arXiv subjects

Cheongho Han

Publications and source records attributed to Cheongho Han.

At least 217 records · Page 12Linked to original sources

arXiv2003

Stellar Contribution to the Galactic Bulge Microlensing Optical Depth

We estimate the optical depth to self-lensing by stars in the Galactic bulge using the HST star counts of Holtzman et al and Zoccali et al as extrapolated by Gould into the brown-dwarf and remnant regimes and deprojected along the line of sight using the model of Dwek et al. We find a self-lensing optical depth tau(bulge-bulge)=0.98 x 10^{-6}. When combined with the lensing of bulge stars by foreground stars in the disk, this yields tau(bulge-total)=1.63 x 10^{-6}, in reasonable agreement with the estimates of tau=2.13 +/- 0.40 x 10^{-6} and tau=1.08 +/- 0.30 x 10^{-6} based on observations of clump giants by the MACHO and EROS collaborations.

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arXiv2003

Probing Structures of Distant Extrasolar Planets with Microlensing

Planetary companions to the source stars of a caustic-crossing binary microlensing events can be detected via the deviation from the parent light curves created when the caustic magnifies the star light reflecting off the atmosphere or surface of the planets. The magnitude of the deviation is delta_p e_p rho_p^{-1/2}, where e_p is the fraction of starlight reflected by the planet and rho_p is the angular radius of the planet in units of angular Einstein ring radius. Due to the extraordinarily high resolution achieved during the caustic crossing, the detailed shapes of these perturbations are sensitive to fine structures on and around the planets. We consider the signatures of rings, satellites, and atmospheric features on caustic-crossing microlensing light curves. We find that, for reasonable assumptions, rings produce deviations of order 10% delta_p, whereas satellites, spots, and zonal bands produce deviations of order 1% delta_p. We consider the detectability of these features using current and future telescopes, and find that, with very large apertures (>30m), ring systems may be detectable, whereas spots, satellites, and zonal bands will generally be difficult to detect. We also present a short discussion of the stability of rings around close-in planets, noting that rings are likely to be lost to Poynting-Robertson drag on a timescale of order 10^5 years, unless they are composed of large (>>1 cm) particles, or are stabilized by satellites.

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arXiv2002

Angular Radii of Stars via Microlensing

We outline a method by which the angular radii of giant and main sequence stars in the Galactic bulge can be measured to a few percent accuracy. The method combines ground-based photometry of caustic-crossing bulge microlensing events, with a handful of precise astrometric measurements of the lensed star during the event, to measure the angular radius of the source, theta_*. Dense photometric coverage of one caustic crossing yields the crossing timescale dt. Less frequent coverage of the entire event yields the Einstein timescale t_E and the angle phi of source trajectory with respect to the caustic. The photometric light curve solution predicts the motion of the source centroid up to an orientation on the sky and overall scale. A few precise astrometric measurements therefore yield theta_E, the angular Einstein ring radius. Then the angular radius of the source is obtained by theta_*=theta_E(dt/t_E) sin(phi). We argue that theta_* should be measurable to a few percent accuracy for Galactic bulge giant stars using ground-based photometry from a network of small (1m-class) telescopes, combined with astrometric observations with a precision of ~10 microarcsec to measure theta_E. We find that a factor of ~50 times fewer photons are required to measure theta_E to a given precision for binary-lens events than single-lens events. Adopting parameters appropriate to the Space Interferometry Mission (SIM), ~7 min of SIM time is required to measure theta_E to ~5% accuracy for giant sources in the bulge. For main-sequence sources, theta_E can be measured to ~15% accuracy in ~1.4 hours. With 10 hrs of SIM time, it should be possible to measure theta_* to ~5% for \~80 giant stars, or to 15% for ~7 main sequence stars. A byproduct of such a campaign is a significant sample of precise binary-lens mass measurements.

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arXiv2002

Direct Lens Imaging of Galactic Bulge Microlensing Events

Recently, from the Hubble Space Telescope (HST) images of one of the Large Magellanic Cloud (LMC) events taken 6.3 years after the original lensing measurement, Alcock et al. were able to directly image the lens. Although the first resolved lens was identified for an LMC event, much more numerous lenses are expected to be resolved for Galactic bulge events. In this paper, we estimate the fraction of Galactic bulge events whose lenses can be directly imaged under the assumption that all bulge events are caused by normal stars. For this determination, we compute the distribution of lens proper motions of the currently detected Galactic bulge events based on standard models of the geometrical and kinematical distributions of lenses and their mass function. We then apply realistic criteria for lens resolution, and the result is presented as a function of the time elapsed after an original lensing measurement, $Δt$. If followup observations are performed by using an instrument with a resolving power of θ_{PSF}=0.1'', which corresponds to that of HST equipped with the new Advanced Camera for Surveys, we estimate that lenses can be resolved for ~ 3% and 22% of disk-bulge events and for ~0.3% and 6% of bulge self-lensing events after $Δt=10$ and 20 years, respectively. The fraction increases substantially with the increase of the resolving power. If the instrument has a resolution of θ_{PSF}=0.05'', which can be achieved by the {\it Next Generation Space Telescope}, we estimate that lenses can be resolved for $\sim 22%$ and 45% of disk-bulge events and for ~6% and 23% of bulge self-lensing events after Δt=10 and 20 years, respectively.

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arXiv2002

On the Feasibility of Detecting Satellites of Extrasolar Planets via Microlensing

Although many methods of detecting extra-solar planets have been proposed and successful implementation of some of these methods enabled a rapidly increasing number of exoplanet detections, little has been discussed about the method of detecting satellites around exoplanets. In this paper, we test the feasibility of detecting satellites of exoplanets via microlensing. For this purpose, we investigate the effect of satellites in the magnification pattern near the region of the planet-induced perturbations by performing realistic simulations of Galactic bulge microlensing events. From this investigation, we find that although satellites can often cause alterations of magnification patterns, detecting satellite signals in lensing light curves will be very difficult because the signals are seriously smeared out by the severe finite source effect even for events involved with source stars with small angular radii.

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arXiv2002

Astrometric Microlensing: A Channel to Detect Multiple Lens Systems

If a source star is gravitationally microlensed by a multiple lens system, the resulting light curve can have significant deviations from the standard form of a single lens event. The chance to produce significant deviations becomes important when the separations between the component lenses are equivalent to the combined angular Einstein ring radius of the system. For multiple lens systems composed of more than two lenses, however, this condition is difficult to meet because the orbits of such systems are unstable. Even if events are caused by a multiple lens system with stable orbits where a pair of lenses are closely located and the other component (third body) has a wide separation from the pair, photometrically identifying the lens multiplicity will be difficult because the event will be identified either by a binary lens event caused by the close pair lenses or a single lens event caused by the third body. In this paper, we show that if a seemingly binary lens event is astrometrically followed up by using future high precision interferometers, the existence of an additional third body can be identified via a repeating event. We show that the signatures of third bodies can be unambiguously identified from the characteristic distortions they make in the centroid shift trajectories. We also show that due to the long range astrometric effect of third bodies, the detection efficiency will be considerable even for third bodies with large separations from their close lens pairs.

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arXiv2002

Effect of a wide binary companion to the lens on the astrometric behavior of gravitational microlensing events

In this paper, we investigate the effect of a wide binary companion of the lens on the astrometric behavior of Galactic gravitational microlensing events and compare it to the effect on the photometric behavior. We find that the wide binary companion of the lens can affect the centroid motion of images substantially even if the corresponding light curve appears to be the one of a standard single point-mass lens event. The relatively significant effect of the wide binary lens on the astrometric lensing behavior, on one side, calls for careful consideration of the lens binarity in analyzing the future astrometric lensing data. On the other side, larger astrometric effect of the companion makes astrometric lensing an efficient method to detect binary lenses over a broad range of separations.

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arXiv2002

A New Channel to Search for Extra-solar Systems with Multiple Planets via Gravitational Microlensing

Gaudi, Naber & Sackett pointed out that if an event is caused by a lens system containing more than two planets, all planets will affect the central region of the magnification pattern, and thus the existence of the multiple planets can be inferred by detecting additionally deformed anomalies from intensive monitoring of high magnification events. Unfortunately, this method has important limitations in identifying the existence of multiple planets and determining their parameters due to the degeneracy of the resulting light curve anomalies from those induced by a single planet and the complexity of multiple planet lensing models. In this paper, we propose a new channel to search for multiple planets via microlensing. The method is based on the fact that the anomalies induced by multiple planets are well approximated by the superposition of those of the single planet systems where the individual planet-primary pairs act as independent lens systems. Then, if the source trajectory passes both of the outer deviation regions induced by the individual planets, one can unambiguously identify the existence of the multiple planets. We illustrate that the probability of successively detecting light curve anomalies induced by two Jovian-mass planets located in the lensing zone through this channel will be substantial. Since the individual anomalies can be well modeled by much simpler single planet lensing models, the proposed method has an important advantage of allowing one to accurately determine the parameters of the individual planets.

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arXiv2002

Variation of Spot-induced Anomalies in Caustic-crossing Binary Microlensing Event Light Curves

We investigate the pattern of anomalies in the light curves of caustic-crossing binary microlensing events induced by spot(s) on the lensed source star. For this purpose, we perform simulations of events with various models of spots. From these simulations, we find that the spot-induced anomalies take various forms depending on the physical state of spots, which is characterized by the surface brightness contrast, the size, the number, the umbra/penumbra structure, the shape, and the orientation with respect to the sweeping caustic. We also examine the feasibility of distinguishing the two possibly degenerate types of anomalies caused by a spot and a transiting planet and find that the degeneracy in many cases can be broken from the characteristic multiple deviation feature in the spot-induced anomaly pattern caused by the multiplicity of spots.

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arXiv2001

Astrometric Detection of Double Gravitational Microlensing Events

If a gravitational microlensing event is caused by a widely separated binary lens and the source approaches both lens components, the source flux is successively magnified by the individual lenses: double microlensing events. If events are observed astrometrically, double lensing events are expected to occur with an increased frequency due to the long range astrometric effect of the companion. We find that although the trajectory of the source star image centroid shifts of an astrometric double lensing event has a distorted shape from both of the elliptical ones induced by the individual single lens components, event duplication can be readily identified by the characteristic loop in the trajectory formed during the source's passage close to the companion. We determine and compare the probabilities of detecting double lensing events from both photometric and astrometric lensing observations by deriving analytic expressions for the relations between binary lensing parameters to become double lensing events. From this determination, we find that for a given set of the binary separation and the mass ratio the astrometric probability is roughly an order higher than the photometric probability. Therefore, we predict that a significant fraction of events that will be followed up by using future high precision interferometeric instruments will be identified as double lensing events.

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arXiv2001

Properties of Planet-induced Deviations in the Astrometric Microlensing Centroid Shift Trajectory

In this paper, we investigate the properties of the planet-induced deviations in the trajectory of the microlensed source star centroid motion (astrometric curve) and the correlations between the astrometric and photometric deviations. For this, we construct vector field maps of excess centroid shifts. Fromthe investigation of the maps, we find that the astrometric deviation is closely correlated with the photometric one. The astrometric deviation increases as the photometric deviation increases and $Δ\deltavec$ is directed towards the planet when the light curve has positive deviation and vice versa. We also present excess centroid shift maps for lens systems with various values of the planetary separation, planet/primary mass ratio, and source size to show the changes in the pattern of excess centroid shifts with these parameters.

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arXiv2001

The Effect of a Binary Source Companion on the Astrometric Microlensing Behavior

If gravitational microlensing occurs in a binary-source system, both source components are magnified, and the resulting light curve deviates from the standard one of a single source event. However, in most cases only one source component is highly magnified and the other component (the companion) can be treated as a simple blending source: blending approximation. In this paper, we show that, unlike the light curves, the astrometric curves, representing the trajectories of the source image centroid, of an important fraction of binary-source events will not be sufficiently well modeled by the blending effect alone. This is because the centroid shift induced by the source companion endures to considerable distances from the lens. Therefore, in determining the lens parameters from astrometric curves to be measured by future high-precision astrometric instruments, it will be important to take the full effect of the source companion into consideration.

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arXiv2001

Properties of Microlensing Light Curve Anomalies Induced by Multiple Planets

In this paper, we show that the pattern of microlensing light curve anomalies induced by multiple planets are well described by the superposition of those of the single-planet systems where the individual planet-primary binary pairs act as independent lens systems. Since the outer deviation regions around the planetary caustics of the individual planets occur in general at different locations, we find that the pattern of anomalies in these regions are hardly affected by the existence of other planet(s). This implies that even if an event is caused by a multiple planetary system, a simple single-planet lensing model is good enough for the description of most anomalies caused by the source passage of the outer deviation regions. Detection of the anomalies resulting from the source trajectory passing both the outer deviation regions caused by more than two planets will provide a new channel of detecting multiple planets.

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arXiv2001

Astrometric Method to Break the Photometric Degeneracy between Binary-source and Planetary Microlensing Perturbations

An extra-solar planet can be detected by microlensing because the planet can perturb the smooth lensing light curve created by the primary lens. However, it was shown by Gaudi that a subset of binary-source events can produce light curves that closely resemble those produced by a significant fraction of planet/star lens systems, causing serious contamination of a sample of suspected planetary systems detected via microlensing. In this paper, we show that if a lensing event is observed astrometrically, one can unambiguously break the photometric degeneracy between binary-source and planetary lensing perturbations. This is possible because while the planet-induced perturbation in the trajectory of the lensed source image centroid shifts points away from the opening of the unperturbed elliptical trajectory, while the perturbation induced by the binary source companion points always towards the opening. Therefore, astrometric microlensing observations by using future high-precision interferometers will be important for solid confirmation of microlensing planet detections.

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arXiv2001

Another Channel to detect Close-in Binary Companions via Gravitational Microlensing

Gaudi & Gould (1997) showed that close companions of remote binary systems can be efficiently detected by using gravitational microlensing via the deviations in the lensing light curves induced by the existence of the lens companions. In this paper, we introduce another channel to detect faint close-in binary companions by using microlensing. This method utilizes a caustic-crossing binary lens event with a source also composed of binary stars, where the companion is a faint star. Detection of the companion is possible because the flux of the companion can be highly amplified when it crosses the lens caustic. The detection is facilitated since the companion is more amplified than the primary because it, in general, has a smaller size than the primary, and thus experiences less finite source effect. The method is extension of the previous one suggested to detect close-in giant planets by Graff & Gaudi (2000) and Lewis & Ibata (2000) and further developed by Ashton & Lewis (2001). From the simulations of realistic Galactic bulge events, we find that companions of K-type main sequence or brighter can be efficiently detected from the current type microlensing followup observations by using the proposed method. We also find that compared to the method of detecting lens companions for which the efficiency drops significantly for binaries with separations $\lesssim 0.2$ of the angular Einstein ring radius, the proposed method has an important advantage of being able to detect companions with substantially smaller separations down to order of 0.01 θ_E.

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arXiv2001

Galactic Bulge Pixel Lensing Events

Gould & DePoy proposed a pixel lensing survey towards the Galactic bulge using a small aperture (~65 mm) camera with a large pixel size (10") detector and deliberately degraded optics achieving 30" PSF. In this paper, we estimate the event rate of this pixel lensing survey expected under various detection criteria, which are characterized by the threshold signal-to-noise ratio, (S/N)_{th}, and event duration, t_{dur,th}, and investigate the characteristics of the detectable events. From this investigation, we find that the event rate varies significantly in the range ~6/yr - 120/yr depending strongly on the imposed detection criteria, implying that to maximize event detections it will be essential to identify events by diligently inspecting light variations and to promptly conduct followup observations for the identified events. Compared to events detectable from classic lensing surveys, the events detectable from the pixel lensing survey will generally involve brighter source stars and have higher amplifications. For the pixel lensing events detectable under the criteria of (S/N)_{th}=10 and t_{dur,th}=6 hr, we find that the baseline brightness of source stars will be in average ~2 mag brighter than those of classic lensing events and ~90% will have amplifications A> 20 and ~40% will be extreme EMEs with A>200. Therefore, followup observations of the pixel lensing events will provide high quality data, which enable one to precisely determine the lensing parameters and obtain extra-information about the lenses and source stars. Especially, high amplifications events with A>20 will be important targets for high-efficiency planet detections and one can uniquely determine the mass, distance, and transverse speed of individual lenses for EMEs.

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arXiv2000

On the Astrometric Behavior of Binary Microlensing Events

Despite the suspected binarity for a significant fraction of Galactic lenses, the current photometric surveys detected binary microlensing events only for a small fraction of the total events. The detection efficiency is especially low for non-caustic crossing events, which comprise majority of the binary lensing events, due to the absence of distinctive features in their light curves combined with small deviations from the standard light curve of a single point-mass event. In addition, even they are detected, it will be difficult to determine the solution of the binary lens parameters due to the severe degeneracy problem. In this paper, we investigate the properties of binary lensing events expected when they are astrometrically observed by using high precision interferometers. For this, we construct vector field maps of excess centroid shifts, which represent the deviations of the binary lensing centroid shifts from those of a single lensing events as a function of source position. From the analysis of the maps, we find that the excess centroid shifts are substantial in a considerably large area around caustics. In addition, they have characteristic sizes and directions depending strongly on the source positions with respect to the caustics and the resulting trajectories of the light centroid (astrometric trajectories) have distinctive features, which can be distinguished from the deviations caused by other reasons. We classify the types of the deviations and investigate where they occur. Due to the strong dependency of the centroid shifts on the lens system geometry combined with the distinctive features in the observed astrometric trajectories, astrometric binary lensing observations will provide an important tool that can probe the properties of Galactic binary lens population.

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arXiv2000

Detectability of the Parallax-induced Deviations in the Astrometric Centroid Shift Trajectories of Gravitational Microlensing Events

The uncertainty of the lens mass can be substantially reduced if it is determined from the lens proper motion obtained from astrometric measurements of the source image centroid shifts, $\vdelta\vtheta_c$, by using high precision interferometers. However, for the complete resolution of the lens parameter degeneracy it is required to determine the lens parallax by measuring the parallax-induced deviations in the centroid shifts trajectory, $\vDelta \vdelta \vtheta_c$. In this paper, we investigate the detectabilities of $\vdelta\vtheta_c$ and $\vDelta\vdelt\vtheta_c$ by determining the distributions of the maximum centroid shifts and the average maximum deviations expected for different types of Galactic microlensing events caused by various masses. From this investigation, we find that as long as sources are bright enough for astrometric observations it is expected that $\vdeltavxtheta_c$ for most events caused by masses greater than 0.1 M_\odot regardless of the event types can be easily detected from observations by using not only the Space Interfeormetry Mission (SIM, with a detection threshold ~ 3 μ-as) but also the ground-based interferometers (with a threshold ~30 μ-as). However, detection of $\vDelta\vdelta\vtheta_c$ from ground-based observations will be difficult for nearly all Galactic bulge self-lensing events, and will be restricted only for small fractions of disk-bulge and halo-LMC events, for which the deviations are relatively large. From observations by using the SIM, on the other hand, detecting $\vDelta\vdelta\vtheta_c$ will be possible for majority of disk-bulge and halo-LMC events and even for some fraction of bulge self-lensing events. For the complete resolution of the lens parameter degeneracy, therefore, SIM observations (or equivalent) will be essential.

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