• The Bulletin of The Korean Astronomical Society
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• v.43 no.2
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• pp.40.1-40.1
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• 2018

Among Type I supernovae, which show no evidence for hydrogen lines in spectra, Type Ib/c supernovae lack of strong Si absorption lines and are involved with massive progenitors. While strong helium absorption lines are present in Type Ib supernovae, narrow helium emission lines also can appear in some Type Ib that are often called Type Ibn supernovae (SNe Ibn). We consider helium giant stars as a promising progenitor candidate for SN Ibn and suggest the evolutionary scenario through binary systems using MESA code. In our models the range of primary mass is 11 - 20 solar mass, mass ratio is 0.5 - 0.9, and initial period is 1.5 / 1.7 / 2.0 / 2.5 / 3.0 day. In particular, we find that the evolution of the secondary star can overtake the primary through mass transfer from the secondary to the primary, which is so-called 'reverse case B' mass transfer. In such systems the secondary star may undergo a supernova explosion earlier than the primary star. In this case, the primary star evolves towards a single helium giant to become a SN Ibn progenitor. These cases are more frequent in relatively low initial primary mass.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.38-38
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• 2014

Despite the great success that stellar evolution theory have enjoyed during the last 50 years, new challenges are emerging with recent observations of supernovae: many aspects of supernovae cannot be easily explained by the standard scenarios on supernova progenitors. A few examples include the red supergiant problem - the dearth of Type IIP supernova progenitors with masses higher than about 16 Msun, the non-detection of Type Ib/c supernova progenitors despite very deep searches in pre-supernova optical images, the unexpected blue colors of some Type IIn supernova progenitors, and the exotic stellar explosions of both ultra-faint and super-luminous types that have been only recently discovered. By confronting these observations with new stellar evolution models, we are making significnt progress in better understanding the role of metallicity, rotation and binary interactions for the pre-supernova evolution of massive stars. In this talk, I will give a brief review on the recent observational constraints on supernova progenitors and a progress report on several research projects that deal with pair-instability supernovae from the local Universe, type Ib/c supernovae from massive binary systems, and some peculiar stellar explosions like SN2012Z.

• Journal of The Korean Astronomical Society
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• v.56 no.1
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• pp.11-22
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• 2023

We report the result of a high-resolution spectroscopic study on seven magnesium (Mg) enhanced stars. The high Mg abundances in these stars imply that they were born in an environment heavily affected by the nucleosynthesis products of massive stars. We measure abundances of 16 elements including Mg and they show various abundance patterns implying their diverse origin. Three of our program stars show a very high Mg to Si ratio ([Mg/Si] ≈ 0.18-0.25), which might be well explained by fall-back supernovae or by supernovae with rapid rotating progenitors having an initial mass higher than about 20 M_⊙. Another three of our program stars have high light to heavy s-process element ratios ([Y/Ba] ≈ 0.30-0.44), which are consistent with the theoretical prediction of the nucleosynthesis in rapidly rotating massive stars with an initial mass of about M = 40 M_⊙. We also report a star having both high Y ([Y/Fe] = 0.2) and Ba ([Ba/Fe] = 0.28) abundance ratios, and it also shows the highest Zn abundance ratio ([Zn/Fe] = 0.27) among our sample, implying the nucleosynthesis by asymmetric supernova explosion induced by very rapid rotation of a massive progenitor having an initial mass between 20 M_⊙ ≲ M ≲ 40 M_⊙. A relative deficiency of odd-number elements, which would be a signature of the pair-instability nucleosynthesis, is not found in our sample.

• Publications of The Korean Astronomical Society
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• v.8 no.1
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• pp.169-177
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• 1993

Observed spectra of supernovae allow the empirical classification of supernovae into two basic categories, Type I with little or no evidence of hydrogen, and Type II with obvious evidence for hydrogen. The broad class of Type I can be subdivided depending on whether helium or silicon and other intermediate mass elements is observed. Understanding the physical processes that underlie these classifications---the progenitor evolution. the explosion mechanism, and end products---requires calculation of radiative transfer and model spectra. While most Type II occur in evolved massive stars that undergo core collapse. some may span the dividing line between degenerate and non-degenerate carbon burning and involve both core collapse and thermonuclear explosion. Type Ia are still most plausibly explained as thermonuclear explosions in carbon/oxygen white dwarfs in binary systems. Type Ib reveal helium atmospheres and are probably the result of core collapse in the helium core of a massive star that has lost its hydrogen envelope to a binary companion or to a wind. Type Ic supernovae are probably related to Type Ib but have also lost their helium envelope to reveal a mantle rich in oxygen.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.59.1-59.1
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• 2020

Extremely metal-poor (EMP; [Fe/H] < -3.0) stars are thought to be genuine second-generation of stars because they were born from relatively pristine gas chemically enriched by one or two supernovae. So, the EMP stars presumably originated from outside the Milky Way (MW) are important tracers for the early chemical evolution and assembly history of the MW. In this study, we present the preliminary results on the early assembly history of the MW inferred by associating the dynamical properties of our EMP stars with those of known substructures in the MW. We also explore the star formation history of the progenitor galaxies of our EMP stars by investigating the elemental abundances of the EMP stars associated with the substructure.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.42.4-43
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• 2020

The hydrogen-rich envelope mass of a dying massive star is the key factor that determines the type and properties of the resulting supernova. Emulating wind-driven mass loss of single stars with the MESA(Modules for Experiments in Stellar Astrophysics) stellar evolution code, we made a grid of models for a large parameter space of initial mass (12 M⊙ to 30M⊙), metallicity (solar, LMC and SMC), hydrogen envelope mass (0.01M⊙ to 10M⊙) for progenitor stars in their final step of evolution. Our results suggest the final luminosity of the progenitor is largely determined by the initial mass, which means there is luminosity degeneracy for stars with the same initial mass but with different hydrogen-rich envelope masses. Since we can break this degeneracy by correcting luminosity with surface gravity (spectroscopic HR diagram), we can infer the exact mass property of an observed progenitor. The surface temperature drastically varies near the envelope mass of ~0.1M⊙ and surface temperature of ~10000 K, where the demarcation between the hydrogen-rich envelope and the helium core lies, which explains the rarity of 'white' supergiants. There also exists a discontinuity in the chemical composition of the progenitor envelope around this critical hydrogen-rich envelope mass of ~0.1 M⊙, which can be tested in future observations of "flash spectroscopy" of supernovae.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.61.4-61.4
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• 2019

The key science goal of the KMTNet Supernova Program (KSP) is to detect and study the early explosions of supernovae using one fifth of the KMTNet time. The BVI-band observations of the nearby target fields mostly closer than 30 Mpc distance and the follow-up spectroscopy provide valuable information on the early phase of the supernovae. These data can also be used for the studies of optical transients such as novae, dwarf novae, variable stars, and active galactic nuclei. Stacked images of several hundred images obtained from the time domain observations can be used for the search of low surface brightness galaxies reaching $28mag\;arcsec^{-2}$. Results and status of the KSP including ${\geq}20$ infant supernovae and ${\geq}100$ faint dwarf galaxies will be presented in this talk.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.76.1-76.1
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• 2016

During the last decade, high-resolution spectra of many very metal-poor (VMP) stars have been observed and their surface compositions have been measured. The abundance patterns of the VMP stars strongly constrain the nucleosynthesis of Pop III stars because they born from material enriched by supernovae or wind ejecta of Pop III stars. The observations show overabundances of light elements like C, N, O, Na, Mg and Al and very low $C^{12}/C^{13}$ ratios. These results indicate that mixing between the H-burning and He-burning region occurred in Pop III stars. To explain these observational results, we performed 1D stellar evolution simulations for non-rotating Pop III stars with ZAMS masses ranging from $20M_{\Box}$ to $50M_{\Box}$ and various overshooting parameters. In our grid calculation, convective mixing between helium burning layers and the hydrogen burning shell generally occurred in models with masses less than $40M_{\Box}$ without rotation and these models show an excess of light element abundances. From this result, it is expected that we could explain the observed abundance patterns with convective mixing in non-rotating massive Pop III stars and we do not necessarily have to invoke rotational mixing.

• Journal of The Korean Astronomical Society
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• v.29 no.spc1
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• pp.273-276
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• 1996

The world wide efforts for detecting gravitational waves, the detectors in vogue and the expected astrophysical sources of gravitational waves will be discussed. Ground based detectors especially, the resonant bar detectors and laser interferometers will be described with a brief mention of the space based detector (the LISA project). Astrophysical sources of gravitational waves such as coalescing binaries, supernovae, pulsars/ rotating neutron stars, stochastic background will be discussed in the context of detection.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.89-91
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• 2015

Observed spectra of stars around the Sun have indicated that the Sun is located in a gas cavity, extending to 100pc. This gas cavity is called the "Local Bubble". The density of the interstellar medium (ISM) in the local bubble is about one tenth that of the average for the ISM in the Milky Way. Furthermore, some structures such as gas planes and strings in the local bubble are probably the result of supernovae. These, due to their low temperatures, can not be observed in the visible and infrared. The only way to do so is to measure the spectra of nearby stars so that the light of stars passing through the local bubble is absorbed by existing gas and the resulting spectral lines from absorption can be measured. In this study, we use binary stars to trace the local bubble structures through lines such as the Na I Doublet. First, we determined the observed spectral lines of stars by HARPS and FEROS echelle spectrographs. Then, we made synthetic spectra with the ATLAS9 code. Finally, the difference between the observational and synthetic spectra confirms the existence of the Na I Doublet in the local ISM.