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1968년 ~ 2024년까지 1,211 건의 한국천문학회지를 격월간 확인하실 수 있습니다.
- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,211건 (DB Construction : 1,211 Articles)
총 게시글 1,211건
페이지 102/122
1011
- PARK SEOK JAE
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.71-75
- 1995
- 원문 바로보기
In the previous work we made a long term evolution code for the central black hole in an active galactic nucleus under the assumption that the Blandford-Znajek process is the source of the emission. Using our code we get the evolution of the angular velocity of the precession for a supermassive black hole. We consider a hole at the center of an axisymmetric, ellipsoidal galactic nucleus. Our numerical results show that, only for the cases such that the stellar density or the mass of the black hole is large enough, the precession of the black hole - presumably the precession of the galactic jet - is interestingly large.
1012
- Jung, Jae-Hun
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.109-117
- 1995
- 원문 바로보기
The morphology and strength of the primordial magnetic field which is generated spontaneously in the early universe are studied for three models: (1) inflation (2) primordial magnetized bubble and (3) primordial turbulence models, We calculate the power spectra of magnetic field that are scale-free and proportional to <TEX>$k^{1.5},k^{3{\sim}4}$</TEX> and <TEX>$k^{2/3}$</TEX>, respectively. The configurations of magnetic field having these power spectra are visualized. To constrain the present strength of the primordial magnetic field we calculate the anisotropy of the microwave background radiation in Bianchi type I universe with globally homogeneous magnetic field. From the COBE limit of the quadrupole moment of <TEX>$({\delta}T/T)_{l=2}$</TEX> the present strength of horizen-scale magnetic fields <TEX>$B_p$</TEX> is constrained to be less than <TEX>$9{\times}10^{-8}G$</TEX>.
1013
- LA DAILE
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.61-65
- 1995
- 원문 바로보기
We discuss a model4-dimensional Friedmann cosmology which may have evolved from a model of 4+D dimensions which admits spontaneous compactification of D dimensions (or N-dimensional variants of the Brans-Dicke (BD) theory). The BD parameter appearing in dimensional reduction is negative <TEX>$-1<\omega<0$</TEX> (for the N-dimensional variants of the BD theory, <TEX>$-1.5{\leq}{\omega})$</TEX>. We find that if there had been inflationary transtion to the standard big-bang model, the Universe can undergoe a polar-type expansion during when the gravitational coupling becomes negative. The unique feature is that for the negative w, the density parameter of the post-inflationary Universe falls in a range 0<0<1 even if the Universe is geometrically flat (k = 0).
1014
- SEON KWANG IL
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.45-53
- 1994
- 원문 바로보기
We present the calculation of X -ray spectra produced through Compton scattering of soft X-rays by hot electrons in the spherical shell geometry, using fully relativistic Monte Carlo simulation. With this model, we show that the power-law component, which has been observed in the low luminosity state of low-mass X-ray binaries (LMXBs), is explained physically. From a spectral. analysis, we find that spectral hardness is mainly due to the relative contribution of scattered component. In addition, we see that Wi en spectral features appear when the plasma is optically thick, especially in the high energy range, <TEX>$E{\gtrsim}100keV$</TEX>. We suggest that after a number of scattering the escape probability approaches an asymptotic form depending on the geometry of the scattering medium rather than on the initial photon spectrum.
1015
- MOON DAE-SIK
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.81-102
- 1994
- 원문 바로보기
We have decomposed the 11-cm radio continuum emission of the W51 complex into thermal and non-thermal components. The distribution of the thermal emission has been determined by analyzing HI, CO, and IRAS <TEX>$60-{\mu}m$</TEX> data. We have found a good correlation between the 11-cm thermal continuum and the 60- 11m emissions, which is used to obtain the thermal and non-thermal 11-cm continuum maps of the W51 complex. Most of the thermal continuum is emanating from the compact H II regions and their low-density ionized envelopes in W51A and W51B. All the H II regions, except G49.1-0.4 in W51B, have associated molecular clumps. The thermal radio continuum fluxes of the compact H II regions are proportional to the CO fluxes of molecular clumps. This is consistent with the previous results that the total mass of stars in an H II region is proportional to the mass of the associated molecular clump. According to our result, there are three non-thermal continuum sources in W51: G49.4-0.4 in W51A, a weak source close to G49.2-0.3 in W51B, and the shell source W51C. The non-thermal flux of G49.5-0.4 at 11-cm is <TEX>$\~28 Jy$</TEX>, which is <TEX>$\~25\%$</TEX> of its total 11-cm flux. The radio continuum spectrum between 0.15 and 300 GHz also suggests an excess emission over thermal free-free emission. We show that the excess emission can be described as a non-thermal emission with a spectral index <TEX>${\alpha}{\simeq}-1.0 (S_v{\propto}V^a)$</TEX> attenuated by thermal free-free absorptions at low-frequencies. The non-thermal source close to G49.2-0.3 is weak <TEX>$(\~9 Jy)$</TEX>. The nature of the source is not known and the reality of the non-thermal emission needs to be confirmed. The non~thermal shell source W51C has a 11-cm flux of <TEX>$\~130Jy$</TEX> and a spectral index <TEX>${\alpha}{\simeq}-0.26$</TEX>.
1016
- JUNG HEE
- Journal of the Korean astronomical society = 천문학회지
- 27, n.2
- pp.177-190
- 1994
- 원문 바로보기
We have examined the photometric evolution of elliptical galaxies, using stellar evolutionary models covering the wide ranges of metallicity and mass, and the different IMFs (simple IMP & time-dependent bimodal IMF). The model with a time-dependent bimodal IMF can reproduce the observed integrated magnitudes and colors at all wavelengths. The computed model shows that the star formation in elliptical galaxies is still going on, although the number of newly born stars is very small. The chemical evolutionary effect is clearly seen in the C-M diagram of computed elliptical galaxies.
1017
- MINH Y. C.
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.55-60
- 1994
- 원문 바로보기
The 3mm transitions to CO, <TEX>$^{13}CO,\;CS,\;HCO^+$</TEX>, and HCN have been observed toward the compact HII regions in W58 using the 14m Daeduk Radio Telescope (DRT). Some of the observed lines show high-velocity wings resulted from outflowing materials of the compact HII regions. We derive the beam averaged column densities of the observed species and compare their relative abundances. The <TEX>$HCO^+$</TEX> abundance appears to be smaller by about an order of magnitude than those of 'typical' quiet molecular clouds. CS may be a good reference molecule in comparing relative abundances in different physical conditions.
1018
- LEE YOUNGUNG
- Journal of the Korean astronomical society = 천문학회지
- 27, n.2
- pp.147-158
- 1994
- 원문 바로보기
We have obtained high angular resolution maps toward a molecular cloud associated with an HII region S287 and studied mainly kinematics of the cloud. The mapped region is 1.5 square degrees of the cloud in the transitions of <TEX>$^{12}CO\;and\;^{13}CO\;J=1-0$</TEX>. We have obtained a large range of mass, <TEX>$1.3\times10^4M_\bigodot$</TEX>, to <TEX>$7.2{\times}10^4M_{\bigodot}$</TEX> using three different techniques. The S287 molecular cloud shows a very disturbed feature: velocity field of the cloud is very complicated, and shows several arcs. It is likely that the southern part of cloud is being disrupted by the residing HII region S287 as well as external perturbing sources. In addition to an HII region, five bipolar outflows are also disturbing the molecular gas significantly. The large virial mass and the very disturbed morphology may reflect the fact that the cloud is not gravitationally bound system, as in the case of nearby giant molecular cloud (GMC) G216-2.5. The several arc structure and the filamentary features are possibly driven by external strong stellar winds, and these external perturbing sources may be driving the second generation of star-forming activities on the edges of the S287 molecular cloud.
1019
- KIM W. -T.
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.13-29
- 1994
- 원문 바로보기
We have constructed a 3-dim hydrodynamics code called BTSPH. The fluid dynamics part of the code is based on the smoothed particle hydrodynamics (SPH), and for its Poisson solver the binary tree (BT) scheme is employed. We let the smoothing length in the SPH algorithm vary with space and time, so that resolution of the calculation is considerably enhanced over the version of SPH with fixed smoothing length. The binary tree scheme calculates the gravitational force at a point by collecting the monopole forces from neighboring particles and the multipole forces from aggregates of distant particles. The BTSPH is free from geometric constraints, does not rely on grids, and needs arrays of moderate size. With the code we have run the following set of test calculations: one-dim shock tube, adiabatic collapse of an isothermal cloud, small oscillation of an equilibrium polytrope of index 3/2, and tidal encounter of the polytrope and a point mass perturber. Results of the tests confirmed the code performance.
1020
- LEE HYUN-A
- Journal of the Korean astronomical society = 천문학회지
- 27, n.2
- pp.103-117
- 1994
- 원문 바로보기
To extend the work of Gott, Park, and Lee (1989), statistical properties of gravitational lensing in a wide variety of cosmological models involving non-zero cosmological constant is investigated, using the redshifts of both lens and source and observed angular separation of images for gravitational lens systems. We assume singular isothermal sphere as lensing galaxy in homogenous and isotropic FriedmannLemaitre-Robertson- Walker universe, Schechter luminosity function, standard angular diameter distance formula and other galaxy parameters used in Fukugita and Turner (1991). To find the most adequate flat cosmological model and put a limit on the value of dimensionless cosmological constant <TEX>$\lambda_0$</TEX>, the mean value of the angular separation of images, probability distribution of angular separation and cumulative probability are calculated for given source and lens redshifts and compared with the observed values through several statistical methods. When there is no angular selection effect, models with highest value of <TEX>$\lambda_0$</TEX> is preferred generally. When the angular selection effects are considered, the preferred model depends on the shape of the selection functions and statistical methods; yet, models with large <TEX>$\lambda_0$</TEX> are preferred in general. However, the present data can not rule out any of the flat universe models with enough confidence. This approach can potentially select out best model. But at the moment, we need more data.