• 천문학회지
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• 제35권2호
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• pp.75-85
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• 2002

We present N-body simulations of globular clusters including gravitational field of the Galaxy, in order to study effects of tidal field systematically on the shape of outer parts of globular clusters using NBODY6. The Galaxy is assumed to be composed of central bulge and outer halo. We mvestigate the cluster of multi-mass models with a power-law initial mass function (IMF) starting with different initial masses, initial number of particles, different slopes of the IMF and different orbits of the cluster. We have examined the general evolution of the clusters, the shape of outer parts of the clusters, density profiles and the direction of tidal tails. The density profiles appear to become somewhat shallower just outside the tidal boundary consistent with some observed data. The position angle of the tidal tall depends on the location in the Galaxy as well as the direction of the motion of. clusters. We found that the clusters become more elongated at the apogalacticon than at the pengalacticon. The tidal tails may be used to trace the orbital paths of globular clusters.

• 천문학회보
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• 제38권2호
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• pp.69.2-69.2
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• 2013

A galaxy cluster is an important laboratory to study the large scale structure in the Universe and the galaxy evolution. In order to identify candidate galaxy clusters at z~1, we have used deep and wide optical-NIR datasets based on IMS, UKIDSS DXS and CFHTLS wide covering ${\sim}20deg^2$ in the SA22 field. We measure the angular two-point correlation function of the candidate clusters and investigate the star formation activity of the member galaxies. Based on bias factor and halo mass function, candidate clusters have the average halo mass of > $10^{14}h^{-1}M_{\odot}$. At z~1, the star formation rate of cluster galaxies is similar to that of field galaxies, which indicates the environmental quenching is not so significant at z~1 as the local Universe.

• 천문학회지
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• 제29권1호
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• pp.53-62
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• 1996

31 white dwarfs in 10 open clusters are examined, and their maximum mass and the upper mass limit of their progenitors are obtained as $1.22\pm0.02M_{\bigodot}\;and\;7.2\pm0.4M_{\bigodot}$ respectively, suggesting that the upper mass limit of white dwarfs is less than 8M_{\bigodot}$ The final mass of white dwarfs shows no clear correlation with the initial mass of their progenitors, and it is found that a deficient gap of initial mass exists between $\~4\;and\;~5.2M_{\bigodot}$. This gap seems to correspond to the mass range for carbon detonation or deflagration. The total expected numbers of white dwarfs are $11\~22$ in Hyades with 7 known white dwarfs and 17 in Praesepe with 8 known white dwarfs. These known white dwrfs are all younger than the others in both clusters. But one known white dwarf in Pleiades is older one among $2\~3$ expected white dwarfs.

• 천문학회지
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• 제23권2호
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• pp.116-121
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• 1990

It has been suggested that there could be a large number of primordial black holes which were formed in the early universe. We analyze the growth of such a primordial black hole following two different accretion rates - the Eddington accretion rate and the Bondi accretion rate - at the center of a host star like the sun. We find that a primordial black hole with M < ${\sim}10^{17}\;g$ cannot substantially grow in any case throughout the lifetime of a host star. If M > ${\sim}10^{17}\;g$, the evolution of a host star depends entirely on the mode of accretion, but it ends as a black hole in either case. Since more stars may have primordial black holes at the center of a galaxy this may result in a cluster of such black holes, and the cluster may eventually collapse to produce a single supermassive black hole.

• 천문학회보
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• 제39권2호
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• pp.55.2-55.2
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• 2014

Role of environments is one of today's most widely discussed and debated topic in the field of extra-galactic astronomy. Extreme morphology-density relations found in low-mass galaxies are considered to be the result of an effective role played by environment in the evolution of these galaxies. I will present the results from our dedicated study of early-type dwarf galaxies (dEs) in different environments using imaging and spectroscopic data. We find that Virgo cluster dEs have a variety of structural and kinematic properties. A significant fraction of dEs possesses disk features, such as spiral arm and bar, while a central nucleus seems to be universal in these low mass galaxies. We also find that a majority of dEs are fast rotator and their rotation curves are much steeper than that of spiral galaxies of similar mass. Finally I will discuss how the different environmental mechanisms, i.e., gas-stripping or tidal interaction, can contribute to form heterogeneous dEs in Virgo cluster.

• 천문학회보
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• 제41권2호
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• pp.56.2-56.2
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• 2016

We present chemical abundances for about 800 red giant stars in Omega Centauri, based on medium-resolution spectra obtained using Hydra multi-fiber spectrograph at the CTIO 4-m telescope. Our sample covers 14.2 < V < 15.0, and is almost unbiased against colors (and therefore metallicity). The metallicity distribution function (MDF) constructed from our data has an overall shape and local peaks that approximately match those for brighter giant stars in Johnson et al.. We also find that more metal-rich cluster members are more concentrated in the cluster center, which is consistent with previous studies. On the other hand, we find no clear evidence for such a spatial dependence with respect to alpha elemental abundance ([${\alpha}/Fe$]).

• 천문학회지
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• 제40권4호
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• pp.91-97
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• 2007

The Fokker-Planck (FP) model is one of the commonly used methods for studies of the dynamical evolution of dense spherical stellar systems such as globular clusters and galactic nuclei. The FP model is numerically stable in most cases, but we find that it encounters numerical difficulties rather often when the effects of tidal shocks are included in two-dimensional (energy and angular momentum space) version of the FP model or when the initial condition is extreme (e.g., a very large cluster mass and a small cluster radius). To avoid such a problem, we have developed a new integration scheme for a two-dimensional FP equation by adopting an Alternating Direction Implicit (ADI) method given in the Douglas-Rachford split form. We find that our ADI method reduces the computing time by a factor of ${\sim}2$ compared to the fully implicit method, and resolves problems of numerical instability.

• 천문학회보
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• 제42권1호
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• pp.51.4-52
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• 2017

We perform hydrodynamical simulations of a late-type galaxy experiencing frequent high-speed encounters with intruding galaxies, called "galaxy harassment". Specifically, we simulate a Milky Way-like galaxy colliding consecutively with six twice-massive early-type galaxies containing hot diffuse gas on their halos, with various impact parameters ranging from 65 kpc/h to 15 kpc/h at the relative speed of about 1500 km/s. We show that galaxy-galaxy encounters play a significant role in a cluster environment in gas stripping and star formation quenching through hydrodynamic interactions of late-type galaxies with cluster early-type galaxies.

• 천문학회보
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• 제43권1호
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• pp.59.2-59.2
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• 2018

Recent studies of galaxy clusters have shown that the galaxy clusters in dense environment tend to have lower star formation rate in local universe with z < 1. However, this correlation is not significant in galaxy clusters with z > 1. The study of galaxy clusters around z=1 can yield insight into cosmological galaxy evolution. Nevertheless, the identification of galaxy clusters beyond the scope of immediate local universe requires wide field data in optical and near-infrared bands. By incorporating data from Canada-France-Hawaii Telescope Legacy Survey(CFHTLS) and Infrared Medium-Deep Survey(IMS), the photometric redshifts of galaxies in CFHTLS W2 field were calculated. Using spatial distribution and photometric redshifts, the galaxies in the field were divided into redshift bins. The image of each redshift bin was analyzed by measuring the number density within proper distance of 1Mpc. By comparing high density regions in consecutive redshift bins, we identified the cluster candidates and mapped the large-scale structure within the CFHTLS W2 field.

• Nuclear Engineering and Technology
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• 제36권3호
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• pp.229-236
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• 2004

Radiation hardening is a multiscale phenomenon involving various processes over a wide range of time and length. We present a multiscale model for estimating the amount of radiation hardening in pressure vessel steel in the environment of a light water reactor. The model comprises two main parts: molecular dynamics (MD) simulation and a point defect cluster (PDC) model. The MD simulation was used to investigate the primary damage caused by displacement cascades. The PDC model mathematically formulates interactions between point defects and their clusters, which explains the evolution of microstructures. We then used a dislocation barrier model to calculate the hardening due to the PDCs. The key input for this multiscale model is a neutron spectrum at the inner surface of reactor pressure vessel steel of the Younggwang Nuclear Power Plant No.5. A combined calculation from the MD simulation and the PDC model provides a convenient tool for estimating the amount of radiation hardening.