• The Bulletin of The Korean Astronomical Society
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• v.40 no.2
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• pp.34.2-34.2
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• 2015

We aim to investigate the formation of primordial globular clusters (GCs) in the isolated dwarf galaxy (${\sim}10^{10}M_{sun}$) with cosmological zoom-in simulations. For this, we modified cosmological hydrodynamic code, GADGET-3, in a way to include the radiative heating/cooling that enables gas particles cool down to T~10K, reionization (z < 8.9) of the Universe, UV shielding ($n_{shield}$ > $0.014cm^{-3}$), and star formation. Our simulation starts in a cubic box of a side length 1Mpc/h with 17 million particles from z = 49. The mass of each dark matter (DM) and gas particle is $M_{DM}=4.1{\times}10^3M_{sun}$ and $M_{gas}=7.9{\times}10^2M_{sun}$, respectively, thus the GC candidates can be resolved with more than hundreds particles. We found the following results: 1) mini halos with the more interactions before merging into the main halo form the more stars and thus have the higher star mass fraction ($M_{star}/M_{total}$), 2) the mini halos with the high $M_{star}/M_{total}$ can survive longer and thus spiral into closer to the galactic center, 3) the majority of them spiral into bulge, but some of them can survive until the last as baryon-dominated system, like the GC.

• Journal of Astronomy and Space Sciences
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• v.24 no.1
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• pp.31-38
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• 2007

We have investigated the evolutionary status of 85 Peg within the framework of standard evolutionary theory. 85 Peg has been known to be a visual and spectroscopic binary system in the solar neighborhood. In spite of the accurate information of the total mass (${\sim}1.5M_{\odot}$) and the distance (${\sim}12pc$) from the HIPPARCOS parallax, it has been undetermined an individual mass, therefore the evolved status of the system. Moreover, the coupled uncertainties of chemical composition and age, make matters worse in predicting an evolutionary status of the system. Nevertheless, we computed the various possible models for 85 Peg, and then calibrated stellar parameters by adjusting to the recent observational data. Our modelling computation has included recently updated input physics and stellar theory such as opacity, equation of state, and chemical diffusion. Through a statistical assessment, we have derived a confident parameter set as the best solution which minimized $X^{2}$ within the observational error domain. Most of all, we found that 85 Peg is not a binary system but a triple system with an unseen companion 85 Peg $B_{b}\;{\sim}0.16M_{\odot}$. The aim of the present paper is (1) to provide a complete modelling of the stellar system based on the evolutionary theory, and (2) to constrain the physical dimensions such as mass, metallicity and age.

• Journal of Astronomy and Space Sciences
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• v.29 no.1
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• pp.47-49
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• 2012

Massive binary stars lose mass by two mechanisms: jet-driven mass loss during periods of active mass transfer and by wind-driven mass loss. Beta Lyrae is an eclipsing, semi-detached binary whose state of active mass transfer provides a unique opportunity to study how the evolution of binary systems is affected by jet-driven mass loss. Roche lobe overflow from the primary star feeds the thick accretion disk which almost completely obscures the mass-gaining star. A hot spot predicted to be on the edge of the accretion disk may be the source of beta Lyrae's bipolar outflows. I present results from spectropolarimetric data taken with the University of Wisconsin's Half-Wave Spectropolarimeter and the Flower and Cook Observatory's photoelastic modulating polarimeter instrument which have implications for our current understanding of the system's disk geometry. Using broadband polarimetric analysis, I derive new information about the structure of the disk and the presence and location of a hot spot. These results place constraints on the geometrical distribution of material in beta Lyrae and can help quantify the amount of mass lost from massive interacting binary systems during phases of mass transfer and jet-driven mass loss.

• Journal of The Korean Astronomical Society
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• v.48 no.3
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• pp.165-175
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• 2015

We investigate physical properties of the nearby (∼ 7.5 pc) astrometric binary μ Cas in the context of standard evolutionary theory. Based on the spectroscopically determined relative abundances ([α/Fe] ≳ +0.4 dex, [Fe/H] ∼ −0.7 dex), all physical inputs such as opacities and equation of state are consistently generated. By combining recent spectroscopic analyses with the astrometric observations from the HIPPARCOS parallaxes and the CHARA array, the evolutionary model grids have been constructed. Through the statistical evaluation of the χ²-minimization among alternative models, we find a reliable evolutionary solution (M_A, M_B, t_age) = (0.74 M_⊙, 0.19 M_⊙, 11 Gyr) which excellently satisfies observational constraints. In particular, we find that the helium abundance of μ Cas is comparable with the primordial helium contents (Y_p ∼ 0.245). On the basis of the well-defined stellar parameters of the primary star, the internal structure and the p-mode frequencies have been estimated. From our seismic computation, μ Cas is expected to have a first order spacing ∆ν ∼ 169 μHz. The ultimate goal of this study is to describe physical processes inside a low-mass star through a complete modelling from the spectroscopic observation to the evolutionary computation.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.88.1-88.1
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• 2014

Helium mass in the envelope is one of the most important properties in progenitors of type Ib/c supernovae (SNe Ib/c), since SN Ib/c progenitors are distinguished by the presence of He I lines. However, previous progenitor models do not reproduce the required He mass limit($M_{He}$ < $0.14M_{\odot}$) suggested by a spectroscopic analysis of SN Ib/c. In this work, we investigated the effect of overshooting on the evolution of pure helium stars, focusing on the final He mass in the envelope, $M_{He,f}$. We used the MESA code to calculate single helium star models with the initial masses of $M_{init}=5{\sim}30M_{\odot}$, Z=0.02, 0.04 and overshooting parameters of $f_{ov}=0{\sim}0.4$. The final He mass $M_{He,f}$ decreases as $f_{ov}$ increases, due to larger burning core compared to weak overshooting models. Dependence of the final mass $M_{He,f}$ on overshooting is strongest for models with $M_{init}=7{\sim}10M_{\odot}$, and this effect originates from accelerated mass loss during transition between WNE and WC/O phase. However, $M_{He,f}$ exceeds $0.27M_{\odot}$ for all models, which still doesn't meet the criteria of $M_{He}$ < $0.14M_{\odot}$. This implies that mass loss during the post helium burning phase must be enhanced dramatically compared to what the standard models predict.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.92.2-92.2
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• 2014

NGC 6822 is a dwarf irregular galaxy in the Local Group. Unlike clouds in the Large Magellanic Cloud and the Small Magellanic Cloud, molecular clouds in NGC 6822 are not influenced by the Galactic tidal force. Therefore the star forming processes are only dictated by local conditions. Hubble V is the brightest of the several bright H II region complexes in NGC 6822. The core of Hubble V, surrounded by a molecular cloud complex, contains compact clusters of bright blue stars. During the commissioning runs of the new high-resolution near-infrared spectrometer, IGRINS (Immersion GRating near-INfrared Spectrometer), we observed Hubble V and detected many emission lines from the H II regions and from the photodissociation region at the interface between the ionized gas and the molecular cloud. In this presentation, we report preliminary results of the IGRINS observations. We discuss the implications of the observed lines ratios and kinematics for our understanding of the evolution of star forming molecular clouds.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.78.1-78.1
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• 2014

An ionization front (IF) surrounding an H II region is a sharp interface through which a cold neutral gas makes transition to a warm ionized phase by absorbing UV photons from central massive stars. We investigate the structure and instability of a plane-parallel D-type IF threaded by magnetic fields parallel to the front. We find that magnetic fields increase the maximum propagation speed of the IFs, while reducing the expansion factor, defined as the density ratio of neutral to ionized phases. IFs become unstable to distortional perturbations due to gas expansion across the fronts, exactly analogous to the Darrieus-Landau instability of ablation fronts in terrestrial flames. The growth rate of the IF instability is proportional linearly to the perturbation wavenumber as well as the upstream flow speed. The IF instability is stabilized by gas compressibility and becomes completely quenched when the front is D-critical. The instability is also stabilized by magnetic pressure when the perturbations propagate in the direction perpendicular to the fields. When the perturbations propagate in the direction parallel to the fields, on the other hand, it is magnetic tension that reduces the growth rate, completely suppressing the instability when ${\beta}$ < 1.5, with ${\beta}$ denoting the square of the ratio of the sound speed to the Alfven speed in the pre-IF region. When the front experiences an acceleration, the IF instability cooperates with the Rayleigh-Taylor instability to make the front more unstable. We discuss potential effects of IF instability on the evolution and dynamics of IFs in the interstellar medium.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.42.2-42.2
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• 2017

The Large Magellanic Cloud (LMC) is a unique target to study the detail structures of molecular clouds and star-forming regions, due to its proximity and face-on orientation from us. Most part of the astrophysical subjects for the LMC have been investigated, but the magnetic field is still veiling despite its role in the evolution of the interstellar medium (ISM) and in the main force to influence the star formation process. Measuring polarization of the background stars behind interstellar medium allows us to describe the existence of magnetic fields through the polarization vector map. In this presentation, I introduce the near-infrared polarimetric results for the $39^{\prime}{\times}69^{\prime}$ field of the northeastern region of the LMC and the N159/N160 star-forming complex therein. The polarimetric observations were conducted at IRSF/SIRPOL 1.4 m telescope. These results allow us to examine both the global geometry of the large-scale magnetic field in the northeastern region and the close structure of the magnetic field in the complex. Prominent patterns of polarization vectors mainly follow dust emission features in the mid-infrared bands, which imply that the large-scale magnetic fields are highly involved in the structure of the dust cloud in the LMC. In addition, local magnetic field structures in the N159/N160 star-forming complex are investigated with the comparison between polarization vectors and molecular cloud emissions, suggesting that the magnetic fields are resulted from the sequential formation history of this complex. I propose that ionizing radiation from massive stellar clusters and the expanding bubble of the ionized gas and dust in this complex probably affect the nascent magnetic field structure.

• Journal of The Korean Astronomical Society
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• v.40 no.4
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• pp.153-155
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• 2007

We have analyzed H and $K_s$-band images of the Arches cluster obtained using the NIRC2 instrument on Keck with the laser guide star adaptive optics (LGS AO) system. With the help of the LGS AO system, we were able to obtain the deepest ever photometry for this cluster and its neighborhood, and derive the background-subtracted present-day mass function (PDMF) down to $1.3M_{\bigodot}$ for the 5"-9" annulus of the cluster. We find that the previously reported turnover at $6M_{\bigodot}$ is simply due to a local bump in the mass function (MF), and that the MF continues to increase down to our 50 % completeness limit ($1.3M_{\bigodot}$) with a power-law exponent of ${\Gamma}$ = -0.91 for the mass range of 1.3 < M/$M_{\bigodot}$ < 50. Our numerical calculations for the evolution of the Arches cluster show that the ${\Gamma}$ values for our annulus increase by 0.1-0.2 during the lifetime of the cluster, and thus suggest that the Arches cluster initially had ${\Gamma}$ of $-1.0{\sim}-1.1$, which is only slightly shallower than the Salpeter value.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.2
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• pp.44-44
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• 2010

We study the effect of massive neutrinos on the evolution of the early mini-halos ($M\sim10^6h^{-1}M{\odot}at$ z~20) where the first stars may have formed. In the framework of the extended Press-Schechter formalism, we evaluate analytically the rates of merging of the mini-halos into zero-dimensional larger halos and one-dimensional mini-filaments. It is shown that the halo-to-filament merging rate increases with the neutrino mass fraction $f_v$ while the halo-to-halo merging rate decreases. Comparing the cases of $f_v$=0 and 0.10, the halo-to-filament merging rate for $f_v$=0.10 is 3 times larger than the other. The distribution of the epochs of the longest-axis collapse of these first filaments is also derived and found to reach a sharp maximum at z~8-9. Once the first mini-filaments form, they would provide bridges along which the matter and gas more rapidly accrete onto the constituent halos, causing the early formation of the first galaxies and rapid growth of their central blackholes. Furthermore, the longest axis collapse of these first mini-filaments would spur the supermassive blackholes to power the ultra-luminous high-z quasars.