Byeong-Gon Park

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.

KMTNET: A NETWORK OF 1.6 M WIDE-FIELD OPTICAL TELESCOPES INSTALLED AT THREE SOUTHERN OBSERVATORIES

The Korea Microlensing Telescope Network (KMTNet) is a wide-field photometric system installed by the Korea Astronomy and Space Science Institute (KASI). Here, we present the overall technical specifications of the KMTNet observation system, test observation results, data transfer and image processing procedure, and finally, the KMTNet science programs. The system consists of three 1.6 m wide-field optical telescopes equipped with mosaic CCD cameras of 18k by 18k pixels. Each telescope provides a 2.0 by 2.0 square degree field of view. We have finished installing all three telescopes and cameras sequentially at the Cerro-Tololo Inter-American Observatory (CTIO) in Chile, the South African Astronomical Observatory (SAAO) in South Africa, and the Siding Spring Observatory (SSO) in Australia. This network of telescopes, which is spread over three different continents at a similar latitude of about -30 degrees, enables 24-hour continuous monitoring of targets observable in the Southern Hemisphere. The test observations showed good image quality that meets the seeing requirement of less than 1.0 arcsec in I-band. All of the observation data are transferred to the KMTNet data center at KASI via the international network communication and are processed with the KMTNet data pipeline. The primary scientific goal of the KMTNet is to discover numerous extrasolar planets toward the Galactic bulge by using the gravitational microlensing technique, especially earth-mass planets in the habitable zone. During the non-bulge season, the system is used for wide-field photometric survey science on supernovae, asteroids, and external galaxies.

Design and early performance of IGRINS (Immersion Grating Infrared Spectrometer)

The Immersion Grating Infrared Spectrometer (IGRINS) is a compact high-resolution near-infrared cross-dispersed spectrograph whose primary disperser is a silicon immersion grating. IGRINS covers the entire portion of the wavelength range between 1.45 and 2.45μm that is accessible from the ground and does so in a single exposure with a resolving power of 40,000. Individual volume phase holographic (VPH) gratings serve as cross-dispersing elements for separate spectrograph arms covering the H and K bands. On the 2.7m Harlan J. Smith telescope at the McDonald Observatory, the slit size is 1ʺ x 15ʺ and the plate scale is 0.27ʺ pixel. The spectrograph employs two 2048 x 2048 pixel Teledyne Scientific and Imaging HAWAII-2RG detectors with SIDECAR ASIC cryogenic controllers. The instrument includes four subsystems; a calibration unit, an input relay optics module, a slit-viewing camera, and nearly identical H and K spectrograph modules. The use of a silicon immersion grating and a compact white pupil design allows the spectrograph collimated beam size to be only 25mm, which permits a moderately sized (0.96m x 0.6m x 0.38m) rectangular cryostat to contain the entire spectrograph. The fabrication and assembly of the optical and mechanical components were completed in 2013. We describe the major design characteristics of the instrument including the system requirements and the technical strategy to meet them. We also present early performance test results obtained from the commissioning runs at the McDonald Observatory.

The Angstrom Project: a microlensing survey of the structure and composition of the bulge of the Andromeda galaxy

The Andromeda Galaxy Stellar Robotic Microlensing Project (The Angstrom Project) aims to use stellar microlensing events to trace the structure and composition of the inner regions of the Andromeda Galaxy (M31). We present microlensing rate and time-scale predictions and spatial distributions for stellar and sub-stellar lens populations in combined disc and barred bulge models of M31. We show that at least half of the stellar microlenses in and around the bulge are expected to have characteristic durations between 1 and 10 d, rising to as much as 80 per cent for brown-dwarf dominated mass functions. These short-duration events are mostly missed by current microlensing surveys that are looking for Macho candidates in the M31 dark matter halo. Our models predict that an intensive monitoring survey programme, such as Angstrom, which will be able to detect events of durations upwards of a day, could detect around 30 events per season within ∼5 arcmin of the M31 centre due to ordinary low-mass stars and remnants. This yield increases to more than 60 events for brown-dwarf dominated mass functions. The overall number of events and their average duration are sensitive diagnostics of the bulge mass, in particular the contribution of low-mass stars and brown dwarfs. The combination of an inclined disc, an offset bar-like bulge, and differences in the bulge and disc luminosity functions results in a four-way asymmetry in the number of events expected in each quadrant defined by the M31 disc axes. The asymmetry is sensitive to the bar prolongation, orientation and mass.

SEJONG OPEN CLUSTER SURVEY (SOS). 0. TARGET SELECTION AND DATA ANALYSIS

Star clusters are superb astrophysical laboratories containing cospatial and coeval samples of stars with similar chemical composition. We initiate the Sejong Open cluster Survey (SOS) - a project dedicated to providing homogeneous photometry of a large number of open clusters in the SAAO Johnson-Cousin’ UBVI system. To achieve our main goal, we pay much attention to the observation of standard stars in order to reproduce the SAAO standard system. Many of our targets are relatively small sparse clusters that escaped previous observations. As clusters are considered building blocks of the Galactic disk, their physical properties such as the initial mass function, the pattern of mass segregation, etc. give valuable information on the formation and evolution of the Galactic disk. The spatial distribution of young open clusters will be used to revise the local spiral arm structure of the Galaxy. In addition, the homogeneous data can also be used to test stellar evolutionary theory, especially concerning rare massive stars. In this paper we present the target selection criteria, the observational strategy for accurate photometry, and the adopted calibrations for data analysis such as color-color relations, zero-age main sequence relations, Sp-Mv relations, Sp ? T eff relations, Sp ? color relations, and T eff ? BC relations. Finally we provide some data analysis such as the determination of the reddening law, the membership selection criteria, and distance determination.

GRAVITATIONAL MICROLENSING: A TOOL FOR DETECTING AND CHARACTERIZING FREE-FLOATING PLANETS

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.

Technical specifications of the KMTNet observation system

Korea Astronomy and Space Science Institute (KASI) has officially started a project to construct an astronomical widefield survey system, namely KMTNet (Korea Micro-lensing Telescope Network), from January 2009. Its primary scientific goal is to discover numerous extra-solar planets, especially earth-mass planets, using the gravitational microlensing technique. This goal requires continuous photometric observations with high cadence of about 10 minutes for tens of millions of stars in dense fields toward the Galactic bulge. KMTNet will comprise three identical systems at southern observatories with different time zones. Each observing system consists of a 1.6 m wide-field optical telescope and a 20k by 20k mosaic CCD camera, which covers a 2 by 2 degrees square field of view. In this proceeding, we present technical specifications, designs and fabrication schedule of the KMTNet system.

MASS MEASUREMENTS of ISOLATED OBJECTS from SPACE-BASED MICROLENSING

We report on the mass and distance measurements of two single-lens events from the 2015 Spitzer microlensing campaign. With both finite-source effect and microlens parallax measurements, we find that the lens of OGLE-2015-BLG-1268 is very likely a brown dwarf (BD). Assuming that the source star lies behind the same amount of dust as the Bulge red clump, we find the lens is a 45 ± 7 M_J BD at 5.9 ± 1.0 kpc. The lens of of the second event, OGLE-2015-BLG-0763, is a 0.50 ± 0.04 M_M☉ star at 6.9 ± 1.0 kpc. We show that the probability to definitively measure the mass of isolated microlenses is dramatically increased once simultaneous ground- and space-based observations are conducted.

Asteroseismology of the PG 1159 star PG 0122+200

Context. The variable pre-white dwarf PG 1159 stars (GW Vir) are g-mode non-radial pulsators. Asteroseismology puts strong constraints on their global parameters and internal structure. PG 0122+200 defines the red edge of the instability strip and its evolutionary timescale is predicted to be dominated by neutrino emission. Its study offers the opportunity to better understand the instability mechanism and to validate the physics of the neutrino production in dense plasma. Aims. To achieve such a goal requires determining precisely its fundamental parameters. This is the goal of this paper. Methods. We present new multi-site photometric observations obtained in 2001 and 2002. Together with previous data, they allow us to detect 23 frequencies, composed of 7 triplets and 2 single frequencies, which are used to constrain its internal structure and derive its fundamental parameters. Results. All the observed frequencies correspond to � = 1 g-modes. The period distribution shows a signature of mode trapping from which we constrain the He-rich envelope mass fraction to be –6.0 ≤ log(qy) ≤− 5.3. The comparison of the mode trapping amplitudes among GW Vir stars suggests that the mass-loss efficiency must decrease significantly below Teff ≤ 140 kK. We measure an average period spacing of 22.9 s from which we derive a mass of 0.59 ± 0.02 M� . From the triplets we measure a mean rotational splitting of 3.74 µHz and a rotational period of 1.55 days. We derive an upper limit to the magnetic field of B ≤ 4 × 10 3 G. The luminosity (logL/L� = 1.3 ± 0.5) and the distance (D = 0.7 +1.0 −0.4 kpc) are only weakly constrained due to the large uncertainty on the spectroscopically derived surface gravity and the absence of a measured parallax. Conclusions. From the asteroseismic mass, the ratio of the neutrino luminosity on the photon luminosity is 1.6 ± 0.2 confirming that the PG 0122+200 evolutionary time scale should be dominated by neutrino cooling. A measurement of u P for the largest amplitude untrapped modes should verify this prediction.

Microlensing Detections of Planets in Binary Stellar Systems

We demonstrate that microlensing can be used for detecting planets in binary stellar systems. This is possible because in the geometry of planetary binary systems, in which the planet orbits one of the binary components and the other binary star is located at a large distance, both planet and secondary companion produce perturbations in a common region around the planet-hosting binary star, and thus the signatures of both planet and binary companion can be detected in the light curves of high-magnification lensing events. We find that identifying planets in binary systems is optimized when the secondary is located within a certain range that depends on the type of the planet. The proposed method can detect planets with masses down to one-tenth of the Jupiter mass in binaries with separations 100 AU. These ranges of planet masses and binary separations are not covered by other methods, and thus microlensing would be able to make the planetary binary sample richer.