• Journal of The Korean Astronomical Society
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• v.33 no.2
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• pp.111-121
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• 2000

Many models of globular cluster formation assume the presence of cold dense clouds in early universe. Here we re-examine the Fall & Rees (1985) model for formation of proto-globular cluster clouds (PGCCs) via thermal instabilities in a protogalactic halo. We first argue, based on the previous study of two-dimensional numerical simulations of thermally unstable clouds in a stratified halo of galaxy clusters by Real et al. (1991), that under the protogalactic environments only nonlinear (${\delta}{\ge}1$) density inhomogeneities can condense into PGCCs without being disrupted by the buoyancy-driven dynamical instabilities. We then carry out numerical simulations of the collapse of overdense douds in one-dimensional spherical geometry, including self-gravity and radiative cooling down to T = $10^4$ K. Since imprinting of Jeans mass at $10^4$ K is essential to this model, here we focus on the cases where external UV background radiation prevents the formation of $H_2$ molecules and so prevent the cloud from cooling below $10^4$ K. The quantitative results from these simulations can be summarized as follows: 1) Perturbations smaller than $M_{min}\~(10^{5.6}\;M{\bigodot})(nh/0.05cm^{-3})^{-2}$ cool isobarically, where nh is the unperturbed halo density, while perturbations larger than $M_{min}\~(10^8\;M{\bigodot})(nh/0.05cm^{-3})^{-2}$ cool isochorically and thermal instabilities do not operate. On the other hand, intermediate size perturbations ($M_{min} < M_{pgcc} < M_{max}$) are compressed supersonically, accompanied by strong accretion shocks. 2) For supersonically collapsing clouds, the density compression factor after they cool to $T_c = 10^4$ K range $10^{2.5} - 10^6$, while the isobaric compression factor is only $10^{2.5}$. 3) Isobarically collapsed clouds ($M < M_{min}$) are too small to be gravitationally bound. For supersonically collapsing clouds, however, the Jeans mass can be reduced to as small as $10^{5.5}\;M_{\bigodot}(nh/0.05cm^{-3})^{-1/2}$ at the maximum compression owing to the increased density compression. 4) The density profile of simulated PGCCs can be approximated by a constant core with a halo of $p{\infty} r^{-2}$ rather than a singular isothermal sphere.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.43.4-44
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• 2016

The presence of multiple stellar populations is now well established in most globular clusters (GCs) in the Milky Way. The origin of this phenomenon, however, is yet to be understood. In this respect, the study of NGC 2808, an intriguing GC which hosts subpopulations with extreme helium abundances, would help to resolve this problem. In order to investigate chemical abundance patterns among different subpopulations in this GC, we have performed low-resolution spectroscopy for the red-giant-branch (RGB) stars and measured CN & CH bands, and Ca line strength. We have identified at least three subpopulations from the CN abundance distribution. This GC shows CN-CH anti-correlation following the general trend among "normal" GCs. In addition, we have cross-matched our results with the high-resolution data in literature, and found a tight correlation between CN strength and sodium abundance. However, CN is anticorrelated with oxygen abundance, as expected from the well known N(&Na)-O anticorrelation. In this talk, we will discuss the implication of these results.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.37.2-37.2
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• 2019

Recent investigations of the double red clump in the color-magnitude diagram of the Milky Way bulge cast serious doubts on the structure and formation origin of the outer bulge. Unlike previous interpretation based on an X-shaped bulge, stellar evolution models and CN-band observations have suggested that this feature is another manifestation of the multiple stellar population phenomenon observed in globular clusters (GCs). This new scenario requires a significant fraction of the outer bulge stars with chemical patterns uniquely observed in GCs. Here we show from homogeneous high-quality spectroscopic data that the red giant branch stars in the outer bulge ($>5.5^{\circ}$ from the Galactic center) are clearly divided into two groups according to Na abundance in the [Na/Fe] - [Fe/H] plane. The Na-rich stars are also enhanced in Al, while the differences in O and Mg are not observed between the two Na groups. The population ratio and the Na and Al differences between the two groups are also comparable with those observed in metal-rich GCs. Since these chemical patterns and characteristics are only explained by stars originated in GCs, this is compelling evidence that the outer bulge was mostly assembled from disrupted proto-GCs in the early history of the Milky Way. We will also discuss the implications of this result on the formation of the early-type galaxies in general.

• Journal of The Korean Astronomical Society
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• v.31 no.2
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• pp.95-99
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• 1998

We report preliminary results of an on-going survey of optically selected barred galaxies with $^{12}CO$(J=1-0) line. The entire sample is composed of about 100 bright barred galaxies ($B_T{\le}13$) with small inclination angle. Most of the galaxies are relatively nearby with receding speed less than 10,000 km/sec. In the first observing run, we have observed central parts of 18 galaxies and detected CO emissions from 5 galaxies (NGC521, 2525, 4262, 4900, and 7479). Most of these galaxies are not observed with CO previously, except for NGC7479 which has been studied at various wavelengths. The peak antenna temperature of detected galaxies ranges from about 30 to 300 mK.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.30.2-30.2
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• 2013

The mean color of globular cluster (GCs) systems in early-type galaxies (ETGs) is, in general, bluer than the integrated color of field stars in their host galaxies. Recently, Goudfrooij & Kruijssen (2013) reported that even red GCs in the ETGs show bluer colors than their host field stars and suggested the different initial mass function (IMF) for red GCs and field stars to explain the observed offset in color. Here we suggest an alternative scenario that explains the observed color offsets between red GCs in ETGs and the field stars in the parent galaxies without invoking to the variation of the IMF. We find that the inclusion of second-generation (SG) helium-enhanced populations in the model fully explains the observed color offset between red GCs and field stars in the host galaxies. We have also tested the effect of the IMF slope on our models, but the effect is relatively small compared to the effect of the SG population. Our new model suggests that, in order to explain far-UV strong metal-rich GCs in M87 and the observed color offset between metal-rich GCs and the field stars in ETGs simultaneously, the inclusion of the SG populations with enhanced helium abundance is a more natural solution than the model that only adopted variations in the IMF.