Detecting Earth-Mass Planets with Gravitational Microlensing

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