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1968년 ~ 2023년까지 1,187 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 : 1,187건 (DB Construction : 1,187 Articles)
총 게시글 1,187건
페이지 62/119
611
- Lee, Chang-Won
- Journal of the Korean astronomical society = 천문학회지
- 36, n.4
- pp.271-282
- 2003
- 원문 바로보기
We present results of a <TEX>$H^{13}CN$</TEX> J=1-0 mapping survey of molecular clouds toward the Galactic Center (GC) region of <TEX>$-1.6^{\circ}{\le}{\iota}{\le}2^{\circ}$</TEX> and <TEX>$-0.23^{\circ}{\le}b{\le}0.30^{\circ}$</TEX> with 2' grid resolution. The <TEX>$H^{13}CN$</TEX> emissions show similar distribution and velocity structures to those of the <TEX>$H^{12}CN$</TEX> emissions, but are found to better trace the feature saturated with <TEX>$H^{12}CN$</TEX> (1-0). The bright components among multi-components of <TEX>$H^{12}CN$</TEX> line profiles usually appear in the <TEX>$H^{13}CN$</TEX> line while most of the dynamically forbidden, weak <TEX>$H^{12}CN$</TEX> components are seldom detected in the <TEX>$H^{13}CN$</TEX> line. We also present results of other complementary observations in <TEX>$^{12}CO$</TEX> (J=1-0) and <TEX>$^{13}CO$</TEX> (J=1-0) lines to estimate physical quantities of the GC clouds, such as fractional abundance of HCN isotopes and mass of the GC cloud complexes. We confirm that the GC has very rich chemistry. The overall fractional abundance of <TEX>$H^{12}CN$</TEX> and <TEX>$H^{13}CN$</TEX> relative to <TEX>$H_2$</TEX> in the GC region is found to be significantly higher than those of any other regions, such as star forming region and dark cloud. Especially cloud complexes nearer to the GC tend to have various higher abundance of HCN. Total mass of the HCN molecular clouds within <TEX>$[{\iota}]{\le}6^{\circ}$</TEX> is estimated to be <TEX>${\~}2 {\times}10^7\;M_{\bigodot}$</TEX> using the abundances of HCN isotopes, which is fairly consistent with previous other estimates. Masses of four main complexes in the GC range from a few <TEX>$10^5$</TEX> to <TEX>${\~}10^7\;M_{\bigodot}$</TEX> All the HCN spectra with multi-components for the four main cloud complexes were investigated to compare the line widths of the complexes. The largest mode (45 km <TEX>$s^{-1}$</TEX>) of the FWHM distributions among the complexes is in the Clump 2. The value of the mode tends to be smaller at the farther complexes from the GC.
612
- KIM SAM
- Journal of the Korean astronomical society = 천문학회지
- 36, n.1
- pp.21-31
- 2003
- 원문 바로보기
ASTRO-F /FIS will carry out all sky survey in the wavelength from 50 to 200 <TEX>${\mu}m$</TEX>. At far infrared, stars and galaxies may not be good calibration sources because the IR fluxes could be sensitive to the dust shell of stars and star formation activities of galaxies. On the other hand, asteroids could be good calibration sources at far infrared because of rather simple spectral energy distribution. Recent progresses in thermal models for asteroids enable us to calculate the far infrared flux fairly accurately. We have derived the Bond albedos and diameters for 559 asteroids based on the IRAS and ground based optical data. Using these thermal parameters and standard thermal model, we have calculated the spectral energy distributions of asteroids from 10 to 200 <TEX>${\mu}m$</TEX>. We have found that more than <TEX>$70\%$</TEX> of our sample asteroids have flux errors less than <TEX>$10\%$</TEX> within the context of the best fitting thermal models. In order to assess flux uncertainties due to model parameters, we have computed SEDs by varing external parameters such as emissivity, beaming parameter and phase integral. We have found that about 100 asteroids can be modeled to be better than <TEX>$5.8\%$</TEX> of flux uncertainties. The systematic effects due to uncertainties in phase integral are not so important.
613
- CHAE JONGCHUL
- Journal of the Korean astronomical society = 천문학회지
- 36, n.1
- pp.33-41
- 2003
- 원문 바로보기
Recent observations have shown that coronal magnetic fields in the northern (southern) hemisphere tend to have negative (positive) magnetic helicity. There has been controversy as to whether this hemispheric pattern is of surface or sub-surface origin. A number of studies have focused on clarifying the effect of the surface differential rotation on the change of magnetic helicity in the corona. Meanwhile, recent observational studies reported the existence of transient shear flows in active regions that can feed magnetic helicity to the corona at a much higher rate than the differential rotation does. Here we propose that such transient shear flows may be driven by the torque produced by either the axial or radial expansion of the coronal segment of a twisted flux tube that is rooted deeply below the surface. We have derived a simple relation between the coronal expansion parameter and the amount of helicity transferred via shear flows. To demonstrate our proposition, we have inspected Yohkoh soft X-ray images of NOAA 8668 in which strong shear flows were observed. As a result, we found that the expansion of magnetic fields really took place in the corona while transient shear flows were observed in the photosphere, and the amount of magnetic helicity change due to the transient shear flows is quantitatively consistent with the observed expansion of coronal magnetic fields. The transient shear flows hence may be understood as an observable manifestation of the pumping of magnetic helicity out of the interior portions of the field lines driven by the expansion of coronal parts as was originally proposed by Parker (1974).
614
- Ann, Hong-Bae
- Journal of the Korean astronomical society = 천문학회지
- 36, n.4
- pp.261-270
- 2003
- 원문 바로보기
We have conducted a V-band CCD surface photometry of 68 disk galaxies to analyze the bulge morphology of nearby spirals. We classify bulges into four types according to their ellipticities and the misalignments between the major axis of the bulge and those of the disk and the bar: spherical, oblate, pseudo triaxial, and triaxial. We found that one third of the bulges are triaxial and they are preponderant in barred galaxies. The presence of the triaxial bulges in a significant fraction of unbarred galaxies as well as in barred galaxies might support the secular evolution hypothesis which postulates that the bar driven mass inflow leads to the formation of triaxial bulges and the destruction of bars when sufficient mass is accumulated in the central regions.
615
- TANIGUCHI YOSHIAKI
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.123-144
- 2003
- 원문 바로보기
The 8m class telescopes in the ground-based optical astronomy together with help from the ultra-sharp eye of the Hubble Space Telescope have enabled us to observe forming galaxies beyond red shift z = 5. In particular, more than twenty Ly<TEX>$\alpha$</TEX>-emitting galaxies have already been found at z > 5. These findings provide us with useful hints to investigate how galaxies formed and then evolved in the early universe. Further, detailed analysis of Ly<TEX>$\alpha$</TEX> emission line profiles are useful in exploring the nature of the intergalactic medium because the trailing edge of cosmic reionization could be close to z <TEX>$\~$</TEX> 6 -7, at which forming galaxies have been found recently. We also discuss the importance of superwinds from forming galaxies at high redshift, which has an intimate relationship between galaxies and the intergalactic medium. We then give a review of early cosmic star formation history based on recent progress in searching for Ly<TEX>$\alpha$</TEX>-emitting young galaxies beyond red shift 5.
616
- AHN KYUNGJIN
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.89-95
- 2003
- 원문 바로보기
Observations of dark matter dominated dwarf and low surface brightness disk galaxies favor density profiles with a flat-density core, while cold dark matter (CDM) N-body simulations form halos with central cusps, instead. This apparent discrepancy has motivated a re-examination of the microscopic nature of the dark matter in order to explain the observed halo profiles, including the suggestion that CDM has a non-gravitational self-interaction. We study the formation and evolution of self-interacting dark matter (SIDM) halos. We find analytical, fully cosmological similarity solutions for their dynamics, which take proper account of the collisional interaction of SIDM particles, based on a fluid approximation derived from the Boltzmann equation. The SIDM particles scatter each other elastically, which results in an effective thermal conductivity that heats the halo core and flattens its density profile. These similarity solutions are relevant to galactic and cluster halo formation in the CDM model. We assume that the local density maximum which serves as the progenitor of the halo has an initial mass profile <TEX>${\delta}M / M {\propto} M^{-{\epsilon}$</TEX>, as in the familiar secondary infall model. If <TEX>$\epsilon$</TEX> = 1/6, SIDM halos will evolve self-similarly, with a cold, supersonic infall which is terminated by a strong accretion shock. Different solutions arise for different values of the dimensionless collisionality parameter, <TEX>$Q {\equiv}{\sigma}p_br_s$</TEX>, where <TEX>$\sigma$</TEX> is the SIDM particle scattering cross section per unit mass, <TEX>$p_b$</TEX> is the cosmic mean density, and <TEX>$r_s$</TEX> is the shock radius. For all these solutions, a flat-density, isothermal core is present which grows in size as a fixed fraction of <TEX>$r_s$</TEX>. We find two different regimes for these solutions: 1) for <TEX>$Q < Q_{th}({\simeq} 7.35{\times} 10^{-4}$</TEX>), the core density decreases and core size increases as Q increases; 2) for <TEX>$Q > Q_{th}$</TEX>, the core density increases and core size decreases as Q increases. Our similarity solutions are in good agreement with previous results of N-body simulation of SIDM halos, which correspond to the low-Q regime, for which SIDM halo profiles match the observed galactic rotation curves if <TEX>$Q {\~} [8.4 {\times}10^{-4} - 4.9 {\times} 10^{-2}]Q_{th}$</TEX>, or <TEX>${\sigma}{\~} [0.56 - 5.6] cm^2g{-1}$</TEX>. These similarity solutions also show that, as <TEX>$Q {\to}{\infty}$</TEX>, the central density acquires a singular profile, in agreement with some earlier simulation results which approximated the effects of SIDM collisionality by considering an ordinary fluid without conductivity, i.e. the limit of mean free path <TEX>${\lambda}_{mfp}{\to} 0$</TEX>. The intermediate regime where <TEX>$Q {\~} [18.6 - 231]Q_{th}$</TEX> or <TEX>${\sigma}{\~} [1.2{\times}10^4 - 2.7{\times}10^4] cm^2g{-1}$</TEX>, for which we find flat-density cores comparable to those of the low-Q solutions preferred to make SIDM halos match halo observations, has not previously been identified. Further study of this regime is warranted.
617
- CHAE JONGCHUL
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.13-20
- 2003
- 원문 바로보기
Observations have indicated that magnetic reconnect ion may occur frequently in the photosphere and chromosphere as well as in the solar corona. The observed features include cancelling magnetic features seen in photospheric magnetograms, and different kinds of small-scale activities such as UV explosive events and EUV jets. By integrating the observed parameters of these features with the Sweet-Parker reconnect ion theory, an attempt is made to clarify the nature of chromospheric magnetic reconnection. Our results suggest that magnetic reconnect ion may be occurring at many different levels of the photosphere and chromosphere without a preferred height and at a faster speed than is predicted by the Sweet-Parker reconnect ion model using the classical value of electric conductivity. Introducing an anomalous magnetic diffusivity 10-100 times the classical value is one of the possible ways of explaining the fast reconnect ion as inferred from observations.
618
- SAKURAI T.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.7-12
- 2003
- 원문 바로보기
We have studied the magnetic helicity of active regions by using the data from (1) the photo-electric magnetograph of the Okayama Observatory (1983-1995) and (2) the video magnetograph of NAOJ/Mitaka (1992-2000). The latitude distribution of helicity showed a tendency that the regions in the north (south) hemisphere have negative (positive) helicities, respectively, which is already known as the hemispheric sign rule. If we look into the sign of helicity as a function of time, the sign rule was less definite or was reversed sometimes in the sunspot minimum phase. We also studied the relation between the magnetic helicity and the sunspot tilt angles, and found that these two quantities are positively correlated, which is opposite to the expectation of a theoretical model. The implications of this cycle-phase dependence of helicity signs and the correlation between magnetic he Ii city and sunspot tilt angles are discussed.
619
- FLETCHER ANDRE B.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.177-187
- 2003
- 원문 바로보기
Active galactic nuclei (AGNs) are distant, powerful sources of radiation over the entire electromagnetic spectrum, from radio waves to gamma-rays. There is much evidence that they are driven by gravitational accretion of stars, dust, and gas, onto central massive black holes (MBHs) imprisoning anywhere from <TEX>$\~$</TEX>1 to <TEX>$\~$</TEX>10,000 million solar masses; such objects may naturally form in the centers of galaxies during their normal dynamical evolution. A small fraction of AGNs, of the radio-loud type (RLAGNs), are somehow able to generate powerful synchrotron-emitting structures (cores, jets, lobes) with sizes ranging from pc to Mpc. A brief summary of AGN observations and theories is given, with an emphasis on RLAGNs. Preliminary results from the imaging of 10000 extragalactic radio sources observed in the MITVLA snapshot survey, and from a new analytic theory of the time-variable power output from Kerr black hole magnetospheres, are presented. To better understand the complex physical processes within the central engines of AGNs, it is important to confront the observations with theories, from the viewpoint of analyzing the time-variable behaviours of AGNs - which have been recorded over both 'short' human (<TEX>$10^0-10^9\;s$</TEX>) and 'long' cosmic (<TEX>$10^{13} - 10^{17}\;s$</TEX>) timescales. Some key ingredients of a basic mathematical formalism are outlined, which may help in building detailed Monte-Carlo models of evolving AGN populations; such numerical calculations should be potentially important tools for useful interpretation of the large amounts of statistical data now publicly available for both AGNs and RLAGNs.
620
- JANG M.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.145-149
- 2003
- 원문 바로보기
LIST is the Lyman-<TEX>$\alpha$</TEX> Imaging Solar Telescope, a project funded by the Korean government to fly on the second Korean Science and Technology research Satellite (STSat-2) due to launch in December 2005. The Principal Investigator is Dr. Minhwan Jang of Kyung-Hee University and of the Space Payload Research Center (SPARC), a consortium of Korean universities and institutions formed to develop scientific research projects in space. The purpose of the LIST project is to design, build, and operate an instrument on STSat-2 which will make images of the Sun from Earth orbit at the wavelength of the Hydrogen Lyman-a emission line at 121.6 nm. LIST has a simple design concept comprised of a small telescope to image the full disk of the Sun onto a CCD detector and a set of filters to isolate the 121.6 nm wavelength.