Yoon-Hyun Ryu

Bio: Yoon-Hyun Ryu is an academic researcher from Korea Astronomy and Space Science Institute. The author has contributed to research in topics: Gravitational microlensing & Planet. The author has an hindex of 15, co-authored 114 publications receiving 826 citations. Previous affiliations of Yoon-Hyun Ryu include Harvard University & Chungbuk National University.

Properties of Central Caustics in Planetary Microlensing

Abstract: To maximize the number of planet detections, current microlensing follow-up observations are focusing on high-magnification events that have a higher chance of being perturbed by central caustics. In this paper, we investigate the properties of central caustics and the perturbations that they induce. We derive analytic expressions for the location, size, and shape of the central caustic as a function of the star-planet separation, s, and the planet/star mass ratio, q, under the planetary perturbative approximation and compare the results with those based on numerical computations. While it has been known that the size of the planetary caustic is ∝q1/2, we find from this work that the dependence of the size of the central caustic on q is linear, i.e., ∝q, implying that the central caustic shrinks much more rapidly with the decrease of q compared to the planetary caustic. The central caustic size also depends on the star-planet separation. If the size of the caustic is defined as the separation between the two cusps on the star-planet axis (horizontal width), we find that the dependence of the central caustic size on the separation is ∝(s + s-1). While the size of the central caustic depends both on s and on q, its shape, defined as the vertical/horizontal width ratio, c, is solely dependent on the planetary separation, and we derive an analytic relation between c and s. Due to the smaller size of the central caustic, combined with a much more rapid decrease of its size with the decrease of q, the effect of finite source size on the perturbation induced by the central caustic is much more severe than the effect on the perturbation induced by the planetary caustic. As a result, we find that although giant planets with q 10-3 can be detected from the planet-search strategy of monitoring high-magnification events, detecting signals of Earth-mass planets with q ~ 10-5 will be very difficult. Although the central caustics of a pair of planets with separations s and s-1 are identical to linear order, we find that the magnification patterns induced by a pair of degenerate caustics of planets with q 10-3 are different to the level of being noticed in observations with 2% photometry. Considering that the majority of planets that would be detected by the strategy of monitoring high-magnification events are giant planets, we predict that the s ↔ s-1 degeneracy could be broken for a majority of planetary events from observations with good enough precision.

Gravitational Microlensing: A Tool for Detecting and Characterizing Free-Floating Planets

Abstract: Various methods have been proposed to search for extrasolar planets. Compared to the other methods, microlensing has unique applicabilities to the detections of Earth-mass and free-floating planets. However, the microlensing method is seriously flawed by the fact that the masses of the detected planets cannot be uniquely determined. Recently, Gould, Gaudi, & Han introduced an observational setup that enables one to resolve the mass degeneracy of the Earth-mass planets. The setup requires a modest adjustment to the orbit of an already proposed microlensing planet-finder satellite combined with ground-based observations. In this paper, we show that a similar observational setup can also be used for the mass determinations of free-floating planets with masses ranging from several Earth masses to several Jupiter masses. If the proposed observational setup is realized, future lensing surveys will play important roles in the studies of Earth-mass and free-floating planets, which are the populations of planets that have not been previously probed.

KMT-2017-BLG-0165Lb: A Super-Neptune mass planet Orbiting a Sun-like Host Star

Abstract: We report the discovery of a low mass-ratio planet $(q = 13\times10^{-4})$, ie, 25 times higher than the Neptune/Sun ratio The planetary system was discovered from the analysis of the KMT-2017-BLG-0165 microlensing event, which has an obvious short-term deviation from the underlying light curve produced by the host of the planet Although the fit improvement with the microlens parallax effect is relatively low, one component of the parallax vector is strongly constrained from the light curve, making it possible to narrow down the uncertainties of the lens physical properties A Bayesian analysis yields that the planet has a super-Neptune mass $(M_{2}=34_{-12}^{+15}~M_{\oplus})$ orbiting a Sun-like star $(M_{1}=076_{-027}^{+034}~M_{\odot})$ located at $45~{\rm kpc}$ The blended light is consistent with these host properties The projected planet-host separation is $a_{\bot}={345_{-095}^{+098}}~{\rm AU}$, implying that the planet is located outside the snowline of the host, ie, $a_{sl}\sim21~{\rm AU}$ KMT-2017-BLG-0165Lb is the sixteenth microlensing planet with mass ratio $q 33$ is quite severe Alternatively, the distribution is also suggestive of a "pile-up" of planets at Neptune-like mass ratios, below which there is a dramatic drop in frequency

OGLE-2017-BLG-1434Lb: Eighth q<1×10-4 Mass-Ratio Microlens Planet Confirms Turnover in Planet Mass-Ratio Function

Abstract: We report the discovery of a cold Super-Earth planet (mp=4.4±0.5 M⊕) orbiting a low-mass (M=0.23±0.03) M⊙ dwarf at projected separation a⊥=1.18±0.10 a.u., i.e., about 1.9 times the distance the snow line. The system is quite nearby for a microlensing planet, DL=0.86±0.09 kpc. Indeed, it was the large lens-source relative parallax πrel=1.0 mas (combined with the low mass M) that gave rise to the large, and thus well-measured, "microlens parallax" πE∝(πrel/M)1/2 that enabled these precise measurements. OGLE-2017-BLG-1434Lb is the eighth microlensing planet with planet-host mass ratio q<1×10-4. We apply a new planet-detection sensitivity method, which is a variant of "V/Vmax", to seven of these eight planets to derive the mass-ratio function in this regime. We find dN/d lnq ∝ qp, with p=1.05+0.78-0.68, which confirms the "turnover" in the mass function found by Suzuki et al. relative to the power law of opposite sign n=-0.93±0.13 at higher mass ratios q≳2×10-4. We combine our result with that of Suzuki et al. to obtain p=0.73+0.42-0.34.

A Free-floating or Wide-orbit Planet in the Microlensing Event OGLE-2019-BLG-0551

Abstract: High-cadence observations of the Galactic bulge by the microlensing surveys led to the discovery of a handful of extremely short-timescale microlensing events that can be attributed to free-floating or wide-orbit planets. Here, we report the discovery of another strong free-floating planet candidate, which was found from the analysis of the gravitational microlensing event OGLE-2019-BLG-0551. The light curve of the event is characterized by a very short duration (≾3 days) and a very small amplitude (≾0.1 mag). From modeling of the light curve, we find that the Einstein timescale, t_E = 0.381 ± 0.017 day, is much shorter, and the angular Einstein radius, θ_E = 4.35 ± 0.34 μas, is much smaller than those of typical lensing events produced by stellar-mass lenses (t_E ~ 20 days, θ_E ~ 0.3 mas), indicating that the lens is very likely to be a planetary-mass object. We conduct an extensive search for possible signatures of a companion star in the light curve of the event, finding no significant evidence for the putative host star. For the first time, we also demonstrate that the angular Einstein radius of the lens does not depend on blending in the low-magnification events with strong finite source effects.

The Exoplanet Handbook

Abstract: 1. Introduction 2. Radial velocities 3. Astrometry 4. Timing 5. Microlensing 6. Transits 7. Imaging 8. Host stars 9. Brown dwarfs and free-floating planets 10. Formation and evolution 11. Interiors and atmospheres 12. The Solar System Appendixes References Index.

Frequency of solar-like systems and of ice and gas giants beyond the snow line from high-magnification microlensing events in 2005-2008

Abstract: We present the first measurement of the planet frequency beyond the "snow line," for the planet-to-star mass-ratio interval –4.5 200) microlensing events during 2005-2008. The sampled host stars have a typical mass M_(host) ~ 0.5 M_⊙, and detection is sensitive to planets over a range of planet-star-projected separations (s ^(–1)_(max)R_E, s_(max)R_E), where R_E ~ 3.5 AU(M_(host)/M_⊙)^(1/2) is the Einstein radius and s_(max) ~ (q/10^(–4.3))^(1/3). This corresponds to deprojected separations roughly three times the "snow line." We show that the observations of these events have the properties of a "controlled experiment," which is what permits measurement of absolute planet frequency. High-magnification events are rare, but the survey-plus-follow-up high-magnification channel is very efficient: half of all high-mag events were successfully monitored and half of these yielded planet detections. The extremely high sensitivity of high-mag events leads to a policy of monitoring them as intensively as possible, independent of whether they show evidence of planets. This is what allows us to construct an unbiased sample. The planet frequency derived from microlensing is a factor 8 larger than the one derived from Doppler studies at factor ~25 smaller star-planet separations (i.e., periods 2-2000 days). However, this difference is basically consistent with the gradient derived from Doppler studies (when extrapolated well beyond the separations from which it is measured). This suggests a universal separation distribution across 2 dex in planet-star separation, 2 dex in mass ratio, and 0.3 dex in host mass. Finally, if all planetary systems were "analogs" of the solar system, our sample would have yielded 18.2 planets (11.4 "Jupiters," 6.4 "Saturns," 0.3 "Uranuses," 0.2 "Neptunes") including 6.1 systems with two or more planet detections. This compares to six planets including one two-planet system in the actual sample, implying a first estimate of 1/6 for the frequency of solar-like systems.

The Exoplanet Handbook: Formation and evolution

Abstract: PLANETARY SYSTEMS, the solar system amongst them, are believed to form as inevitable and common byproducts of star formation For orientation, an overview of the processes described in this chapter is as follows The present paradigm starts with star formation in molecular clouds Brown dwarfs are formed as the lowmass tail of this process, although some may be formed as a high-mass tail of planet formation Gas and dust in the collapsing molecular cloud which does not fall directly onto the protostar resides in a relatively long-lived accretion disk which provides the environment for the subsequent stages of planet formation Terrestrial-mass planets are formed within the disk through the progressive agglomeration of material denoted, as it grows in size, as dust, rocks, planetesimals and protoplanets A similar process typically occurring further out in the disk results in the cores of giant planets, which then gravitationally accumulate their mantles of ice and/or gas As the planet-forming bodies grow in mass, growth and dynamics become more dominated by gravitational interactions Towards the final phases, and before the remaining gas is lost through accretion or dispersal, the gas provides a viscous medium at least partially responsible for planetary migration Some migration also occurs during these later stages as a result of gravitational scattering between the (proto-)planets and the residual sea of planetesimals The final structural stabilisation of the planetary system may be affected by planet–planet interactions, until a configuration emerges which may be dynamically stable over billions of years The current observational data for exoplanet systems is broadly compatible with this overall picture Other constraints come from a substantial body of detailed observations of the solar system (Chapter 12) Context and present paradigm An understanding of howplanets formis essential in understanding and interpreting the considerable range of observed planetary system architectures and dynamics Today, the most widely considered solar nebula theory holds that planet formation in the solar system, and by inference in other exoplanet systems, follows on from the process of star formation and accretion disk formation, through the agglomeration of residual material as the protoplanetary disk collapses and evolves

The exoplanet mass-ratio function from the moa-ii survey: discovery of a break and likely peak at a neptune mass

Abstract: We report the results of the statistical analysis of planetary signals discovered in MOA-II microlensing survey alert system events from 2007 to 2012. We determine the survey sensitivity as a function of planet star mass ratio, q, and projected planet star separation, s, in Einstein radius units. We find that the mass-ratio function is not a single power law, but has a change in slope at q approx.10(exp -4), corresponding to approx. 20 Stellar Mass for the median host-star mass of approx. 0.6 M. We find significant planetary signals in 23 of the 1474 alert events that are well-characterized by the MOA-II survey data alone. Data from other groups are used only to characterize planetary signals that have been identified in the MOA data alone. The distribution of mass ratios and separations of the planets found in our sample are well fit by a broken power-law model. We also combine this analysis with the previous analyses of Gould et al. and Cassan et al., bringing the total sample to 30 planets. The unbroken power-law model is disfavored with a p-value of 0.0022, which corresponds to a Bayes factor of 27 favoring the broken power-law model. These results imply that cold Neptunes are likely to be the most common type of planets beyond the snow line.

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.