Cheongho Han

Discovery of a Jupiter/Saturn Analog with Gravitational Microlensing

Searches for extrasolar planets have uncovered an astonishing diversity of planetary systems, yet the frequency of solar system analogs remains unknown. The gravitational microlensing planet search method is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury. We report the detection of a multiple-planet system with microlensing. We identify two planets with masses of ∼0.71 and ∼0.27 times the mass of Jupiter and orbital separations of ∼2.3 and ∼4.6 astronomical units orbiting a primary star of mass ∼0.50 solar mass at a distance of ∼1.5 kiloparsecs. This system resembles a scaled version of our solar system in that the mass ratio, separation ratio, and equilibrium temperatures of the planets are similar to those of Jupiter and Saturn. These planets could not have been detected with other techniques; their discovery from only six confirmed microlensing planet detections suggests that solar system analogs may be common.

Microlens OGLE-2005-BLG-169 Implies That Cool Neptune-like Planets Are Common

We detect a Neptune mass ratio (q 8 ? 10-5) planetary companion to the lens star in the extremely high magnification (A ~ 800) microlensing event OGLE-2005-BLG-169. If the parent is a main-sequence star, it has mass M ~ 0.5 M?, implying a planet mass of ~13 M? and projected separation of ~2.7 AU. When intensely monitored over their peak, high-magnification events similar to OGLE-2005-BLG-169 have nearly complete sensitivity to Neptune mass ratio planets with projected separations of 0.6-1.6 Einstein radii, corresponding to 1.6-4.3 AU in the present case. Only two other such events were monitored well enough to detect Neptunes, and so this detection by itself suggests that Neptune mass ratio planets are common. Moreover, another Neptune was recently discovered at a similar distance from its parent star in a low-magnification event, which are more common but are individually much less sensitive to planets. Combining the two detections yields 90% upper and lower frequency limits f = 0.38 over just 0.4 decades of planet-star separation. In particular, f > 16% at 90% confidence. The parent star hosts no Jupiter-mass companions with projected separations within a factor 5 of that of the detected planet. The lens-source relative proper motion is ? ~ 7-10 mas yr-1, implying that if the lens is sufficiently bright, I 23.8, it will be detectable by the Hubble Space Telescope by 3 years after peak. This would permit a more precise estimate of the lens mass and distance and, so, the mass and projected separation of the planet. Analogs of OGLE-2005-BLG-169Lb orbiting nearby stars would be difficult to detect by other methods of planet detection, including radial velocities, transits, and astrometry.

A jovian-mass planet in microlensing event OGLE-2005-BLG-071

We report the discovery of a several-Jupiter mass planetary companion to the primary lens star in microlensing event OGLE-2005-BLG-071. Precise (<1%) photometry at the peak of the event yields an extremely high signal-to-noise ratio detection of a deviation from the light curve expected from an isolated lens. The planetary character of this deviation is easily and unambiguously discernible from the gross features of the light curve. Detailed modeling yields a tightly-constrained planet-star mass ratio of q=m_p/M=0.0071+/-0.0003. This is the second robust detection of a planet with microlensing, demonstrating that the technique itself is viable and that planets are not rare in the systems probed by microlensing, which typically lie several kpc toward the Galactic center.

MICROLENSING EVENT MOA-2007-BLG-400: EXHUMING THE BURIED SIGNATURE OF A COOL, JOVIAN-MASS PLANET

We report the detection of the cool, Jovian-mass planet MOA-2007-BLG-400Lb. The planet was detected in a high-magnification microlensing event (with peak magnification A max = 628) in which the primary lens transited the source, resulting in a dramatic smoothing of the peak of the event. The angular extent of the region of perturbation due to the planet is significantly smaller than the angular size of the source, and as a result the planetary signature is also smoothed out by the finite source size. Thus, the deviation from a single-lens fit is broad and relatively weak (approximately few percent). Nevertheless, we demonstrate that the planetary nature of the deviation can be unambiguously ascertained from the gross features of the residuals, and detailed analysis yields a fairly precise planet/star mass ratio of , in accord with the large significance () of the detection. The planet/star projected separation is subject to a strong close/wide degeneracy, leading to two indistinguishable solutions that differ in separation by a factor of ~8.5. Upper limits on flux from the lens constrain its mass to be M < 0.75 M ? (assuming that it is a main-sequence star). A Bayesian analysis that includes all available observational constraints indicates a primary in the Galactic bulge with a mass of ~0.2-0.5 M ? and thus a planet mass of ~0.5-1.3 M Jup. The separation and equilibrium temperature are ~5.3-9.7 AU (~0.6-1.1 AU) and ~34 K (~103 K) for the wide (close) solution. If the primary is a main-sequence star, follow-up observations would enable the detection of its light and so a measurement of its mass and distance.

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 Hubble Space Telescope 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 τ(bulge - bulge) = 0.98 × 10-6. When combined with the lensing of bulge stars by foreground stars in the disk, this yields τ(bulge - total) = 1.63 × 10-6, in reasonable agreement with the estimates of τ = 2.13 ± 0.40 × 10-6 and τ = 1.08 ± 0.30 × 10-6 based on observations of clump giants by the MACHO and EROS collaborations.

Masses and Orbital Constraints for the OGLE-2006-BLG-109Lb,c Jupiter/Saturn Analog Planetary System

We present a new analysis of the Jupiter+Saturn analog system, OGLE-2006-BLG-109Lb,c, which was the first double planet system discovered with the gravitational microlensing method. This is the only multi-planet system discovered by any method with measured masses for the star and both planets. In addition to the signatures of two planets, this event also exhibits a microlensing parallax signature and finite source effects that provide a direct measure of the masses of the star and planets, and the expected brightness of the host star is confirmed by Keck AO imaging, yielding masses of , Mb = 231 ± 19 M ⊕, and Mc = 86 ± 7 M ⊕. The Saturn-analog planet in this system had a planetary light-curve deviation that lasted for 11 days, and as a result, the effects of the orbital motion are visible in the microlensing light curve. We find that four of the six orbital parameters are tightly constrained and that a fifth parameter, the orbital acceleration, is weakly constrained. No orbital information is available for the Jupiter-analog planet, but its presence helps to constrain the orbital motion of the Saturn-analog planet. Assuming co-planar orbits, we find an orbital eccentricity of and an orbital inclination of . The 95% confidence level lower limit on the inclination of i > 49° implies that this planetary system can be detected and studied via radial velocity measurements using a telescope of 30 m aperture.

The Extreme Microlensing Event OGLE-2007-BLG-224: Terrestrial Parallax Observation of a Thick-Disk Brown Dwarf

Parallax is the most fundamental technique for measuring distances to astronomical objects. Although terrestrial parallax was pioneered over 2000 years ago by Hipparchus (ca. 140 B.C.E.) to measure the distance to the Moon, the baseline of the Earth is so small that terrestrial parallax can generally only be applied to objects in the Solar System. However, there exists a class of extreme gravitational microlensing events in which the effects of terrestrial parallax can be readily detected and so permit the measurement of the distance, mass, and transverse velocity of the lens. Here we report observations of the first such extreme microlensing event OGLE-2007-BLG-224, from which we infer that the lens is a brown dwarf of mass M = 0.056 ± 0.004 M ☉, with a distance of 525 ± 40 pc and a transverse velocity of 113 ± 21 km s–1. The velocity places the lens in the thick disk, making this the lowest-mass thick-disk brown dwarf detected so far. Follow-up observations may allow one to observe the light from the brown dwarf itself, thus serving as an important constraint for evolutionary models of these objects and potentially opening a new window on substellar objects. The low a priori probability of detecting a thick-disk brown dwarf in this event, when combined with additional evidence from other observations, suggests that old substellar objects may be more common than previously assumed.

Properties of Central Caustics in Planetary Microlensing

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.

Properties of Planetary Caustics in Gravitational Microlensing

Although some of the properties of the caustics in planetary microlensing have been known, our understanding of them is mostly from scattered information based on numerical approaches. In this paper, we conduct a comprehensive and analytic analysis of the properties of the planetary caustics, which are one of the two sets of caustics in planetary microlensing, those located away from the central star. Under the perturbative approximation, we derive analytic expressions for the location, size, and shape of the planetary caustic as a function of the star-planet separation and the planet/star mass ratio. Based on these expressions combined with those for the central caustic, which is the other set of caustics and is located close to the central star, we compare the similarities and differences between the planetary and central caustics. We also present the expressions for the size ratio between the two types of caustics and for the condition of the merging of the two types of caustics. These analytic expressions will be useful in understanding the dependence of the planetary lensing behavior on the planet parameters and thus in interpreting the planetary lensing signals.

The Intrinsic Shapes of Low Surface Brightness Dwarf Irregular Galaxies and Comparison to Other Types of Dwarf Galaxies

In this paper, we measure the ellipticities of 30 low surface brightness (LSB) dwarf irregular (dI) galaxies and compare the ellipticity distribution with that of 80 dwarf elliptical (dEs) and 62 blue-compact dwarfs (BCDs). We find that the ellipticity distribution of LSB dIs is very similar to that of BCDs, and marginally different from that of dEs. We then determine the distribution of intrinsic shapes of dI galaxies and compare this to the distributions of other types of dwarf galaxies under various assumptions. First, we assume that LSB dIs are either all oblate or all prolate, and use a nonparametric analysis to find the best-fitting distribution of intrinsic shapes. With this assumption, we find that the scarcity of nearly circular LSB dIs implies, at the 99% confidence level, that they cannot be a population of randomly oriented oblate or prolate objects, implying that LSB dIs are highly unlikely to be disk-shaped systems. Next, we assume that dIs are triaxial, and use a parametric analysis to find permissible distributions of intrinsic shapes. We find that if the intrinsic axis ratios β and γ are distributed according to a Gaussian with means β0 and γ0 and a common standard deviation of σ, the best-fitting set of parameters for LSB dIs is (β0, γ0, σ) = (0.66, 0.50, 0.15), and the best fit for BCDs is (β0, γ0, σ) = (0.66, 0.55, 0.16), while the best fit for dEs is (β0, γ0, σ) = (0.78, 0.69, 0.24). The dIs and BCDs thus have very similar shape distributions, given this triaxial hypothesis, while the dEs peak at a somewhat more spherical shape. Therefore, our results provide strong observational evidence to support the evolutionary scenario in which the three types of dwarf galaxy have a close relation with each other.