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- 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 39/122
381
- 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.
382
- 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.
383
- 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.
384
- Ahn, Kwang-Su
- Journal of the Korean astronomical society = 천문학회지
- 41, n.2
- pp.39-47
- 2008
- 원문 바로보기
Spectral line profiles of filaments/prominences to be observed by the Fast Imaging Solar Spectrograph (FISS) are studied. The main spectral lines of interests are <TEX>$H{\alpha}$</TEX>, Ca II 8542, and Ca II K. FISS has a high spectral resolving power of <TEX>$2{\times}10^5$</TEX>, and supports simultaneous dual-band recording. This instrument will be installed at the 1.6m New Solar Telescope (NST) of Big Bear Solar Observatory, which has a high spatial resolution of 0.065' at 500nm. Adopting the cloud model of radiative transfer and using the model parameters inferred from pre-existing observations, we have simulated a set of spectral profiles of the lines that are emitted by a filament on the disk or a prominence at the limb. Taking into account the parameters of the instrument, we have estimated the photon count to be recorded by the CCD cameras, the signal-to-noise ratios, and so on. We have also found that FISS is suitable for the study of multi-velocity threads in filaments if the spectral profiles of Ca II lines are recorded together with <TEX>$H{\alpha}$</TEX> lines.
385
- Minh, Y.C.
- Journal of the Korean astronomical society = 천문학회지
- 41, n.5
- pp.139-145
- 2008
- 원문 바로보기
The molecular cloud, embedding AFGL 2591, has a 'head-and-tail' structure with a total mass of <TEX>${\sim}\;1800\;M_{\odot}$</TEX>, about half of the mass (<TEX>${\sim}\;900\;M_{\odot}$</TEX>) in the head (size <TEX>${\sim}\;1.2\;pc$</TEX> in diameter), and another half in the envelope (<TEX>${\sim}\;3.5\;pc$</TEX> in the east-west direction). We found a new cloud in the direction toward north-east from AFGL 2591 (projected distance <TEX>${\sim}\;2.4\;pc$</TEX>), which is probably associated with the AFGL 2591 cloud. The <TEX>$^{12}CO$</TEX> spectrum clearly shows a blue-shifted high-velocity wing at around the velocity <TEX>$-20\;{\sim}\;-10\;km\;s^{-1}$</TEX>, but it is not clear whether this high-velocity component has a bipolar nature in our observations. The observed CN spectra also show blue-shifted wing component but the existence of the red-shifted component is not clear, either. In some CN and HCN spectra, the highvelocity components appear as a different velocity component, not a broad line-wing component. The dense cores, traced by CN and HCN, exist in the 'head' of the AFGL 2591 cloud with an elongated morphology roughly in the north-south direction with a size of about 0.5 pc. The abundance ratio between CN and HCN is found to be about 2 - 3 within the observed region, which may suggest a possibility that this core is being affected by the embedded YSOs or by possible shocks from outside.
386
- Takeda, Yoichi
- Journal of the Korean astronomical society = 천문학회지
- 41, n.4
- pp.83-98
- 2008
- 원문 바로보기
In an attempt of clarifying the connection between the photospheric abundance anomalies and the stellar rotation as well as of exploring the nature of 'normal A' stars, the abundances of seven elements (C, O, Si, Ca, Ti, Fe, and Ba) and the projected rotational velocity for 46 A-type field stars were determined by applying the spectrum-fitting method to the high-dispersion spectral data obtained with BOES at BOAO. We found that the peculiarities(underabundances of C, O, and Ca; an overabundance of Ba) seen in slow rotators efficiently decrease with an increase of rotation, which almost disappear at <TEX>$v_esin\;i{\gtrsim}100km\;s^{-1}$</TEX>. This further suggests that stars with sufficiently large rotational velocity may retain the original composition at the surface without being altered. Considering the subsolar tendency(by several tenths dex below) exhibited by the elemental abundances of such rapidly-rotating (supposedly normal) A stars, we suspect that the gas metallicity may have decreased since our Sun was born, contrary to the common picture of galactic chemical evolution.
387
- 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.
388
- 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.
389
- Sim, Chae-Kyung
- Journal of the Korean astronomical society = 천문학회지
- 41, n.3
- pp.65-76
- 2008
- 원문 바로보기
We have investigated the optical properties of the global haze on Titan from spectra recorded between 7100 and <TEX>$9200{\AA}$</TEX>, where <TEX>$CH_4$</TEX> absorption bands of various intensities occur. The Titan spectra were obtained on Feb. 23, 2005 (UT), near the times of the Cassini T3 flyby and Huygens probe, using an optical echelle spectrograph (BOES) on the 1.8-m telescope at Bohyunsan Observatory in Korea. In order to derive the optical properties of the haze as a function of altitude, we developed an inversion radiative-transfer program using an atmospheric model of Titan and laboratory <TEX>$CH_4$</TEX> absorption coefficients available from the literature. The derived extinction coefficients of the haze increase toward the surface, and the coefficients at shorter wavelengths are greater than those at longer wavelengths for the 30 - 120 km altitude range, indicating that the Titanian haze becomes optically thin toward the longer wavelength range. Total optical depths of the haze are estimated to be 1.4 and 1.2 for the 7270 - <TEX>$7360{\AA}$</TEX> and 8940 - <TEX>$9150{\AA}$</TEX> ranges, respectively. Based on the Huygens/DISR data set, Tomasko et al. (2005) reported total optical depths of 2.5 - 3.5 at <TEX>$8290{\AA}$</TEX>, depending on the assumed fractal aggregate particle model. The total optical depths based on our results are smaller than those of Tomasko et al., but they partially overlap with their results if we consider a large uncertainty from possible variations of the <TEX>$CH_4$</TEX> mixing ratio over Titan's disk. We also derived the single scattering albedo of the haze particles as a function of altitude: it is less than 0.5 at altitudes higher than <TEX>${\sim}150\;km$</TEX>, and approaches 1.0 toward the surface. This behavior suggests that, at altitudes above <TEX>${\sim}150\;km$</TEX>, the average particle radius is smaller than the wavelengths, whereas near the surface, it becomes comparable or greater.
390
- Seon, Kwang-Il
- Journal of the Korean astronomical society = 천문학회지
- 41, n.4
- pp.99-107
- 2008
- 원문 바로보기
When a new counting experiment is proposed, it is crucial to predict whether the desired source signal will be detected, or how much observation time is required in order to detect the signal at a certain significance level. The concept of the a priori prediction of the detection limit in a newly proposed experiment should be distinguished from the a posteriori claim or decision whether a source signal was detected in an experiment already performed, and the calculation of statistical significance of a measured source signal. We formulate precise definitions of these concepts based on the statistical theory of hypothesis testing, and derive an approximate formula to estimate quickly the a priori detection limit of expected Poissonian source signals. A more accurate algorithm for calculating the detection limits in a counting experiment is also proposed. The formula and the proposed algorithm may be used for the estimation of required integration or observation time in proposals of new experiments. Applications include the calculation of integration time required for the detection of faint emission lines in a newly proposed spectroscopic observation, and the detection of faint sources in a new imaging observation. We apply the results to the calculation of observation time required to claim the detection of the surface thermal emission from neutron stars with two virtual instruments.