• Title, Summary, Keyword: stars: formation

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PHOTOELECTRIC OBSERVATIONS OF EXTREMELY YOUNG OPEN CLUSTERS

  • Kwon, Suk-Minn;Lee, See-Woo
    • Journal of The Korean Astronomical Society
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    • v.16 no.1
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    • pp.7-17
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    • 1983
  • During the period between January and November in 1982, UBV photoelectric observations were made for 48 stars in NGC 2264, 66 stars in IC 1805 and 22 stars in IC 348. From these observations, various physical parameters such as distance, mean color excess, total-to-selective extinction ratio and mean age of the clusters were determined. Making use of these parameters, the star formation rates were examined for IC 348 and NGC 2264. The overall formation rate is found to be increase rapidly during the period of the active star formation. The age spread (ranging from $5\times10^6$ yrs to $10^7$ yrs) of stars in a given cluster appears to be real which occurs in the extremely young open clusters.

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Young Open Clusters: Their Uses in Star Formation Studies

  • Lim, Beomdu
    • The Bulletin of The Korean Astronomical Society
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    • v.40 no.2
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    • pp.28.1-28.1
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    • 2015
  • Open clusters are one of stellar systems consisting of a few hundreds to thousands of stars. The cluster members are, in general, believed to be a coeval stellar population at the same distance, and therefore they have almost the same properties in chemical composition and kinematics. Owing to these advantages, the clusters are utilized in many astronomy studies, such as the calibrations of distance and stellar age scales, assessments of stellar evolution theories, and the chemical evolution of the Galactic disk. Young open clusters are, inter alia, superb objects to study star formation process as most of stars are known to be formed in clusters. In this talk, I will review the uses of these young open clusters in star formation studies based on the ongoing work of our research group on the stellar initial mass function, an age spread problem, mass accretion rate of pre-main sequence stars, and a feedback of high-mass stars on surroundings.

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On the Use of the Number Count of Blue Horizontal-Branch Stars to Infer the Dominant Building Blocks of the Milky Way Halo

  • Chung, Chul;Lee, Young-Wook;Pasquato, Mario
    • The Bulletin of The Korean Astronomical Society
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    • v.40 no.2
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    • pp.57.3-57.3
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    • 2015
  • The formation of the Milky Way stellar halo is thought to be the result of merging and accretion of building blocks such as dwarf galaxies and massive globular clusters. Recently, Deason et al. (2015) suggested that the Milky Way outer halo formed mostly from big building blocks, such as dwarf spheroidal galaxies, based on the similar number ratio of blue straggler (BS) stars to blue horizontal-branch (BHB) stars. Here we demonstrate, however, that this result is seriously biased by not taking into detailed consideration on the formation mechanism of BHB stars from helium enhanced second-generation population. In particular, the high BS-to-BHB ratio observed in the outer halo fields is most likely due to a small number of BHB stars provided by GCs rather than to a large number of BS stars. This is supported by our dynamical evolution model of GCs which shows preferential removal of first generation stars in GCs. Moreover, there are sufficient number of outer halo GCs which show very high BS-to-BHB ratio. Therefore, the BS-to-BHB number ratio is not a good indicator to use in arguing that more massive dwarf galaxies are the main building blocks of the Milky Way outer halo. Several lines of evidence still suggest that GCs can contribute a signicant fraction of the outer halo stars.

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Discovery of a significant population of CN-enhanced red clump stars in the Milky Way bulge: Implications for the formation of early-type galaxies

  • Lee, Young-Wook;Hong, Seungsoo;Lim, Dongwook
    • The Bulletin of The Korean Astronomical Society
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    • v.42 no.2
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    • pp.43.2-43.2
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    • 2017
  • We report our discovery of a significant population of CN-enhanced red clump stars in the classical bulge component of the Milky Way. Since CN-rich stars trace a population with enhanced Helium, Nitrogen, and Sodium originated in globular clusters (GCs), this is a direct evidence that proto-GCs were the major building blocks in the formation of the classical bulges and early-type galaxies in the hierarchical merging paradigm.

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Narrow-band Ca Photometry for Dwarf Spheroidal Galaxies: Recent Results and Future Work

  • Kim, Hak-Sub;Han, Sang-Il;Yoon, Suk-Jin
    • The Bulletin of The Korean Astronomical Society
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    • v.44 no.2
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    • pp.75.2-75.2
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    • 2019
  • This poster introduces the ongoing "Narrow-band Ca Photometry for Dwarf Spheroidal Galaxies" project and presents the latest results. The project aims to explain the formation and evolution of dwarf spheroidal galaxies by examining the structural properties of stellar populations as a function of metallicity. To overcome the lack of stars with known spectroscopic metallicities for dwarf spheroidal galaxies, we apply the hk index as a photometric metallicity indicator to three galaxies-Draco, Sextans, and Canes Venatici I. For all three galaxies, we found that metal-poor and metal-rich groups of red-giant-branch stars have distinct spatial distributions, in which metal-rich stars are centrally concentrated while metal-poor stars are relatively dispersed. In Sextans, we found an off-centered peak of metal-poor stars which is presumed to be a disrupting star cluster in this galaxy. We will discuss the implications of our results for the dwarf galaxy formation and possible directions on future work of this project.

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SEARCH FOR DEBRIS DISKS BY AKARI AND IRSF

  • Takeuchi, Nami;Ishihara, Daisuke;Kaneda, Hidehiro;Oyabu, Shinki;Kobayashi, Hiroshi;Nagayama, Takahiro;Onaka, Takashi;Fujiwara, Hideaki
    • Publications of The Korean Astronomical Society
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    • v.32 no.1
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    • pp.73-75
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    • 2017
  • Debris disks are important observational clues to understanding on-going planetary system formation. They are usually identified by significant mid-infrared excess on top of the photospheric emission of a central star on the basis of prediction from J-, H-, and Ks-band fluxes and the stellar model spectra. For bright stars, 2MASS near-infrared fluxes suffer large uncertainties due to the near-infrared camera saturation. Therefore we have performed follow-up observations with the IRSF 1.4 m near-infrared telescope located in South Africa to obtain accurate J-, H-, and Ks-band fluxes of the central stars. Among 754 main-sequence stars which are detected in the AKARI $18{\mu}m$ band, we have performed photometry for 325 stars with IRSF. As a result, we have successfully improved the flux accuracy of the central stars from 9.2 % to 0.5 % on average. Using this dataset, we have detected $18{\mu}m$ excess emission from 57 stars in our samples with a $3{\sigma}$ level. We find that some of them have high ratios of the excess to the photospheric emission even around very old stars, which cannot be explained by the current planet-formation theories.

TRIGGERED HIGH MASS STAR FORMATION

  • Kurtz, Stan
    • Journal of The Korean Astronomical Society
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    • v.40 no.4
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    • pp.137-140
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    • 2007
  • Triggered star formation is not a new idea - it's been around for at least the last 30 years. Although it has never disappeared from the scene, it seems to be enjoying something of a renaissance in recent years, in both observational and theoretical studies. Here we give a brief discussion of the background of triggered star formation, describe some of our own recent observational efforts in this regard, and briefly mention some initial conclusions that may be drawn.