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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
- 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.
382
- Choi, Youn-Su
- Journal of the Korean astronomical society = 천문학회지
- 41, n.6
- pp.163-172
- 2008
- 원문 바로보기
Using the spectral data in the 3700 to <TEX>$10050{\AA}$</TEX> wavelength range secured with the Hamilton Echelle Spectrograph (HES) at the Lick observatory, we have investigated the expansion velocities and the physical conditions of the elliptical planetary nebula NGC 6803. Various forbidden and permitted lines, e.g. HI, HeI, HeII, [OIII], [NII], [ArIII], and [SII], indicate complicated but systematic physical conditions variation: electron temperatures <TEX>$T_{\varepsilon}\;{\sim}\;9000$</TEX> - 11000 K and electron number densities <TEX>$N_{\varepsilon}\;{\sim}\;2000$</TEX> - <TEX>$9000\;cm^{-3}$</TEX>. The line profile analysis of these ions also indicates the systematic change or the acceleration of the expansion velocities in the range of 10 - <TEX>$22\;km\;s^{-1}$</TEX>. We show that the velocity gradient and physical condition found in various ions are closely related to the prolate ellipsoidal structure of NGC 6803. The expansion velocity and the ionic abundance of <TEX>$O^{2+}$</TEX> were derived based on the OII and [OIII] lines. In spite of the discrepancy of ionic abundances derived by the two cases and their line profiles, the expansion velocities of them agree well. We find that the ratios of the red to blue line component of the HeII & OII lines are different from those of the [OIII] or other forbidden lines that indicates a possible involvement of emission of HeII & OII lines. This subtle difference and the different physical condition of the lines are likely to be caused by the elongated geometry and the latitude dependence of the emission zone.
383
- Kyeong, Jae-Mann
- Journal of the Korean astronomical society = 천문학회지
- 41, n.6
- pp.147-155
- 2008
- 원문 바로보기
We present UBV I photometry of the old open cluster NGC 1193. Color-magnitude diagrams (CMDs) of this cluster show a well defined main sequence and a sparse red giant branch. For the inner region of r < 50', three blue straggler candidates are newly found in addition to the objects Kaluzny (1988) already found. The color-color diagrams show that the reddening value toward NGC 1193 is E(B - V ) = <TEX>$0.19{\pm}0.04$</TEX>. From the ultraviolet excess measurement, we derived the metallicity to be [Fe/H]= <TEX>$-0.45{\pm}0.12$</TEX>. A distance modulus of <TEX>${(m\;-\;M)}_0$</TEX> = <TEX>$13.3{\pm}0.15$</TEX> is obtained from zero age main sequence fitting with the empirically calibrated Hyades isochrone of Pinsonneault et al. (2004). CMD comparison with the Padova isochrones by Bertelli et al. (1994) gives an age of log t = <TEX>$9.7{\pm}0.1$</TEX>.
384
- 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.
385
- Lee, Young-Ung
- Journal of the Korean astronomical society = 천문학회지
- 41, n.6
- pp.157-161
- 2008
- 원문 바로보기
We have estimated the fractal dimension of the molecular clouds associated with the Hii region Sh 156 in the Outer Galaxy. We selected the <TEX>$^{12}CO$</TEX> cube data from the FCRAO CO Survey of the Outer Galaxy. Using a developed code within IRAF, we identified slice-clouds (2-dimensional clouds in velocity-channel maps) with two threshold temperatures to estimate the fractal dimension. With the threshold temperatures of 1.8 K, and 3 K, we identified 317 slice-clouds and 217 slice-clouds, respectively. There seems to be a turn-over location in fractional dimension slope around NP (area; number of pixel) = 40. The fractal dimensions was estimated to be D = <TEX>$1.5\;{\sim}\;1.53$</TEX> for <TEX>$NP\;{\geq}\;40$</TEX>, where <TEX>$P\;{\propto}\;A^{D/2}$</TEX> (P is perimeter and A is area), which is slightly larger than other results. The sampling rate (spatial resolution) of observed data must be an important parameter when estimating fractal dimension. Fractal dimension is apparently invariant when varying the threshold temperatures applied to slice-clouds identification.
386
- 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.
387
- 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.
388
- 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.
389
- 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.
390
- 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.