• Journal of The Korean Astronomical Society
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• v.38 no.2
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• pp.267-270
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• 2005

We investigate the spectral energy distributions (SEDs) of low mass-loss rate O-rich asymptotic giant branch (AGB) stars using the infrared observational data including the Infrared Space Observatory (ISO) data. Comparing the results of detailed radiative transfer model calculations with observations, we find that the dust formation temperature is much lower than 1000 K for standard dust shell models. We find that the superwind model with a density-enhanced region can be a possible alternative dust shell model for LMOA stars.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.68.3-68.3
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• 2019

Even though the initial mass function (IMF) of the first generation of stars played important roles in reionization of the universe, subsequent star formation, and chemical enrichment of the universe, it is still very uncertain. In this study, among the several indirect ways of estimating the IMF of the population III (Pop III) stars, we make use of extremely metal-poor (EMP; [Fe/H] < -3.0) stars in the Milky Way, in order to infer the characteristic mass range of Pop III stars. As the progenitors of many of the EMP stars are known to be Pop III stars, we attempt to construct the characteristic mass range of the progenitors (e.g., Pop III stars) of the EMP stares by comparing their observed abundance pattern of various chemical elements with chemical yields from supernova models.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.83-86
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• 2015

We present observational results characterizing molecular outflows from very low-mass objects in ${\rho}$ Ophiuchi and Taurus. Our results provide us with important implications that clarify the formation process of very low-mass objects.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.49.1-49.1
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• 2017

In order to investigate the origin of multiple stellar populations in the halo and bulge of the Milky Way, we have constructed chemical evolution models for the low-mass proto-Galactic subsystems such as globular clusters (GCs). Unlike previous studies, we assume that supernova blast waves undergo blowout without expelling the pre-enriched gas, while relatively slow winds of massive stars, together with the winds and ejecta from low and intermediate mass asymptotic giant branch stars, are all locally retained in these less massive systems. We first applied these models to investigate the origin of super-helium-rich red clump stars in the metal-rich bulge as recently suggested by Lee et al. (2015). We find that chemical enrichments by the winds of massive stars can naturally reproduce the required helium enhancement (dY/dZ = 6) for the second generation stars. Disruption of these "building blocks" in a hierarchical merging paradigm would have provided helium enhanced stars to the bulge field. Interestingly, we also find that the observed Na-O anticorrelation in metal-poor GCs can be reproduced, when multiple episodes of starbursts are allowed to continue in these subsystems. Specific star formation history with decreasing time intervals between the stellar generations, however, is required to obtain this result, as would be expected from the orbital evolution of these subsystems in a proto-Galaxy. The "mass budget problem" is also much alleviated by our models without ad-hoc assumptions on star formation efficiency and initial mass function.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.46.1-46.1
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• 2018

In order to investigate the origin of multiple stellar populations in the halo and bulge of the Milky Way, we have constructed chemical evolution models for the low-mass proto-Galactic subsystems such as globular clusters. Unlike previous studies, we assume that supernova blast waves undergo blowout without expelling the pre-enriched gas, while relatively slow winds of massive stars, together with the winds and ejecta from low and intermediate mass asymptotic-giant-branch stars, are all locally retained in these less massive systems. We find that the observed Na-O anti-correlations in metal-poor GCs can be reproduced when multiple episodes of starbursts are allowed to continue in these subsystems. A specific form of star formation history with decreasing time intervals between the stellar generations, however, is required to obtain this result, which is in good agreement with the parameters obtained from our stellar evolution models for the horizontal-branch. The "mass budget problem" is also much alleviated by our models without ad-hoc assumptions on star formation efficiency and initial mass function. We also applied these models to investigate the origin of super helium-rich red clump stars in the metal-rich bulge as recently suggested by Lee et al. (2015). We find that chemical enrichments by the winds of massive stars can naturally reproduce the required helium enhancement (dY/dZ = 6) for the second-generation stars. Disruption of proto-globular clusters in a hierarchical merging paradigm would have provided helium enhanced stars to the bulge field.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.76.3-76.3
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• 2019

Intracluster stars are believed to be gravitationally bound to a galaxy cluster, however, not to individual cluster galaxies. Their presence is observed as diffuse light typically in the central region extended from the brightest cluster galaxy. The diffuse light, often referred to as intracluster light (ICL), is difficult to quantify in distant high-redshift galaxy clusters because of the significant surface brightness dimming although ICL observations in high-redshift clusters provide powerful constraints on the origin of intracluster stars. In this poster, we present ICL study of the distant galaxy cluster SPT2106-5844 at z=1.132 with Hubble Space Telescope IR imaging data. With careful control of systematics, we successfully quantify the total amount of the ICL, measure the color profile, and obtain its two-dimensional distribution. Our measurement of the high abundance of the intracluster stars in this young cluster favors the ICL formation scenario, wherein production of intracluster stars are predominantly associated with the BCG formation.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.69.2-69.2
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• 2019

Recent high resolution IR observations reveal that molecular clouds are filamentary and such a structure is ubiquitous over various star-forming environments, and it is clear that filaments play a crucial role in the formation of cores and stars. However, the formation process of dense cores in the filaments are still unknown. To investigate this issue in detail, we have carried out TRAO FUNS (TRAO survey of nearby Filamentary molecular clouds, the Universal Nursery of Stars) toward various star forming filamentary molecular clouds. In this presentation, we will report the first look results of filaments and dense cores in MCLD 123.5+24.9 and IC 5146, which are known as a quiescent, non-star-forming region and an active, high-mass star forming region, respectively. By comparing the kinematic properties of filaments and dense cores in different star forming environments, we verified the formation scenario of filaments and dense core, i.e., gravoturbulent fragmentation via supersonic motions.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.67.3-67.3
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• 2019

Massive stars play an important role in terms of their feedback, but their formation process is poorly understood. Direct observational evidence for the formation of massive stars through accretion disks is rare. Hence the detection of disks in massive young stellar objects (MYSOs), if any, could be important to constrain the formation process of massive stars. The inner gaseous disk can be observed by the high-resolution near-infrared spectroscopy. We observed a MYSO, Min 2-62, using IGRINS and detected a double peak feature, which could be an evidence of a rotating disk, in the Bracket and Pfund series lines. We report the preliminary observational results of Min 2-62 with IGRINS.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.76.2-76.2
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• 2012

We present the first simulations of cosmic reionization that include the first stars and their radiative feedback that limited their formation, in a volume large enough to capture the spatial variations that affected the process and its observability. We show hat these first stars made reionization begin much earlier than without, and was reatly extended, which boosts the intergalactic electron-scattering optical depth and the large-angle polarization fluctuations of the cosmic microwave background (CMB) significantly. Although within current WMAP uncertainties, this will enable Planck see he signature of the first stars at high redshift, currently undetectable by other probes.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.72.1-72.1
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• 2010

Gas materials in the inner Galactic disk continuously migrate toward the Galactic center (GC) due to interactions with the bar potential, magnetic fields, stars, and other gaseous materials. In case of the Milky Way, those in forms of molecules appear to accumulate around 200 pc from the center (the central molecular zone, CMZ) to form stars there and further inside. The bar potential in the GC is thought to be responsible for such acculmulation of molecules and subsequent star formation, which is believed to have been continous throughout the lifetime of the Galaxy. We present 3-D hydrodynamic simulations of the CMZ that consider self-gravity, radiative cooling, and supernova feedback, and discuss the efficiency and role of the star formation in that region. We find that the gas accumulated in the CMZ by a bar potential of the inner bulge effectively turns into stars, supporting the idea that the stellar cusp inside the central 200 pc is a result of the sustained star formation in the CMZ. The obtained star formation rate in the CMZ, 0.03-0.1 Msun, is consistent with the recent estimate based on the mid-infrared observations by Yusef-Zadeh et al. We discuss the secular evolution of nuclear bulges in general, based on our results.