Doeon Kim

Bio: Doeon Kim is an academic researcher from Chungbuk National University. The author has contributed to research in topics: Gravitational microlensing & Planet. The author has an hindex of 15, co-authored 66 publications receiving 640 citations. Previous affiliations of Doeon Kim include Korea Astronomy and Space Science Institute.

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.

VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis

Abstract: Context. Until recently, the 3D shape, and therefore density (when combining the volume estimate with available mass estimates), and surface topography of the vast majority of the largest (D ≥ 100 km) main-belt asteroids have remained poorly constrained. The improved capabilities of the SPHERE/ZIMPOL instrument have opened new doors into ground-based asteroid exploration.Aims. To constrain the formation and evolution of a representative sample of large asteroids, we conducted a high-angular-resolution imaging survey of 42 large main-belt asteroids with VLT/SPHERE/ZIMPOL. Our asteroid sample comprises 39 bodies with D ≥ 100 km and in particular most D ≥ 200 km main-belt asteroids (20/23). Furthermore, it nicely reflects the compositional diversity present in the main belt as the sampled bodies belong to the following taxonomic classes: A, B, C, Ch/Cgh, E/M/X, K, P/T, S, and V.Methods. The SPHERE/ZIMPOL images were first used to reconstruct the 3D shape of all targets with both the ADAM and MPCD reconstruction methods. We subsequently performed a detailed shape analysis and constrained the density of each target using available mass estimates including our own mass estimates in the case of multiple systems.Results. The analysis of the reconstructed shapes allowed us to identify two families of objects as a function of their diameters, namely “spherical” and “elongated” bodies. A difference in rotation period appears to be the main origin of this bimodality. In addition, all but one object (216 Kleopatra) are located along the Maclaurin sequence with large volatile-rich bodies being the closest to the latter. Our results further reveal that the primaries of most multiple systems possess a rotation period of shorter than 6 h and an elongated shape (c∕a ≤ 0.65). Densities in our sample range from ~1.3 g cm−3 (87 Sylvia) to ~4.3 g cm−3 (22 Kalliope). Furthermore, the density distribution appears to be strongly bimodal with volatile-poor (ρ ≥ 2.7 g cm−3) and volatile-rich (ρ ≤ 2.2 g cm−3) bodies. Finally, our survey along with previous observations provides evidence in support of the possibility that some C-complex bodies could be intrinsically related to IDP-like P- and D-type asteroids, representing different layers of a same body (C: core; P/D: outer shell). We therefore propose that P/ D-types and some C-types may have the same origin in the primordial trans-Neptunian disk.

The Possibility of Detecting Planets in the Andromeda Galaxy

Abstract: The Angstrom project is using a global network of 2 m class telescopes to conduct a high-cadence pixel microlensing survey of the bulge of the Andromeda galaxy (M31), with the primary aim of constraining its underlying bulge mass distribution and stellar mass function. Here we investigate the feasibility of using such a survey to detect planets in M31. We estimate the efficiency of detecting signals produced by planets with various masses and separations from the host star. We find that for a ~5MJ planet that is located within the lensing zone (~1-3 AU), detection is possible above 3 σ with detection efficiency ~6%. This corresponds to the yearly detection rate of ~3fLZ planets, where fLZ is the probability that a planet exists in the lensing zone. It is expected that most events with detectable planets are associated with giant source stars, and thus source size will have a significant effect on the planet detection efficiency. We also find that the planetary perturbations will be in nearly all cases caused by central caustics, and thus observational strategies focusing on these central perturbations would maximize planet detections. A dramatic improvement in the efficiency of ~30%-50% is expected if follow-up observations on an 8 m telescope are made possible by a real-time alert system.

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.