• Publications of The Korean Astronomical Society
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• v.7 no.1
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• pp.89-96
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• 1992

Recent spectroscopic observations indicate concentration of dark masses in the nuclei of nearby galaxies. This has been usually interpreted as the presence of massive black holes in these nuclei. Alternative explanations such as the dark cluster composed of low mass stars (brown dwarfs) or dark stellar remnants are possible provided that these systems can be stably maintained for the age of galaxies. For the case of low mass star cluster, mass of individual stars can grow to that of conventional stars in collision time scale. The requirement of collision time scale being shorter than the Hubble time gives the minimum cluster size. For typical conditions of M31 or M32, the half-mass radii of dark clusters can be as small as 0.1 arcsecond. For the case of clusters composed of stellar remnants, core-collapse and post-collapse expansion are required to take place in longer than Hubble time. Simple estimates reveal that the size of these clusters also can be small enough that no contradiction with observational data exists for the clusters made of white dwarfs or neutron stars. We then considered the possible outcomes of interactions between the black hole and the surrounding stellar system. Under typical conditions of M31 or M32, tidal disruption will occur every $10^3$ to $10^4$ years. We present a simple scenario for the evolution of stellar debris based on basic principles. While the accretion of stellar material could produce large amount of radiation so that the mass-to-light ratio can become too small compared to observational values it is too early to rule out the black hole model because the black hole can consume most of the stellar debris in time scale much shorter than mean time between two successive tidal disruptions. Finally we outline recent effort to simulate the process of tidal disruption and subsequent evolution of the stellar debris numerically using Smoothed Particle Hydrodynamics technique.

• Journal of The Korean Astronomical Society
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• v.40 no.1
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• pp.17-28
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• 2007

We present wide-field $JHK_s$-band photometric observations of the three compact H II regions G48.9-0.3, G49.0-0.3, and G49.2-0.3 in the active star-forming region W51B. The star clusters inside the three compact H II regions show the excess number of stars in the $J-K_s$ histograms compared with reference fields. While the mean color excess ratio $(E_{J-H}/E_{H-K_s})$ of the three compact H II regions are similar to ${\sim}2.07$, the visual extinctions toward them are somewhat different: ${\sim}17$ mag for G48.9-0.3 and G49.0-0.3; ${\sim}23$ mag for G49.2-0.3. Based on their sizes and brightnesses, we suggest that the age of each compact H II region is ${\leq}2\;Myr$. The inferred total stellar mass, ${\sim}1.4{\times}10^4M_{\odot}$, of W51B makes it one of the most active star forming regions in the Galaxy with the star formation efficiency of ${\sim}10%$.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.39.1-39.1
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• 2014

We present the results of our analysis of the RR Lyrae (RRL) variable stars detected in two transition-type dwarf galaxies (dTrans), ESO294-G010 and ESO410-G005 in the Sculptor group, which is known to be one of the closest neighboring galaxy groups to our Local Group. Using deep archival images from the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope (HST), we have identified a sample of RR Lyrae candidates in both dTrans galaxies [219 RRab (RR0) and 13 RRc (RR1) variables in ESO294-G010; 225 RRab and 44 RRc stars in ESO410-G005]. The metallicities of the individual RRab stars are calculated via the period-amplitude-[Fe/H] relation derived by Alcock et al. This yields mean metallicities of <[Fe/H]>_{ESO294} = -1.77 +/- 0.03 and <[Fe/H]>_{ESO410} = -1.64+/- 0.03. The RRL metallicity distribution functions (MDFs) are investigated further via simple chemical evolution models; these reveal the relics of the early chemical enrichment processes for these two dTrans galaxies. In the case of both galaxies, the shapes of the RRL MDFs are well-described by pre-enrichment models. This suggests two possible channels for the early chemical evolution for these Sculptor group dTrans galaxies: 1) The ancient stellar populations of our target dwarf galaxies might have formed from the star forming gas which was already enriched through "prompt initial enrichment" or an "initial nucleosynthetic spike" from the very first massive stars, or 2) this pre-enrichment state might have been achieved by the end products from more evolved systems of their nearest neighbor, NGC 55.

• Journal of The Korean Astronomical Society
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• v.56 no.2
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• pp.195-199
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• 2023

This paper is written as a follow-up observations to reinterpret the radial velocity (RV) of HD 36384, where the existence of planetary systems is known to be ambiguous. In giants, it is, in general, difficult to distinguish the signals of planetary companions from those of stellar activities. Thus, known exoplanetary giant hosts are relatively rare. We, for many years, have obtained RV data in evolved stars using the high-resolution, fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO). Here, we report the results of RV variations in the M giant HD 36384. We have found two significant periods of 586 d and 490 d. Considering the orbital stability, it is impossible to have two planets at so close orbits. To determine the nature of the RV variability variations, we analyze the HIPPARCOS photometric data, some indicators of stellar activities, and line profiles. A significant period of 580 d was revealed in the HIPPARCOS photometry. H_α EW variations also show a meaningful period of 582 d. Thus, the period of 586 d may be closely related to the rotational modulations and/or stellar pulsations. On the other hand, the other significant period of 490 d is interpreted as the result of the orbiting companion. Our orbital fit suggests that the companion was a planetary mass of 6.6 M_J and is located at 1.3 AU from the host.

• Journal of The Korean Astronomical Society
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• v.15 no.1
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• pp.1-7
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• 1982

The luminosity functions of 12 open clusters are derived for which their membership and the colors of their individual stars have been established by detailed proper motion study and high quality photometric work. The resulting luminosity functions of these clusters are presented and discussed.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.144.1-144.1
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• 2011

We explore the metallicity distribution of the Galactic halo based on the SDSS ugriz photometry. We use empirically calibrated sets of stellar isochrones to determine distances and metallicities of individual main-sequence stars in the halo. At heliocentric distances greater than 5 kpc, we find that the in situ photometric metallicity distribution reveals chemically divided dual halo components, which supports arguments from earlier studies based on the medium resolution spectroscopy. Our finding provides an unbiased estimate of relative fractions of each of these stellar components in the Galactic halo.

• Publications of The Korean Astronomical Society
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• v.32 no.1
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• pp.113-116
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• 2017

A significant number of the parameters of a gamma-ray burst (GRB) and its host galaxy are calculated from the afterglow. There are various methods obtaining extinction values for the necessary correction for galactic foreground. These are: galaxy counts, from HI 21 cm surveys, from spectroscopic measurements and colors of nearby Galactic stars, or using extinction maps calculated from infrared surveys towards the GRB. We demonstrate that AKARI Far-Infrared Surveyor sky surface brightness maps are useful uncovering the fine structure of the galactic foreground of GRBs. Galactic cirrus structures of a number of GRBs are calculated with a 2 arcminute resolution, and the results are compared to that of other methods.

• Journal of The Korean Astronomical Society
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• v.51 no.1
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• pp.17-25
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• 2018

Detecting exoplanets around giant stars sheds light on the later-stage evolution of planetary systems. We observed the M giant HD 18438 and the K giant HD 158996 as part of a Search for Exoplanets around Northern circumpolar Stars (SENS) and obtained 38 and 24 spectra from 2010 to 2017 using the high-resolution Bohyunsan Observatory Echelle Spectrograph (BOES) at the 1.8m telescope of Bohyunsan Optical Astronomy Observatory in Korea. We obtained precise RV measurements from the spectra and found long-period radial velocity (RV) variations with period 719.0 days for HD 18438 and 820.2 days for HD 158996. We checked the chromospheric activities using Ca $\text\tiny{II}$ H and $H{\alpha}$ lines, HIPPARCOS photometry and line bisectors to identify the origin of the observed RV variations. In the case of HD 18438, we conclude that the observed RV variations with period 719.0 days are likely to be caused by the pulsations because the periods of HIPPARCOS photometric and $H{\alpha}$ EW variations for HD 18438 are similar to that of RV variations in Lomb-Scargle periodogram, and there are no correlations between bisectors and RV measurements. In the case of HD 158996, on the other hand, we did not find any similarity in the respective periodograms nor any correlation between RV variations and line bisector variations. In addition, the probability that the real rotational period can be as longer than the RV period for HD 158996 is only about 4.3%. Thus we conclude that observed RV variations with a period of 820.2 days of HD 158996 are caused by a planetary companion, which has the minimum mass of 14.0 $M_{Jup}$, the semi-major axis of 2.1 AU, and eccentricity of 0.13 assuming the stellar mass of $1.8 M_{\odot}$. HD 158996 is so far one of the brightest and largest stars to harbor an exoplanet candidate.

• Journal of The Korean Astronomical Society
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• v.46 no.6
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• pp.253-259
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• 2013

We present near-infrared light curves of HBC 722 after its the September 2010 outburst. We have been monitoring its near-infrared light curves since November 2010 with Korean Astronomy and Space Science Institute Infrared Camera System (KASINICS). HBC 722 exhibits large changes in optical and near-infrared brightness since its outburst. The J, H, and $K_s$ light curves over about 2.5 years show that in all observed bands HBC 722 progressively became fainter until around April 2011, down to J ~10.7, H ~9.9, $K_s$ ~9.3, but it is getting brighter again. Large scatter in the obtained light curve prevents us from finding whether there is any short timescale variation as reported in other optical observations. The near-infrared color of HBC 722 is becoming bluer since its outburst. The pre-outburst Spectral Energy Distribution (SED) of HBC 722 is consistent with that of a slightly reddened Class II YSO with the exception of the extraordinary IR-excess in the far-infrared region.

• Journal of The Korean Astronomical Society
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• v.36 no.2
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• pp.55-60
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• 2003

In Lee, Kang & Byun (2001) the discovery of Raman scattered 6545 A feature was reported in symbiotic stars and the planetary nebula M2-9. The broad emission feature around 6545 A is formed as a result of Raman scattering of He II n = 6 $\to$ n = 2 photons by atomic hydrogen. In this paper, we introduce a method to compute the equivalent width of He II $\lambda$ 1025 line and present an optical spectrum of the symbiotic star RR Telescopii as an example for a detailed illustration. In this spectrum, we pay attention to the broad H$\alpha$ wings and the Raman scattered He II 6545 feature. The broad Ha wings are also proposed to be formed through Raman scattering of continuum around Ly$\beta$ by Lee (2000), and therefore we propose that the equivalent width of the He II $\lambda$ 1025 emission line is obtained by a simple comparison of the strengths of the 6545 feature and the broad H$\alpha$ wings. We prepare a template H$\alpha$ wing profile from continuum radiation around Ly$\beta$ with the neutral scattering region that is supposed to be responsible for the formation of Raman scattered He II 6545 feature. Isolation of the 6545 feature that is blended with [N II] $\lambda$ 6548 is made by using the fact that [N II] $\lambda$ 6584 is always 3 times stronger than [N II] $\lambda$ 6548. We also fit the 6545 feature by a Gaussian which has a width 6.4 times that of the He II $\lambda$ 6527 line. A direct comparison of these two features for RR Tel yields the equivalent width $EW_{Hel025} = 2.3{\AA}$ of He II $\lambda$ 1025 line. Even though this far UV emission line is not directly observable due to heavy interstellar extinction, nearby He II lines such as He II $\lambda$ 1085 line may be observed using far UV space instruments, which will verify this calculation and hence the origins of various features occurring in spectra around H$\alpha$.