• 천문학회지
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• 제43권4호
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• pp.135-139
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• 2010

We present the result of our investigation on the impact of the low Solar abundance of Asplund and collaborators (2004) on the derived ages for the oldest star clusters based on isochrone fittings. We have constructed new stellar models and corresponding isochrones using this new solar mixture with a proper Solar calibration. We have found that the use of the Asplund et al. (2004) metallicity causes the typical ages for old globular clusters in the Milky Way to be increased roughly by 10%. Although this may appear small, it has a significant impact on the interpretation for the formation epoch of Milky Way globular clusters. The Asplund et al. (2004) abundance may not necessarily threaten the current concordance cosmology but would suggest that Milky Way globular clusters formed before the reionization epoch and before the main galaxy body starts to build up. This is in contrast to the current understanding on the galaxy formation.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1999년도 춘계학술대회논문집
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• pp.195-198
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• 1999

For the thixoforming process beside an existing solidus-liquidus interval, the reheating conditions to obtain the globular microstructure are very important. It relies on the control of globular microstructure of semi-solid alloys that contain non-dendritic particles. To obtain the globular microstructure in cross section of billet, the optimal design of the induction coil is necessary. Therefore, in this paper the optimal coil design to minimize electromagnetic end effect will be proposed. The results of coil design were also applied to the reheating process to obtain a fine globular microstructure. Finally, reheating data base of aluminum alloys for thixoforming and FEM model for induction heating based on the optimal coil design have been proposed.

• 천문학회보
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• 제41권1호
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• pp.43.3-43.3
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• 2016

Most globular clusters are now known to have two or more stellar populations with different chemical properties. In order to understand the origin and evolution of multiple stellar populations in these globular clusters, it is necessary to study not only the chemical property, but also the dynamical property. In our previous works (Lim et al. 2015; Han et al. 2015), we have shown that Ca narrow-band photometry can be combined with low-resolution spectroscopy to effectively study the chemical properties of globular clusters. In this talk, we will show our observations are also useful to study the radial distribution of stars in globular clusters with multiple stellar populations, and report our preliminary results.

• 천문학회보
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• 제40권2호
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• pp.33.1-33.1
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• 2015

M85 is a nearby merger remnant galaxy located at the northern part of the Virgo Cluster. Because of its remarkable merging features, it is an interesting object to investigate its formation history. Globular clusters are a great tracer of the formation history of early-type galaxies, so that we study the globular cluster system of M85. It has been already found that there are "intermediate-color" globular clusters as well as blue and red ones based on the photometric survey using CFHT/Megacam. For follow-up research, we obtain the spectra of 21 globular clusters in the central region of M85 using Gemini-N/GMOS. We estimate their ages and metallicities based on the strength of Lick indices. We detect the intermediate-age population (~ 2 Gyr) with solar metallicities, comprising about 50% of the observed globular clusters, as well as old and metal-poor population. It suggests that M85 experienced a major merging event around 2 Gyr ago. We discuss these results regarding to the formation history of M85.

• 천문학논총
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• 제15권spc1호
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• pp.1-13
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• 2000

We review the early historical developement of astronomical spectrographs, properties of emission line spectra of HII regions in spiral galaxies, and absorption line features of galactic globular clusters. Emission line spectra of HII regions within three spiral galaxies NGC 300, NGC 1365, and NGC 7793, which were observed from AAT/IPCS, had been analysed, and we discuss the abundances of elements in HII regions and the radial abundace gradients through the galaxies. The radial UBV color variations of two globular clusters, NGC 1851 and NGC 2808, were examined for correlations with radial variations of several absorption lines in the integrated spectra, which were obtained from SAAO 74 inch telescope and image tube spectrograph. Nine giant star's spectra in NGC 3201 were also obtained and analysed for the radial abundance gradients in the globular cluster. The results show that the presence of a radial color gradient in a globular cluster is correlated with the presence of abundance gradients. Finally, we suggest some scientific programs for the new high dispersion spectrograph, which will be installed to the BOAO 1.8m telescope.

• 천문학회보
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• 제41권2호
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• pp.30.3-31
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• 2016

The M81 Group is the nearest among the known groups of galaxies. HI maps show that all main galaxies of this group are interacting, indicating that this group is in the active phase of formation. On the other hand, wide field imaging shows that globular cluster candidates are found not only in the member galaxies but also between the galaxies in this group. In this study we present preliminary results based on MMT/Hectospec spectroscopy of the globular cluster candidates in this group. We find that the intragroup globular clusters have mostly low metallicity, while the globular clusters in M81 have mostly high metallicity. We will discuss the implication of this result and the kinematics of the globular clusters in relation with the formation history of the M81 Group.

• 천문학회보
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• 제38권1호
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• pp.57.2-57.2
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• 2013

Based on N-body simulations, we find out that significant fraction of dynamically formed BH-BH (10 $M_{\odot}$ and NS-NS (1.4 $M_{\odot$ ecah) binaries are ejected from globular clusters. About 30 percent of compact stars are ejected in the form of binary. The merging time of ejected binary depends on the velocity dispersion of globular cluster. Some of ejected binaries have merging time-scales shorter than Hubble time and are expected to produce gravitational waves that can be detectable by the advanced ground-based interferometers. The merger rates of ejected BH-BH and NS-NS binaries per globular cluster are estimated to be 3.5 and 17 per Gyr, respectively. Assuming the spatial density of globular clusters as 8.4 $h^3$ clusters $Mpc^{-3}$ and extrapolating to the horizon distance of the advanced LIGO-Virgo network, we expect the detection rates solely attributed to BH-BH and NS-NS with cluster origin are to be 42 and 1.7 $yr^{-1}$, respectively. Besides, we find out that BH-NS binary ejection hardly occurs in globular clusters and dynamically formed compact binaries may possibly be the source of short GRBs whose locations are far from host galaxies.

• 천문학회보
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• 제45권1호
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• pp.32.2-32.2
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• 2020

Massive Compact Elliptical Galaxies (MCEGs) found in the local universe are as massive as normal galaxies but extremely compact (M∗ > 1011 Msun, Reff < 1.5 kpc). They are considered to be the relics of red nugget galaxies found at high redshift. They are not likely to have undergone many mergers, keeping their original mass and size. Moreover, it is expected that they host a dominant population of red (metal-rich) globular clusters rather than blue (metal-poor) ones. Indeed, Beasley et al. (2018) found that the color distribution of the cluster system of NGC 1277 is unimodal, showing only a red population. However, NGC 1277 is the only case whose cluster system was studied among MCEGs. In this study, we investigate globular cluster systems of 14 nearby MCEGs with a homogeneous data set of HST/WFC3 F814W/F160W archive images. We detect tens to hundreds of globular clusters in each galaxy and examine their color distributions. Surprisingly, the fractions of red globular clusters are similar to those of normal galaxies, and are much lower than that of NGC 1277. We additionally obtain Gemini/GMOS-N g'r'i' images of PGC 70520, one of the 14 nearby MCEGs, to detect more globular clusters from deeper and wider images. We will discuss the results from the Gemini data combined with the results from the HST data in relation with the formation of MCEGs.

• 한국주조공학회지
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• 제6권1호
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• pp.12-19
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• 1986

This study was undertaken to understand the formation mechanism of the hard spots in high strength brass. To investigate the behavior of the hard spots in the isothermal liquid state with varying silicon content, the rapidly quenched specimens were obtained by suctioning the melt into the silica tube and water quenching. To examine the growth process of the hard spots with holding time, the unidirectional solidification technique was used. The results of this study are summarized as follows: 1) With the addition of Fe in order to get the effects of grain refinement in high strength brass, the two different type of Fe-rich phases are occurred, which are defined as dendritic and globular phase. The chemical composition of the globular phase was different from that of the dendritic phase in that the globular phase contained Si. 2) With increasing Si content, the Fe-rich phase had a tendency to form globular phase. 3) As the holding time increased in the liquid state, globular was also prone to coalesce. The further growth of globular phase to large size was due to reducing the interfacial energy. 4) The primary phase of copper alloy was nucleated preferentially on the dendritic phase. It was noticeable that the dendritic phase acted as a grain refiner. However, the agglomerate (hard spots) which was composed of the globular phase decreased the mechanical properties of high strength brass. 5) Once the hard spots formed in the high strength brass casting, it was very difficult to remove them. This is due to the fact that their meting temperature is higher than the pouring temperature of high strength brass.

• 천문학회지
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• 제29권spc1호
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• pp.139-140
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• 1996