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- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,227건 (DB Construction : 1,227 Articles)
총 게시글 1,227건
페이지 37/123
361
- Yi, Su-Kyoung K.
- Journal of the Korean astronomical society = 천문학회지
- 43, n.4
- pp.135-139
- 2010
- 원문 바로보기
We present the result of our investigation on the impact of the low Solar abundance of Asplund and collaborators (2004) on the derived ages for the oldest star clusters based on isochrone fittings. We have constructed new stellar models and corresponding isochrones using this new solar mixture with a proper Solar calibration. We have found that the use of the Asplund et al. (2004) metallicity causes the typical ages for old globular clusters in the Milky Way to be increased roughly by 10%. Although this may appear small, it has a significant impact on the interpretation for the formation epoch of Milky Way globular clusters. The Asplund et al. (2004) abundance may not necessarily threaten the current concordance cosmology but would suggest that Milky Way globular clusters formed before the reionization epoch and before the main galaxy body starts to build up. This is in contrast to the current understanding on the galaxy formation.
362
- Kim, Sung-Soo
- Journal of the Korean astronomical society = 천문학회지
- 43, n.4
- pp.105-113
- 2010
- 원문 바로보기
We study the dynamical evolution of the M87 globular cluster (GC) system using the most advanced and realistic Fokker-Planck (FP) model.By comparing our FP models with both mass function (MF) and radial distribution (RD) of the observed GC system, we find the best-fit initial (at M87's age of 2-3 Gyr) MF and RD for three GC groups: all GCs, blue GCs, and red GCs. We estimate the initial total mass in GCs to be <TEX>$1.8^{+0.3}_{-0.2}{\times}10^{10}M_{\bigodot}$</TEX>, which is about 100 times larger than that of the Milky Way GC system. We also find that the fraction of the total mass currently in GCs is 34\%. When blue and red GCs are fitted separately, blue GCs initially have a larger total mass and a shallower radial distribution than red GCs. If one assumes that most of the significant major merger events of M87 have ended by the age of 2-3 Gyr, our finding that blue (metal-poor) GCs initially had a shallower radial distribution supports the major merger scenario for the origin of metallicity bimodality.
363
- Kim, Gwan-Jeong
- Journal of the Korean astronomical society = 천문학회지
- 43, n.1
- pp.9-23
- 2010
- 원문 바로보기
In this paper we examined the association of Infrared Dark Cloud (IRDC) cores with YSOs and the geometric properties of the IRDC cores. For this study a total of 13,650 IRDC cores were collected mainly from the catalogs of the IRDC cores published from other studies and partially from our catalog of IRDC cores containing new 789 IRDC core candidates. The YSO candidates were searched for using the GLIMPSE, MSX, and IRAS point sources by the shape of their SED or using activity of water or methanol maser. The association of the IRDC cores with these YSOs was checked by their line-of-sight coincidence within the dimension of the IRDC core. This work found that a total of 4,110 IRDC cores have YSO candidates while 9,540 IRDC cores have no indication of the existence of YSOs. Considering the 12,200 IRDC cores within the GLIMPSE survey region for which the YSO candidates were determined with better sensitivity, we found that 4,098 IRDC cores (34%) have at least one YSO candidate and 1,072 cores among them seem to have embedded YSOs, while the rest 8,102 (66%) have no YSO candidate. Therefore, the ratio of [N(IRDC core with protostars)]/[N(IRDC core without YSO)] for 12,200 IRDC cores is about 0.13. Taking into account this ratio and typical lifetime of high-mass embedded YSOs, we suggest that the IRDC cores would spend about <TEX>$10^4\sim10^5$</TEX> years to form high-mass stars. However, we should note that the GLIMPSE point sources have a minimum detectable luminosity of about <TEX>$1.2 L_{\odot}$</TEX> at a typical IRDC core's distance of ~4 kpc. Therefore, the ratio given here should be a 100ver limit and the estimated lifetime of starless IRDC cores can be an upper limit. The physical parameters of the IRDC cores somewhat vary depending on how many YSO candidates the IRDC cores contain. The IRDC cores with more YSOs tend to be larger, more elongated, and have better darkness contrast than the IRDC cores with fewer or no YSOs.
364
- Kwon, Young-Joo
- Journal of the Korean astronomical society = 천문학회지
- 43, n.4
- pp.123-133
- 2010
- 원문 바로보기
In order to study properties of the pulsation in the infrared emission for long period variables, we collect and analyze the infrared observational data at L band for 12 OH/IR. The observation data cover about three decades including recent data from the ISO and Spitzer. We use the Marquardt-Levenberg algorithm to determine the pulsation period and amplitude for each star and compare them with results of previous investigations at infrared and radio bands. We obtain the relationship between the pulsation periods and the amplitudes at L band. Contrary to the results at K band, there is no difference of the trends in the short and long period regions of the period-luminosity relation at L band. This may be due to the molecular absorption effect at K band. The correlations among the L band parameters, IRAS [12-25] colors, and K band parameters may be explained as results of the dust shell parameters affected by the stellar pulsation. The large scatter of the correlation could be due to the existence of a distribution of central stars with various masses and pulsation modes.
365
- Choi, Yun-Young
- Journal of the Korean astronomical society = 천문학회지
- 43, n.6
- pp.191-200
- 2010
- 원문 바로보기
We present the Korea Institute for Advanced Study Value-Added Galaxy Catalog (KIAS VAGC), a catalog of galaxies based on the Large Scale Structure (LSS) sample of New York University Value-Added Galaxy Catalog (NYU VAGC) Data Release 7. Our catalog supplements redshifts of 10,497 galaxies with 10 < <TEX>$r_P\;{\leq}\;17.6$</TEX> (1455 with 10 < <TEX>$r_P\;{\leq}\;14.5$</TEX>) to the NYU VAGC LSS sample. Redshifts from various existing catalogs such as the Updated Zwicky Catalog, the IRAS Point Source Catalog Redshift Survey, the Third Reference Catalogue of Bright Galaxies, and the Two Degree Field Galaxy Redshift Survey have been put into the NYU VAGC photometric catalog. Our supplementation significantly improves spectroscopic completeness: the area covered by the spectroscopic sample with completeness higher than 95% increases from 2.119 to 1.737 sr. Our catalog also provides morphological types of all galaxies that are determined by the automated morphology classification scheme of Park & Choi (2005), and related parameters, together with fundamental photometry parameters supplied by the NYU VAGC. Our catalog contains matches to objects in the Max Planck for Astronomy (MPA) & Johns Hopkins University (JHU) spectrum measurements (Data Release 7). This new catalog, the KIAS VAGC, is complementary to the NYU VAGC and MPA-JHU catalog.
366
- Isayev, Alexander A.
- Journal of the Korean astronomical society = 천문학회지
- 43, n.5
- pp.161-168
- 2010
- 원문 바로보기
Magnetars are neutron stars possessing a magnetic field of about <TEX>$10^{14}-10^{15}$</TEX> G at the surface. Thermodynamic properties of neutron star matter, approximated by pure neutron matter, are considered at finite temperature in strong magnetic fields up to <TEX>$10^{18}$</TEX> G which could be relevant for the inner regions of magnetars. In the model with the Skyrme effective interaction, it is shown that a thermodynamically stable branch of solutions for the spin polarization parameter corresponds to the case when the majority of neutron spins are oriented opposite to the direction of the magnetic field (i.e. negative spin polarization). Moreover, starting from some threshold density, the self-consistent equations have also two other branches of solutions, corresponding to positive spin polarization. The influence of finite temperatures on spin polarization remains moderate in the Skyrme model up to temperatures relevant for protoneutron stars. In particular, the scenario with the metastable state characterized by positive spin polarization, considered at zero temperature in Phys. Rev. C 80, 065801 (2009), is preserved at finite temperatures as well. It is shown that, above certain density, the entropy for various branches of spin polarization in neutron matter with the Skyrme interaction in a strong magnetic field shows the unusual behavior, being larger than that of the nonpolarized state. By providing the corresponding low-temperature analysis, we prove that this unexpected behavior should be related to the dependence of the entropy of a spin polarized state on the effective masses of neutrons with spin up and spin down, and to a certain constraint on them which is violated in the respective density range.
367
- Kang, Hye-Sung
- Journal of the Korean astronomical society = 천문학회지
- 43, n.2
- pp.25-39
- 2010
- 원문 바로보기
We perform kinetic simulations of diffusive shock acceleration (DSA) in Type Ia supernova remnants (SNRs) expanding into a uniform interstellar medium (ISM). Bohm-like diffusion due to self-excited <TEX>$Alfv\acute{e}n$</TEX> waves is assumed, and simple models for <TEX>$Alfv\acute{e}nic$</TEX> drift and dissipation are adopted. Phenomenological models for thermal leakage injection are considered as well. We find that the preshock gas temperature is the primary parameter that governs the cosmic ray (CR) acceleration efficiency and energy spectrum, while the CR injection rate is a secondary parameter. For SNRs in the warm ISM of <TEX>$T_0\lesssim10^5K$</TEX>, if the injection fraction is <TEX>$\xi\gtrsim10^{-4}K$</TEX>, the DSA is efficient enough to convert more than 20% of the SN explosion energy into CRs and the accelerated CR spectrum exhibits a concave curvature flattening to <TEX>$E^{-1.6}$</TEX>, which is characteristic of CR modified shocks. Such a flat source spectrum near the knee energy, however, may not be reconciled with the CR spectrum observed at Earth. On the other hand, SNRs in the hot ISM of<TEX>$T_{0}\approx10^{6}K$</TEX> with a small injection fraction, <TEX>$\xi$</TEX><<TEX>$10^{-4}$</TEX>, are inefficient accelerators with less than 10% of the explosion energy getting converted to CRs. Also the shock structure is almost test-particle like and the ensuing CR spectrum can be steeper than <TEX>$E^{-2}$</TEX>. With amplified magnetic field strength of order of <TEX>$30{\mu}G$</TEX> <TEX>$Alfv\acute{e}n$</TEX> waves generated by the streaming instability may drift upstream fast enough to make the modified test-particle power-law as steep as <TEX>$E^{-2.3}$</TEX>, which is more consistent with the observed CR spectrum.
368
- Oh, Seung-Kyung
- Journal of the Korean astronomical society = 천문학회지
- 42, n.4
- pp.105-105
- 2009
- 원문 바로보기
369
- Abdel-Rahman, H.I.
- Journal of the Korean astronomical society = 천문학회지
- 42, n.4
- pp.71-79
- 2009
- 원문 바로보기
In this paper, statistical distribution functions are developed for distance determination of stellar groups. This method depends on the assumption that absolute magnitudes and apparent magnitudes follow a Gaussian distribution function. Due to the limits of the integrands of the frequency function of apparent and absolute magnitudes, we introduce Case A, B, and C Gaussian distributions. The developed approaches have been implemented to determine distances to some clusters and stellar associations. The comparison with the distances derived by different authors reveals good agreement.
370
- Moon, Y.J.
- Journal of the Korean astronomical society = 천문학회지
- 42, n.2
- pp.27-32
- 2009
- 원문 바로보기
Recently, we suggested a CME earthward direction parameter as an important geoeffective parameter that has been demonstrated by front-side halo CME data. In this study, we present the geometrical implication of this parameter by comparing with the parameters from a CME cone model. Major results from this study can be summarized as follows. First, we derive an analytic relationship between the cone model parameters(the half angular width of a cone and the angle between the cone axis and the plane of sky) and the earthward direction parameter. Second, we demonstrate a close relationship between the earthward direction parameter and the cone axis angle using 32 front-side full halo CMEs. Third, we found that there is noticeable inconsistency between the cone axis angles estimated from the cone model fitting to the CMEs and from their associated flare positions, implying that the flare position should not be considered as a good earthward direction parameter. Finally we present several advantages of our earthward direction parameter in terms of the forecast of a geomagnetic storm based on CME parameters.