Dong-Jin Kim

Upper Limits on the Number of Small Bodies in Sedna-Like Orbits by the TAOS Project

We present the results of a search for occultation events by objects at distances between 100 and 1000 AU in light curves from the Taiwanese-American Occultation Survey. We searched for consecutive, shallow flux reductions in the stellar light curves obtained by our survey between 2005 February 7 and 2006 December 31 with a total of ~4.5 × 10^9 three-telescope simultaneous photometric measurements. No events were detected, allowing us to set upper limits on the number density as a function of size and distance of objects in Sedna-like orbits, using simple models.

First Results From The Taiwanese-American Occultation Survey (TAOS)

Results from the first 2 years of data from the Taiwanese-American Occultation Survey (TAOS) are presented. Stars have been monitored photometrically at 4 or 5 Hz to search for occultations by small (~3 km) Kuiper Belt objects (KBOs). No statistically significant events were found, allowing us to present an upper bound to the size distribution of KBOs with diameters 0.5 km < D < 28 km.

The TAOS Project: High-Speed Crowded Field Aperture Photometry

We have devised an aperture photometry pipeline for data reduction of image data from the nTaiwanese-American Occultation Survey (TAOS). The photometry pipeline has high computational performance, nand is capable of real-time photometric reduction of images containing up to 1000 stars, within the sampling rate of n5 Hz. The pipeline is optimized for both speed and signal-to-noise performance, and in the latter category it performs nnearly as well as DAOPHOT. This paper provides a detailed description of the TAOS aperture photometry pipeline.

THE TAIWANESE-AMERICAN OCCULTATION SURVEY PROJECT STELLAR VARIABILITY. II. DETECTION OF 15 VARIABLE STARS

The Taiwanese–American Occultation Survey (TAOS) project has collected more than a billion photometric measurements since 2005 January. These sky survey data—covering timescales from a fraction of a second to a few hundred days—are a useful source to study stellar variability. A total of 167 star fields, mostly along the ecliptic plane, have been selected for photometric monitoring with the TAOS telescopes. This paper presents our initial analysis of a search for periodic variable stars from the time-series TAOS data on one particular TAOS field, No. 151 (R.A. = 17h30m6. s7, decl. = 27◦17′30′′, J2000), which had been observed over 47 epochs in 2005. A total of 81 candidate variables are identified in the 3 deg2 field, with magnitudes in the range 8 < R < 16. On the basis of the periodicity and shape of the light curves, 29 variables, 15 of which were previously unknown, are classified as RR Lyrae, Cepheid, δ Scuti, SX Phonencis, semi-regular, and eclipsing binaries.The Taiwanese-American Occultation Survey (TAOS) project has collected more than a billion photometric measurements since 2005 January. These sky survey data-covering timescales from a fraction of a second to a few hundred days-are a useful source to study stellar variability. A total of 167 star fields, mostly along the ecliptic plane, have been selected for photometric monitoring with the TAOS telescopes. This paper presents our initial analysis of a search for periodic variable stars from the time-series TAOS data on one particular TAOS field, No. 151 (RA = 17

A Close Binary Star Resolved from Occultation by 87 Sylvia

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Simultaneous VLBI Astrometry of H2O and SiO Masers toward the Semiregular Variable R Crateris

67, Dec = 27degr17arcmin 30arcsec, J2000), which had been observed over 47 epochs in 2005. A total of 81 candidate variables are identified in the 3 square degree field, with magnitudes in the range 8 < R < 16. On the basis of the periodicity and shape of the lightcurves, 29 variables, 15 of which were previously unknown, are classified as RR Lyrae, Cepheid, delta Scuti, SX Phonencis, semi-regular and eclipsing binaries.

Astrometrically registered maps of H2O and SiO masers toward VX Sagittarii

The star BD +29 1748 was resolved to be a close binary from its occultation by the asteroid 87 Sylvia on 2006 December 18 UT. Four telescopes were used to observe this event at two sites separated by some 80 km. Two flux drops were observed at one site, whereas only one flux drop was detected at the other. From the long-term variation of Sylvia, we inferred the probable shape of the shadow during the occultation, and this in turn constrains the binary parameters: the two components of BD +29 1748 have a projected separation of 0:097-0:140 on the sky with a position angle 104°-110°. The asteroid was clearly resolved, with a size scale ranging from 130 to 290 km as projected onto the occultation direction, consistent with the size dimensions 385 × 265 × 230 km, mea- sured by direct adaptive optics imaging. No occultation was detected for either of the two known moonlets of 87 Sylvia.

KaVA ESTEMA project

We obtained, for the first time, astrometrically registered maps of the 22.2 GHz H2O and 42.8, 43.1, and 86.2 GHz SiO maser emission toward the semiregular b-type variable (SRb) R Crateris, at three epochs (2015 May 21, and 2016 January 7 and 26) using the Korean Very-long-baseline Interferometry Network. The SiO masers show a ring-like spatial structure, while the H2O maser shows a very asymmetric one-side outflow structure, which is located at the southern part of the ring-like SiO maser feature. We also found that the 86.2 GHz SiO maser spots are distributed in an inner region, compared to those of the 43.1 GHz SiO maser, which is different from all previously known distributions of the 86.2 GHz SiO masers in variable stars. The different distribution of the 86.2 GHz SiO maser seems to be related to the complex dynamics caused by the overtone pulsation mode of the SRb R Crateris. Furthermore, we estimated the position of the central star based on the ring fitting of the SiO masers, which is essential for interpreting the morphology and kinematics of a circumstellar envelope. The estimated stellar coordinate corresponds well to the position measured by Gaia.

SiO MASERS AROUND WX PSC MAPPED with the KVN and VERA ARRAY (KaVA)

The supergiant VX Sagittarii is a strong emitter of both H2O and SiO masers. However, previous VLBI observations have been performed separately, which makes it difficult to spatially trace the outward transfer of the material consecutively. Here we present the astrometrically registered, simultaneous maps of 22.2u2009GHz H2O and 43.1/42.8/86.2/129.3u2009GHz SiO masers toward VX Sagittarii. The H2O masers detected above the dust-forming layers have an asymmetric distribution. The multi-transition SiO masers are nearly circular ring, suggesting spherically symmetric wind within a few stellar radii. These results provide the clear evidence that the asymmetry in the outflow is enhanced after the smaller molecular gas clump transform into the inhomogeneous dust layers. The 129.3u2009GHz maser arises from the outermost region compared to that of 43.1/42.8/86.2u2009GHz SiO masers. The ring size of the 129.3u2009GHz maser is maximized around the optical maximum, suggesting that radiative pumping is dominant.The red supergiant VX Sagittarii is a strong emitter of H2O and SiO masers, however its mass loss dynamics are still poorly understood. Here, the authors present astrometrically registered, simultaneous maps of SiO and H2O maser regions, and provide observational evidence for a break in spherical symmetry between the SiO and H2O maser zone.

A study on evolved stars by simultaneous observations of H2O and SiO masers using KVN

The ESTEMA (Expanded Study on Stellar Masers) project is one of three Large Programs of the KaVA (the combined array of the Korean VLBI Network and Japanese VLBI Exploration of Radio Astrometry), and conducted in 2015-2016. It aims to publish a database of the largest sample of VLBI images of circumstellar water (H2O) and silicon-monoxide (SiO) maser sources towards circumstellar envelopes (CSEs) of 80 evolved stars in late AGB to early post-AGB phase. Here we present the specifications of the ESTEMA observations and the planned scientific goals in order to share the basic information of the ESTEMA with astronomical community and encourage future collaborations with the ESTEMA and future follow-up observations for the targeted stars.