• 천문학회보
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• 제46권1호
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• pp.35.2-35.2
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• 2021

Blue straggler stars (BSs) are "rejuvenated" main sequence stars first recognized by Allan Sandage from his observation of the prominent northern globular cluster M3 in the year of 1953. BSs are now known to be present in diverse stellar environments including open clusters, globular clusters, dwarf galaxies, and even the field populations of the Milky Way. This makes them a very useful tool in a wide range of astrophysical applications: Particularly BSs are considered to have a crucial role in the evolution of stellar clusters because they affect on the dynamics, the binary population, and the history of the stellar evolution of the cluster they belong to. Here we report a part of the preliminary results from our ongoing research on the BSs in the two metal-poor globular clusters (GCs) in the Large Magellanic Cloud (LMC), Hodge 11 and NGC1466. Using the high precision multi-band images obtained with the Advanced Camera for Survey (ACS) onboard the Hubble Space Telescope (HST), we extract time-series photometry to search for the signal of periodic variations in the luminosity of the BSs. Our preliminary results confirm that several BSs are intrinsic "short period (0.05 < P < 0.25 days)" variable stars with either pulsating or eclipsing types. We will discuss our investigation on the properties of those variable BS candidates in the context of the formation channels of these exotic main sequence stars, and their roles in the dynamical evolution of the host star clusters.

• 천문학회보
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• 제46권2호
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• pp.37.1-37.1
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• 2021

Observations suggest the star formation in nuclear rings of barred galaxies proceeds episodically in time and sometimes asymmetrically in space. Existing theories and numerical simulations suggest that the episodic star formation is perhaps due to either supernova feedback combined with fluid instabilities or time-varying mass inflow rate. However, it has been challenging to discern what dominates in shaping the star formation history because the effects of the inflow and feedback are blended in global simulations of nuclear rings. To understand their effects separately, we construct semi-global models of nuclear rings, which treat the mass inflow rate as a model parameter. By running simulations with the inflow rates kept constant or oscillating in time, we find that the star formation rate (SFR) of the rings varies coherently with the inflow rate, while the feedback is responsible only for stochastic fluctuations of the SFR within a factor of two. The feedback instead plays an important role in maintaining the vertical dynamical equilibrium and setting the depletion time. While the asymmetry in the inflow does not necessarily lead to the asymmetry in the star formation, we find that the rings undergo a transient period of lopsided star formation when the inflow rate of only one dust lane is suddenly increased.

• 천문학논총
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• 제31권1호
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• pp.1-9
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• 2016

We investigate the records related to the reign style and the calendar day from the epigraphs of the Goryeo dynasty (918 - 1392) in Korea in order to verify and supplement the sexagenary cycle data of the first day in the lunar month of the dynasty. The database of the National Research Institute of Culture Heritage contains a rubbed-copy image, transcription statement, and translation statement for Korean epigraphs as well as 775 epigraphs corresponding to the Goryeo dynasty. The epigraph records are valuable in that, during this time, they were written differently from other historical literature such as the Goryeosa (History of the Goryeo Dynasty), which was compiled in the next dynasty. We find that the Goryeo dynasty, in general, had adopted the reign styles of Chinese dynasties at that time. We also find 159 calendar day records all showing good agreement with the work of Ahn et al. except for dozens of records. Through this study, we can verify the reign styles and the calendar days of the Goryeo dynasty.

• 천문학회보
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• 제41권2호
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• pp.34.3-35
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• 2016

Ram pressure stripping of early-type galaxies has been largely neglected until now because of their gas poor nature. MUSE IFU observation vividly reveal the presence of star-forming blobs and ionised gas tails, around an early-type galaxy in Abell 2670. The galaxy was identified as a post-merger galaxy with disturbed faint features, in MOSAIC 2 deep optical images. The imaging also revealed a series of star-forming blobs, situated in the direction facing away from the cluster centre. Thanks to the revolutionary wide field-of-view of the MUSE, combined with 8.2-m VLT (UT-4) at Cerro Paranal, we could simultaneously obtain IFU spectra of the blobs, as well as the galaxy. The MUSE spectra clearly confirms that the star-forming blobs are associated with the early-type galaxy. Moreover, MUSE reveals long ionised-gas tails, emanating from the galaxy. The quantity of gas indicates a gas rich progenitor has merged with the early-type galaxy. However the direction of the tails and blobs, and the blob morphology, appears to indicate that strong ram-pressure stripping may have stripped out gas brought in by the merger. We will present kinematic structure of the whole system (the galaxy, star-forming blobs, and gas tails), as well as the star formation history of the system, supporting a scenario where a recent galaxy merger is subjected to cluster environmental mechanisms.

• 천문학회보
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• 제43권2호
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• pp.35.3-36
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• 2018

One of the most prevalent knowledge about disk galaxies, which dominate the population of the local Universe, is that they consist of stellar structures with different kinematics, such as thin disk, bulge, and halo. Therefore, investigating when and how these components develop in a galaxy is the key to understanding the evolution of galaxies. Using the NewHorizon simulation, we can resolve the detailed structures of galaxies, in the field environment, from the early Universe where star formation and mergers were most active. We first decompose stellar particles in a galaxy into a disk and a dispersion-dominated, spheroidal, component based on their orbits and then see how these components evolve in terms of mass and structure. At high redshift z~3, galaxies are mostly dispersion-dominated as stars are formed misaligned with the galactic rotational axis. At z=1~2, massive galaxies start to dominantly form disk stars, while less massive galaxies do much later. Furthermore, massive galaxies are forming thinner and larger disks with time, and the preexistent disks are heated or even disrupted to become a part of dispersion-dominated component. Thus, the mass growth of spheroidal components at later epochs is dominated by disrupted stars with disk origins and accreted stars at large radii.

• 천문학회보
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• 제39권2호
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• pp.62.1-62.1
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• 2014

Metallicity distribution of globular clusters (GCs) provides an important clue for star formation history of their host galaxy. With an assumption that GCs are generally old, GC colors have been used as a proxy of GC metallicities. Bimodal GC color distributions observed in most large galaxies have, for decades, been interpreted as bimodal metallicity distributions, indicating the presence of two populations within a galaxy. However, the conventional view has been challenged by a new theory that non-linear GC color-metallicity relations (CMRs) can cause a bimodal color distribution even from a single-peaked metallicity distribution. Using the photometric and spectroscopic data of NGC 5128 GCs in combination with stellar population simulation models, we examine the effect of non-linearity in GC CMRs on the transformation of GC color distributions into metallicity distributions. Although, in some colors, offsets are present between observations and models in the CMRs, their overall shape agrees well for various colors. After the offsets are corrected, the observed spectroscopic metallicity distribution is well reproduced via modeled CMRs from various color distributions having different morphologies. On the other hand, the linearly converted metallicity distributions from GC colors show a significant discrepancy with the observed spectroscopic metallicity distribution. We discuss the implications of our results.

• 천문학회보
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• 제42권1호
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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.

• 천문학회보
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• 제43권1호
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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.

• 천문학회보
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• 제36권2호
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• pp.142.1-142.1
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• 2011

UBVI and $H{\alpha}$ CCD photometry of IC 1848, one of active star-forming regions in Cas OB6, is carried out as a part of Sejong Open cluster Survey (SOS) project. An OB association is an ideal laboratory for studying the triggered star formation and star formation history. Our purposes are to provide deep photometric data up to 21 mag in V and physical parameters of IC 1848. We classify 79 early-type stars and 186 pre-main sequence (PMS) stars as being the members of the cluster using photometric criteria. The IR excess emission PMS stars by Koenig et al. (2008) are also included as members of IC 1848. Total number of members is 414. We derive the interstellar reddening (=$0.659{\pm}0.058$ mag), reddening law ($R_V=4.0{\pm}0.1$), distance modulus ($V_0-M_V=12.0{\pm}0.1$ mag) using the early-type members of IC 1848. We also determine the age of the cluster ($3.5{\pm}1.5$ Myr) by placing the theoretical isochrones on the HR diagram.

• 천문학논총
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• 제30권2호
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• pp.261-263
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• 2015

The metallicity distribution of globular clusters (GCs) provides a crucial clue for the star formation history of their host galaxy. With the assumption that GCs are generally old, GC colors have been used as a proxy for GC metallicities. Bimodal color distributions of GCs observed in most large galaxies have, for decades, been interpreted as bimodal metallicity distributions, indicating the presence of two populations within a galaxy. However, the conventional view has been challenged by a new theory that non-linear GC color-metallicity relations can cause a bimodal color distribution even from a single-peaked metallicity distribution. Using photometric and spectroscopic data of NGC 5128 GCs in combination with stellar population simulation models, we examine the effect of non-linearity in GC color-metallicity relations on transformation of the color distributions into the metallicity distributions. Although in some colors offsets are present between observations and models for the color-metallicity relations, their overall shape agrees well for various colors. After the offsets are corrected, the observed spectroscopic metallicity distribution is well reproduced via modeled color-metallicity relations from various color distributions having different morphologies. We discuss the implications of our results.