• 천문학회보
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• 제35권2호
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• pp.84.1-84.1
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• 2010

Globular clusters (GCs) are known to be one of the oldest objects in the Milky Way. Therefore the dynamical informations of GCs are very important to understand the formation and evolution of our Galaxy. Motion of GCs in the halo of Galaxy can be traced by radial velocities of individual stars and proper motions of GCs. Measuring the radial velocities of stars in GCs has been challenging for decades because the brightness of stars (even for the brightest stars) in GCs are too faint (V>14) to measure the radial velocities. The available large telescopes (D>4m) enable us to observe the spectra of stars in the red giant branch of GCs, and it is now more plausible to measure the radial velocities of stars in GCs. On the contrary it is still very difficult to measure the sky-projected two-dimensional motion of GCs in Galaxy even with the large telescopes because the distance to GCs is quite large (~10kpc) compared to the spatial resolution of present-day large ground-based telescopes. Instruments on-board Hubble Space Telescope are ideal to study the proper motion of GCs thanks to their extremely high spatial resolution (~0.05arcsec). We report a study of proper motion of NGC 104, one of the most metal-rich Milky Way GCs, based-on archival images of NGC 104 observed using HST/ACS. Using the stars in Small Magellanic Cloud as reference coordinate, we are able to measure the proper motions of individual stars in NGC 104 with a high precision. We discuss the internal dynamics of stars in NGC 104 by comparing proper motion results based-on shorter (<1yr) and longer (~7yrs) time durations.

• 천문학논총
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• 제15권spc1호
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• pp.15-30
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• 2000

A large scatter of the chemical abundances among globular cluster red giants has been observed. Especially the chemical elements C, N, O, Na, Mg, and Al vary form star to star within globular clusters. Except for $\omega$ Cen and M22, most globular clusters could be considered to be monometallic of their iron peak elements within error ranges. The variations in light elements among globuar cluster giants appear much more pronounced than in field halo giants of comparable Fe-peak metallicity. It has been found that in general the nitrogen abundance is anticorrelated with both carbon and oxygen, while it is correlated with Na and AI. These intracluster abundance inhomogeneities can be interpreted either by mixing of nucleosythesized material from the deep stellar interior during the red giant branch phase of evolution or by inhomogeneities of primordially processed material, from which the stars were formed. The simple way of distingushing between two senarios is to obtain the element abundances of main-sequence stars in globular clusters, which are too faint for high resolution spectroscopic studies until now. Both 'evolutionary' and 'primodial' origins are accepted for explanations of abundance variations among red giants and CN-CH anticorrelations among main-sequence stars in globular clusters. This paper reviews chemical abundances of light elements among globular cluster giants, with brief reviews of cannonical stellar evolution of low mass stars after main-sequence and deep mixing for abundance variations of cluster giants, and a possible connection between deep mixing and second parameter.

• 천문학회보
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• 제36권2호
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• pp.64-64
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• 2011

We present a new quasi-stellar object (QSO) selection algorithm using a Support Vector Machine, a supervised classification method, on a set of extracted time series features including period, amplitude, color, and autocorrelation value. We train a model that separates QSOs from variable stars, non-variable stars, and microlensing events using 58 known QSOs, 1629 variable stars, and 4288 non-variables in the MAssive Compact Halo Object (MACHO) database as a training set. To estimate the efficiency and the accuracy of the model, we perform a cross-validation test using the training set. The test shows that the model correctly identifies ~80% of known QSOs with a 25% false-positive rate. The majority of the false positives are Be stars. We applied the trained model to the MACHO Large Magellanic Cloud (LMC) data set, which consists of 40 million lightcurves, and found 1620 QSO candidates. During the selection, none of the 33,242 known MACHO variables were misclassified as QSO candidates. In order to estimate the true false-positive rate, we crossmatched the candidates with astronomical catalogs including the Spitzer Surveying the Agents of a Galaxy's Evolution (SAGE) LMC catalog and a few X-ray catalogs. The results further suggest that the majority of the candidates, more than 70%, are QSOs.

• 천문학회보
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• 제43권2호
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• pp.35.3-36
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• 2018

One of the most prevalent knowledge about disk galaxies, which dominate the population of the local Universe, is that they consist of stellar structures with different kinematics, such as thin disk, bulge, and halo. Therefore, investigating when and how these components develop in a galaxy is the key to understanding the evolution of galaxies. Using the NewHorizon simulation, we can resolve the detailed structures of galaxies, in the field environment, from the early Universe where star formation and mergers were most active. We first decompose stellar particles in a galaxy into a disk and a dispersion-dominated, spheroidal, component based on their orbits and then see how these components evolve in terms of mass and structure. At high redshift z~3, galaxies are mostly dispersion-dominated as stars are formed misaligned with the galactic rotational axis. At z=1~2, massive galaxies start to dominantly form disk stars, while less massive galaxies do much later. Furthermore, massive galaxies are forming thinner and larger disks with time, and the preexistent disks are heated or even disrupted to become a part of dispersion-dominated component. Thus, the mass growth of spheroidal components at later epochs is dominated by disrupted stars with disk origins and accreted stars at large radii.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2004년도 한국우주과학회보 제13권1호
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• pp.24-24
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• 2004

Using stellar population synthesis techniques, we explore the photometric signatures of white dwarf progenitor dominated galactic halos, in order to constrain the fraction of halo mass that may be locked-up in white dwarf stellar remnants. We first construct a 10＾9 M_sun stellar halo using the canonical Salpeter initial stellar mass distribution, and then allow for an additional component of low-and intermediate-mass stars, which ultimately give rise to white dwarf remnants. (omitted)

• 천문학회지
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• 제37권5호
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• pp.447-454
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• 2004

During the hierarchical formation of large scale structure in the universe, the progressive collapse and merging of dark matter should inevitably drive shocks into the gas, with nonthermal particle acceleration as a natural consequence. Two topics in this regard are discussed, emphasizing what important things nonthermal phenomena may tell us about the structure formation (SF) process itself. 1. Inverse Compton gamma-rays from large scale SF shocks and non-gravitational effects, and the implications for probing the warm-hot intergalactic medium. We utilize a semi-analytic approach based on Monte Carlo merger trees that treats both merger and accretion shocks self-consistently. 2. Production of $^6Li$ by cosmic rays from SF shocks in the early Galaxy, and the implications for probing Galaxy formation and uncertain physics on sub-Galactic scales. Our new observations of metal-poor halo stars with the Subaru High Dispersion Spectrograph are highlighted.

• 천문학회보
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• 제41권1호
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• pp.66.4-67
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• 2016

Our recent investigation (Lee et al. 2015) suggests that the presence of double red clump in the Milky Way bulge is another manifestation of multiple populations observed in halo globular clusters. The origin of Helium enhancement in the 2nd generation population (G2), however, is not yet fully understood. Here we investigate the origin of this super-Helium-rich population in the framework of self-enrichment scenario. We find that chemical enrichments and pollutions by asymptotic giant branch stars and winds of massive rotating stars can naturally reproduce the observed Helium enhancement. The Helium to metal enrichment ratio appears to be ${\Delta}Y/{\Delta}Z=6$ for G2, while the standard ratio, ${\Delta}Y/{\Delta}Z=2$, is appropriate for G1, which is probably enriched mostly by typeII supernovae.

• 천문학회보
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• 제41권2호
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• pp.54.2-54.2
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• 2016

We present a machine learning approach to estimating stellar atmospheric parameters, effective temperature (Teff), surface gravity (log g), and metallicity ([Fe/H]) for stars observed during the course of the Sloan Digital Sky Survey (SDSS). For training a neural network, we randomly sampled the SDSS data with stellar parameters available from SEGUE Stellar Parameter Pipeline (SSPP) to cover the parameter space as wide as possible. We selected stars that are not included in the training sample as validation sample to determine the accuracy and precision of each parameter. We also divided the training and validation samples into four groups that cover signal-to-noise ratio (S/N) of 10-20, 20-30, 30-50, and over 50 to assess the effect of S/N on the parameter estimation. We find from the comparison of the network-driven parameters with the SSPP ones the range of the uncertainties of 73~123 K in Teff, 0.18~0.42 dex in log g, and 0.12~0.25 dex in [Fe/H], respectively, depending on the S/N range adopted. We conclude that these precisions are high enough to study the chemical and kinematic properties of the Galactic disk and halo stars, and we will attempt to apply this technique to Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), which plans to obtain about 8 million stellar spectra, in order to estimate stellar parameters.

• 천문학논총
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• 제20권1호
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• pp.11-20
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• 2005

We present measurements of interstellar $H_2$ absorption lines in the continuum spectra of 54 early-type stars in the Galactic disk and halo and 3 stars in the Magellanic Clouds. The data were obtained with the Berkeley Extreme and Far-Ultraviolet Spectrometer (BEFS), part of the ORFEUS telescope, which flew on the ORFEUS-SPAS I and II space-shuttle missions in 1993 and 1996, respectively. The spectra extend from the interstellar cutoff at $912{\AA}$ to about $1200{\AA}$ with a spectral resolution of ${\sim}3000$ and statistical signal-to-noise ratios between 10 and 65. Assuming a velocity profile derived from optical observations (when available), we model the column densities N(J) of the rotational levels J = 0 through 5 for each line of sight. Our data reproduce the relationships among molecular and total hydrogen column density, fractional molecular abundance, and reddening first seen in Copernicusobservations of nearby stars (Savage et al. 1977). The results show that most of these molecular clouds have $H_2$ total column densities between $10^{15}cm^{-2}$ and $10^{21}cm^{-2}$, and kinetic temperatures from 21 K to 232 K, with average of 89 K, consistent with the result of Copernicus (Savage et al. 1977).

• 천문학논총
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• 제30권2호
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• pp.183-187
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• 2015

We use ALLWISE data release W1- and W2-band epoch photometry collected by the Wide-Field Infrared Survey Explorer (WISE) to determine slopes of the period-luminosity relations for RR Lyrae stars in 15 globular clusters in the corresponding bands. We further combine these results with V- and K-band photometry of Galactic field RR Lyrae stars to determine the metallicity slopes of the log $P_F-[Fe/H]-M_K$, log $P_F-[Fe/H]-M_{W1}$, and log $P_F-[Fe/H]-M_{W2}$ period-metallicity-luminosity relations. We infer the zero points of these relations and determine the kinematical parameters of thick-disk and halo RR Lyraes via statistical parallax, and estimate the RR Lyrae-based distances to 18 Local-Group galaxies including the center of the Milky Way.