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- 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 37/119
361
- Kim, Yeon-Han
- Journal of the Korean astronomical society = 천문학회지
- 41, n.6
- pp.173-180
- 2008
- 원문 바로보기
Using the MHD coronal seismology technique, we estimated the magnetic field for three spicules observed in 2008 June. For this study, we used the high resolution Ca II H line (<TEX>$3968.5\;{\AA}$</TEX>) images observed by the Hinode SOT and considered a vertical thin flux tube as a spicule model. To our knowledge, this is the first attempt to estimate the spicule magnetic field using the Hinode observation. From the observed oscillation properties, we determined the periods, amplitudes, minimum wavelengths, and wave speeds. We interpreted the observed oscillations as MHD kink waves propagating through a vertical thin flux tube embedded in a uniform field environment. Then we estimated spicule magnetic field assuming spicule densities. Major results from this study are as follows : (1) we observed three oscillating spicules having durations of 5-7 minutes, oscillating periods of 2-3 minutes, and transverse displacements of 700-1000 km. (2) The estimated magnetic field in spicules is about 10-18 G for lower density limit and about 43-76 G for upper density limit. (3) In this analysis, we can estimate the minimum wavelength of the oscillations, such as 60000 km, 56000 km, and 45000 km. This may be due to the much longer wavelength comparing with the height of spicules. (4) In the first event occurred on 2008 June 03, the oscillation existed during limited time (about 250 s). This means that the oscillation may be triggered by an impulsive mechanism (like low atmospheric reconnection), not continuous. Being compared with the ground-based observations of spicule oscillations, our observation indicates quite different one, i.e., more than one order longer in wavelength, a factor of 3-4 larger in wave speed, and 2-3 times longer in period.
362
- Moon, Hong-Kyu
- Journal of the Korean astronomical society = 천문학회지
- 41, n.1
- pp.7-15
- 2008
- 원문 바로보기
We carried out a set of simulations to reproduce the performance of wide-field NEO surveys based on the revised population model of Near Earth Objects (NEOs) constructed by Morbidelli (2006). This is the first time where the new model is carefully compared with discovery statistics, and with the exception of population model, the simulation is identical to the procedure described in Moon et al. (2008). Our simulations show rather large discrepancy between the number of NEO discoveries made by the actual and the simulated surveys. First of all, unlike Bottke et al. (2002)'s, Morbidelli (2006)'s population model overestimates the number of NEOs. However, the latter reproduces orbit distributions of the actual population better. Our analysis suggests that both models significantly underestimate Amors, while overestimating the number of Apollos. Our simulation result implies that substantial modifications of both models are needed for more accurate reproduction of survey observations. We also identify Hungaria region (HU) to be one of the most convincing candidates that supply a large fraction of asteroids to the inner Solar System.
363
- Sung, Eon-Chang
- Journal of the Korean astronomical society = 천문학회지
- 41, n.5
- pp.121-137
- 2008
- 원문 바로보기
We present optical and near-infrared imaging and long-slit spectroscopy for the blue compact dwarf galaxy (BCD) Mrk 49 in the Virgo Cluster. The surface brightness distribution analysis shows that Mrk 49 consists of an off-centered blue bright compact core of r = 10' and a red faint outer exponential envelope. The <TEX>$H_{\alpha}$</TEX> image and color difference suggest that these two components have different stellar populations: a high surface brightness population of massive young stars and an underlying low surface brightness population of older stars. The redder near-infrared colors of the inner most region suggest that the near-infrared flux of Mrk 49 originates from evolved massive stars associated with the current star-forming activity. The total apparent magnitude is <TEX>$B_T\;=\;14.32$</TEX> mag and the mean effective surface brightness is <TEX>${\mu}_{eff}(B)\;=\;21.56$</TEX> mag <TEX>$arcsec^{-2}$</TEX>. Long-slit spectroscopy shows that Mrk 49 rotates apparently as a solid body within r = 10' in a plane at position angle 55 degrees with an amplitude of about <TEX>$20\;km\;sec^{-1}$</TEX>. The measured radial velocity of Mrk 49 was derived as <TEX>$1,535\;km\;sec^{-1}$</TEX>; and the total mass of stars and gases is in the range of 3 to <TEX>$6\;{\times}\;10^9\;M_{\odot}$</TEX>. The mass-to-light ratios for the central region of Mrk 49 in I and B band are estimated 1.0 and 0.5, respectively. The upper limit of the dark matter to visible matter ratio seems to be < 5. The oxygen abundance is <TEX>$12\;+\;\log(O/H)\;=\;8.21\;{\pm}\; 0.1$</TEX> which is about one quarter of the solar value while the relative helium abundance appears to be similar to that of the sun.
364
- Ran, In-Woo
- Journal of the Korean astronomical society = 천문학회지
- 41, n.3
- pp.59-64
- 2008
- 원문 바로보기
To detect exoplanets and study pulsation of K giant stars, we have observed precise RV (radial velocity) of about 55 early K giant (K0 - K4) stars brighter than V = 5 magnitude since 2003 by using BOES, a high resolution Echelle spectrograph attached to the 1.8 m telescope at BOAO (Bohyunsan Optical Astronomy Observatory). We detected periodic RV variation of KO III star <TEX>$\beta$</TEX> Gem (HD 62509) with a period <TEX>$P\;=\;596.6\;{\pm}\;2.3$</TEX> days and a semi-amplitude <TEX>$K\;=\;44.8\;{\pm}\;0.7\;ms^{-1}$</TEX>. If we adopt 1.7 <TEX>$M_{\odot}$</TEX> for the mass of <TEX>$\beta$</TEX> Gem, this yields the minimum mass of the companion m sin i = 2.64 <TEX>$M_{Jupiter}$</TEX>. Our results agree well with Hatzes et al. (2006) and Reffert et al. (2006), and confirm their discovery of a planetary object around <TEX>$\beta$</TEX> Gem. We also confirmed about 192 minutes short period stellar oscillation found by Hatzes and Zechmeister (2007). This is the first report of exoplanet detection using BOES and demonstrates that the RV observation using BOES is accurate and stable enough to detect exoplanets around bright K giant stars.
365
- Kim, Chul-Hee
- Journal of the Korean astronomical society = 천문학회지
- 41, n.2
- pp.17-22
- 2008
- 원문 바로보기
We present an analysis of the soft X-ray emission of MRK 841 to investigate its long-term variation. The light variation of MRK 841 for three different energy bands of soft, medium, and hard values were studied. The maximum variability with a factor of 5 for about two years was confirmed at all three different bands. The light curves exhibit a gradual variation of brightness. In addition to a gradual variation, the short- term or micro variation was also confirmed with a factor of about two for all three different bands. The light variation of each band did not exhibit a correlation between them, but the flare event is strongest in the soft band. The hardness ratio for hard and soft bands shows irregular variation but there was no correlation between them. It was confirmed that there is a gradual decrease of the photon index. Results of our analysis are discussed within the framework of the accretion disk phenomenon.
366
- Kim, Hyouk
- Journal of the Korean astronomical society = 천문학회지
- 41, n.2
- pp.23-37
- 2008
- 원문 바로보기
The high-resolution optical region spectroscopic data of the symbiotic nova AG Peg secured with the Hamilton Echelle Spectrograph at the Lick Observatory, have been analyzed along with the International Ultraviolet Explorer UV archive data. We measure about 700 line intensities in the wavelengths of 3859 to <TEX>$9230{\AA}$</TEX> and identify about 300 lines. We construct pure photoionization models that represent the observed lines and the physical condition for this symbiotic nova. The spectral energy distribution of the ionizing radiation is adopted from stellar model atmospheres. Based on photoionization models, we derive the elemental abundances; C & N appear to be similar to be smaller than the Galactic planetary nebular value while O is enhanced. Our result is compared with the Contini (1997, 2003) who analyzed the UV region spectral data with the shock + ionization model. The Fe abundance appears to be enhanced than that of normal planetary nebulae, which suggests that AG Peg may have formed in the Galactic disk. The models indicate that the temperature of the central star which excite the shell gas may have fluctuated to an unexpected extent during the years 1998 - 2002.
367
- Baek, Kyoung-Min
- Journal of the Korean astronomical society = 천문학회지
- 40, n.1
- pp.1-7
- 2007
- 원문 바로보기
About 10 percent of quasars are known to exhibit deep broad absorption troughs blueward of prominent permitted emission lines, which are usually attributed to the existence of outflows slightly above he accretion disk around the supermassive black hole. Typical widths up to 0.2c of these absorption roughs indicate the velocity scales in which special relativistic effects may not be negligible. Under he assumption of the ubiquity of the broad absorption line region in quasars, the broad emission line flux will exhibit Thomson scattered components from these fast outflows. In this paper, we provide our Monte Carlo calculation of linear polarization of singly Thomson scattered line radiation with the careful considerations of special relativistic effects. The scattering region is approximated by a collection of rings that are moving outward with speeds <TEX>${\upsilon}=c{\beta}<0.2c$</TEX> near the equatorial plane, and the scattered line photons are collected according to its direction and wavelength in the observer's rest frame. We find that the significantly extended red tail appears in the scattered radiation. We also find that the linear degree of polarization of singly Thomson scattered line radiation is wavelength-dependent and hat there are significant differences in the linear degree of polarization from that computed from classical physics in the far red tail. We propose that the semi-forbidden broad emission line C III]1909 may be significantly contributed from Thomson scattering because this line has small resonance scattering optical depth in the broad absorption line region, which leads to distinct and significant polarized flux in this broad emission line.
368
- Kurtz, Stan
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.137-140
- 2007
- 원문 바로보기
Triggered star formation is not a new idea - it's been around for at least the last 30 years. Although it has never disappeared from the scene, it seems to be enjoying something of a renaissance in recent years, in both observational and theoretical studies. Here we give a brief discussion of the background of triggered star formation, describe some of our own recent observational efforts in this regard, and briefly mention some initial conclusions that may be drawn.
369
- Migenes, Victor
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.131-132
- 2007
- 원문 바로보기
The research of OH maser emission sources with high angular resolution is complicated by the effects of interstellar scattering: more over, most of the OH maser sources are located in the galactic plane where the scattering is largest. However, the data available from pulsar studies on the spatial distribution of the amount of scattering indicate that there is a strong non-uniformity in the distribution of the amount of scattering material. There are directions in the galactic plane where the scattering is an order of magnitude higher than the average, as well as directions where the scattering is much lower. The latter provide an opportunity to investigate OH masers with the full angular resolution offered by very long baseline interferometry instruments, like the VLBA, and measure their true angular size, shape and brightness temperature. We have observed approximately 100 OH maser sources, distributed all over the northern hemisphere, with the VLBA in order to study the scattering properties of the interstellar medium.
370
- Lee, Young-Ung
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.133-135
- 2007
- 원문 바로보기
A survey project of TRAO with the fifteen beam array receiver system is presented. A multibeam array receiver system has been purchased from FCRAO, and is being installed on TRAO 14m telescope. The target region of the survey is from <TEX>${\iota}=120^{\circ}{\sim}137^{\circ},\;b=-1^{\circ}{\sim}+ 1^{\circ}$</TEX>, and velocity resolution would be 1 km/sec after smoothing from the original resolution of <TEX>$0.64km\;s^{-1}$</TEX> in the transition of J = 1-0 of <TEX>$^{13}CO$</TEX>. The survey region is a part of the <TEX>$^{12}CO$</TEX> Outer Galaxy Survey(OGS), and would be an extension of the Bell Laboratories <TEX>$^{13}CO$</TEX> Galactic Plane Survey. By combining with the existing <TEX>$^{12}CO$</TEX> database of the Outer Galaxy Survey, we will derive physical properties of identified molecular clouds and will conduct and statistical analysis of the Outer Galalxy molecular clouds. Reduction process and analysis methods will be introduced.