• 천문학논총
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• 제23권2호
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• pp.65-71
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• 2008

It has been considered that 'exhausted eclipses' (日食旣) were total eclipses. However, modern precise calculations show that a significant fraction of such records are not realized to be total. Thus we doubt that the two concepts are equivalent. Here we investigate the meaning of 'exhausted eclipses' in the east-Asian history. We first find that eclipses of magnitude greater than 0.8 were regarded as 'exhausted eclipses' by a Korean astronomer of the 18th-century Choson dynasty. His notion was based upon the definition of 'exhausted eclipses' in the ephemerides of pre-modern Chinese dynasties. According to those ephemerides, the 'exhausted eclipses', whose magnitude is greater than 0.8, have the first contact at the western part of the solar disk and the fourth contact at the eastern part of the solar disk. A simple geometrical calculation shows that such cases really occur when the magnitude of eclipse is greater than 0.7. We pointed out that such an ancient definition might not be impractical for ancient astronomers, because the uncertainty of eclipse magnitude estimated by ancient Chinese ephemerides was 10% and the human sight has a spatial resolution of 1.2 arcmin, which is approximately one twentieth of the Sun's angular diameter.

• 천문학회지
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• 제51권4호
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• pp.111-117
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• 2018

A numerical method is proposed to calculate the response of detectors measuring particle energies from incident isotropic fluxes of electrons and positive ions. The isotropic flux is generated by injecting particles moving radially inward on a hypothetical, spherical surface encompassing the detectors. A geometric projection of the field-of-view from the detectors onto the spherical surface allows for the identification of initial positions and momenta corresponding to the clear field-of-view of the detectors. The contamination of detector responses by particles penetrating through, or scattering off, the structure is also similarly identified by tracing the initial positions and momenta of the detected particles. The relative contribution from the contaminating particles is calculated using GEANT4 to obtain the geometric factor of the instrument as a function of the energy. This calculation clearly shows that the geometric factor is a strong function of incident particle energies. The current investigation provides a simple and decisive method to analyze the instrument geometric factor, which is a complicated function of contributions from the anticipated field-of-view particles, together with penetrating or scattered particles.

• 천문학회지
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• 제35권1호
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• pp.41-57
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• 2002

We have written a code called QDM_sca, which numerically solves the problem of radiative transfer in an anisotropically scattering, spherical atmosphere. First we formulate the problem as a second order differential equation of a quasi-diffusion type. We then apply a three-point finite differencing to the resulting differential equation and transform it to a tri-diagonal system of simultaneous linear equations. After boundary conditions are implemented in the tri-diagonal system, the QDM_sca radiative code fixes the field of specific intensity at every point in the atmosphere. As an application example, we used the code to calculate the brightness of atmospheric diffuse light(ADL) as a function of zenith distance, which plays a pivotal role in reducing the zodiacal light brightness from night sky observations. On the basis of this ADL calculation, frequent uses of effective extinction optical depth have been fully justified in correcting the atmospheric extinction for such extended sources as zodiacal light, integrated starlight and diffuse galactic light. The code will be available on request.

• 천문학회보
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• 제40권2호
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• pp.29.3-30
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• 2015

We introduce an advanced one-to-one galaxy correspondence method that populates dark matter halos with galaxies by tracing merging histories of most bound member particles (MBPs) identified in simulated virialized halos. To estimate the survival time of a satellite galaxy, we adopt several models of tidal-destruction time derived from an analytic calculation, isolated galaxy simulations, and cosmological simulations. We build mock galaxy samples for each model by using a merging tree information of MBPs from our new Horizon Run 4 N-body simulation from z = 12 to 0. For models of galaxy survival time derived from cosmological and isolated galaxy simulations, about 40% of satellites galaxies merged into a certain halo are survived until z = 0. We compare mock galaxy samples from our MBP-galaxy correspondence scheme and the subhalo-galaxy scheme with SDSS volume-limited galaxy samples around z = 0 with $M_r-5{\log}h$ < -21 and -20. Compared to the subhalo-galaxy correspondence method, our method predicts more satellite galaxies close to their host halo center and larger pairwise peculiar velocity of galaxies. As a result, our method reproduces the observed galaxy group mass function, the number of member galaxies, and the two-point correlation functions while the subhalo-galaxy correspondence method underestimates them.

• 천문학회보
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• 제41권2호
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• pp.42.2-43
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• 2016

Electrostatic fluctuations near upper-hybrid frequency, which are sometimes accompanied by multiple-harmonic electron cyclotron frequency bands above and below the upper-hybrid frequency, are common occurrences in the Earth's radiation belt, as revealed through the twin Van Allen Probe spacecraft. In the literature upper-hybrid emissions are used for estimating the background electron density, which in turn can be used to determine the plasmapause locations, but the role of energetic electrons in generating such fluctuations has not been discussed. The present paper carries out detailed analyses of data from the Waves instrument, which is part of the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) suite onboard the Van Allen Probes. Combined with theoretical calculation, it is demonstrated that the peak intensity associated with the upper-hybrid fluctuations is predominantly determined by tenuous but energetic electrons, and that denser and less energetic background electrons do not contribute much to the peak intensity. This finding shows that upper-hybrid fluctuations detected during quiet time are useful not only for the determination of the electron density, but also they contain information on the ambient energetic electron population as well.

• 천문학회지
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• 제29권spc1호
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• pp.263-264
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• 1996

We have solved the radiative transfer problem using a Sobolev approximation with an escape probability method in case of the supersonic expansion of a stellar envelope to an ambient medium. The radiation from the expanding envelope turns out to produce a P-Cygni type profile. In order to investigate the morphology of the theoretical P-Cygni type profile, we have treated $V{\infty},\;V_{sto},\;{\beta}$ (parameter for the velocity field), M and $\epsilon$ (parameter for collisional effect) as model parametrs. We have found that the velocity field and the mass loss rate affect the shapes of the P-Cygni type profiles most effectively. The secondarily important factors are $V{\infty},\;V_{sto}$. The collisional effect tends to make the total flux increase but not so .much in magnitude. We have infered some physical parameters of 68 Cyg, HD24912, and $\xi$ persei such as V$\infty$, M from the model calculation, which shows a good agreement with the observational results.

• 천문학회보
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• 제37권1호
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• pp.39.2-39.2
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• 2012

We present our study on a comparison of dark matter halo merger history from the runs using different numerical simulation codes. To analyze the uncertainty caused by the use of different N-body calculation methods, we compare the results from two cosmological hydrodynamic simulation codes GADGET-2 and RAMSES, which use a TreePM algorithm and the Adaptive Mesh Refinement(AMR) technique respectively. We perform cosmological dark matter-only simulations with the same parameter set and initial condition for both. The dark matter halo mass functions from two simulation runs correspond well with each other, except for lower mass haloes. The discrepancy on the low-mass haloes in turn causes a notable difference in halo merger rate, especially for the case of extremely minor merger. The result from GADGET-2 predicts that most haloes undergo more number of mergers with small haloes than that from RAMSES, independent of halo mass and environment. However, in the context of the study on galaxy evolution, such extreme minor mergers generally do not have strong effects on galaxy properties such as morphology or star formation history. Hence, we suggest that this uncertainty could be quantitatively negligible, and the results from two simulations are reliable even with only minor difference in merger history.

• 천문학회보
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• 제37권1호
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• pp.85.2-85.2
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• 2012

In this paper, we introduce the 3D modeling of the coronal magnetic field in the solar active region by extrapolating from the 2D observational data numerically. First, we introduce a nonlinear force-free field (NLFFF) extrapolation code based on the MHD-like relaxation method implementing the cleaning a numerical error for Div B proposed by Dedner et al. 2002 and the multi-grid method. We are able to reconstruct the ideal force-free field, which was introduced by Low & Lou (1990), in high accuracy and achieve the faster speed in the high-resolution calculation (512^3 grids). Next we applied our NLFFF extrapolation to the solar active region NOAA 10930. First of all, we compare the 3D NLFFF with the flare ribbons of Ca II images observed by the Solar Optical Telescope (SOT) aboard on the Hinode. As a result, it was found that the location of the two foot-points of the magnetic field lines well correspond to the flare ribbon. The result indicates that the NLFFF well capture the 3D structure of magnetic field in the flaring region. We further report the stability of the magnetic field by estimating the twist value of the field line and finally suggest the flare onset mechanism.

• 천문학회보
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• 제42권2호
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• pp.64.2-64.2
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• 2017

The Population III (Pop III) stars are thought to be massive. If massive Pop III stars form binary system and they experience mass transfer via the Roche lobe overflow, this may significantly change the properties of the system. For example, mass transfer in such system may shorten the period of the system, forming short period binary black hole (BBH) system, which is the most promising candidate for recently detected gravitational wave radiation sources. Also, there is an expectation that due to the stripped envelope of donor star by mass transfer, this system can play a significant role in the cosmic reionization by emitting more UV photons. However, this outcome highly depends on the initial properties of the system. We perform grid calculation on Pop III binary models with various initial primary masses (20 ~ 100 solar mass), initial separations, and initial mass ratios (q = 0.5 ~ 0.9). We find that 1) in most cases binary models show no increase in the number of ionizing photons and 2) formation of short period BBH system via mass transfer is highly unlikely.

• 천문학회지
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• 제37권1호
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• pp.55-59
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• 2004

We have developed a two fluid solar wind model from the Sun to 1 AU. Its basic equations are mass, momentum and energy conservations. In these equations, we include a wave mechanism of heating the corona and accelerating the wind. The two fluid model takes into account the power spectrum of Alfvenic wave fluctuation. Model computations have been made to fit observational constraints such as electron($T_e$) and proton($T_p$) temperatures and solar wind speed(V) at 1 AU. As a result, we obtained physical quantities of solar wind as follows: $T_e$ is $7.4{\times}10^5$ K and density(n) is $1.7 {\times}10^7\;cm^{-3}$ in the corona. At 1 AU $T_e$ is $2.1 {\times} 10^5$ K and n is $0.3 cm^{-3}$, and V is $511 km\;s^{-1}$. Our model well explains the heating of protons in the corona and the acceleration of the solar wind.