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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 68/122
671
- PARK BYEONG-GON
- Journal of the Korean astronomical society = 천문학회지
- 35, n.4
- pp.197-208
- 2002
- 원문 바로보기
Five contemporary pre-main sequence (PMS) evolution model grids are compared with the photo-metric data for a nearly complete sample of low-mass members in NGC 2264. From amongst the grids compared, the models of Baraffe et al. (1998) prove to be the most reliable in mass-age distribution. To overcome the limited mass range of the models of Baraffe et al. we derived a simple transformation relation between the mass of a PMS star from Swenson et al. (1994) and that from Baraffe et al., and applied it to the PMS stars in NGC 2264 and the Orion nebula cluster (ONC). The resulting initial mass function (IMF) of the ONC shows that the previous interpretation of the IMF is not a real feature, but an artifact caused by the evolution models adopted. The IMFs of both clusters are in a good agreement with the IMF of the field stars in the solar neighborhood. This result supports the idea proposed by Lada, Strom, & Myers (1993) that the field stars originate from the stars that are formed in clusters and spread out as a result of dynamical dissociation. Nevertheless, the IMFs of OB associations and young open clusters show diverse behavior. For the low-mass regime, the current observations suffer from difficulties in membership assignment and sample incompleteness. From this, we conclude that a more thorough study of young open clusters is necessary in order to make any definite conclusions on the existence of a universal IMF.
672
- LEE SUNGHO
- Journal of the Korean astronomical society = 천문학회지
- 35, n.3
- pp.111-121
- 2002
- 원문 바로보기
We have measured the correlation functions of the optically selected clusters of galaxies in the Abell and the APM catalogs, and of the X-ray clusters in the X-ray-Brightest Abell-type Clusters of galaxies (XBACs) catalog and the Brightest Clusters Sample (BCS). The same analysis method and the same method of characterizing the resulting correlation functions are applied to all observational samples. We have found that the amplitude of the correlation function of the APM clusters is much higher than what has been previously claimed, in particular for richer subsamples. The correlation length of the APM clusters with the richness R <TEX>$\ge$</TEX> 70 (as defined by the APM team) is found to be <TEX>$r_0 = 25.4_{-3.0}^{+3.1}\;h^{-1}$</TEX> Mpc. The amplitude of correlation function is about 2.4 times higher than that of Croft et al. (1997). The correlation lengths of the Abell clusters with the richness class RC <TEX>$\ge$</TEX> 0 and 1 are measured to be <TEX>$r_0 = 17.4_{-1.1}^{+1.2}$</TEX> and <TEX>$21.0_{-2.8}^{+2.8}\;h^{-1}$</TEX> Mpc, respectively, which is consistent with our results for the APM sample at the similar level of richness. The richness dependence of cluster correlations is found to be <TEX>$r_0= 0.40d_c + 3.2$</TEX> where <TEX>$d_c$</TEX> is the mean intercluster separation. This is identical in slope with the Bahcall & West (1992)'s estimate, but is inconsistent with the weak dependence of Croft et al. (1997). The X-ray bright Abell clusters in the XBACs catalog and the X-ray selected clusters in the BCS catalog show strong clustering. The correlation length of the XBACs clusters with <TEX>$L_x {\ge}0.65{\times} 10^{44}\;h^{-2}erg\;s^{-1}$</TEX> is <TEX>$30.3_{-6.5}^{+8.2}\;h^{-1}$</TEX> Mpc, and that of the BCS clusters with <TEX>$L_x {\ge}0.70{\times} 10^{44}\;h^{-2}erg\;s^{-1}$</TEX> is <TEX>$30.2_{-8.9}^{+9.8}\;h^{-1}$</TEX> Mpc. The clustering strength of the X-ray clusters is much weaker than what is expected from the optical clusters.
673
- ANN HONG BAE
- Journal of the Korean astronomical society = 천문학회지
- 35, n.1
- pp.29-34
- 2002
- 원문 바로보기
We have conducted UBVI CCD photometry of an intermediate-age open cluster NGC 559 to investigate the effect of dynamical evolution on the stellar distributions in NGC 559. Our photometry allows better estimates of distance and age of the cluster owing to much deeper photometry (V <TEX>$\le$</TEX> 21) than previous ones. It is found that the luminosity function and mass function as well as the spatial stellar distributions are affected by the dynamical evolution. Mass segregation leads to the central concentration of the high mass stars, which results in the flattened mass function inside the half mass radius.
674
A NEW CHANNEL TO SEARCH FOR EXTRA-SOLAR SYSTEMS WITH MULTIPLE PLANETS VIA GRAVITATIONAL MICROLENSING
- HAN CHEONGHO
- Journal of the Korean astronomical society = 천문학회지
- 35, n.1
- pp.35-40
- 2002
- 원문 바로보기
Gaudi, Naber & Sackett pointed out that if an event is caused by a lens system containing more than two planets, all planets will affect the central region of the magnification pattern, and thus the existence of the multiple planets can be inferred by detecting additionally deformed anomalies from intensive monitoring of high magnification microlensing events. Unfortunately, this method has important limitations in identifying the existence of multiple planets and determining their parameters (the mass ratio and the instantaneous projected separation) due to the degeneracy of the resulting light curve anomalies from those induced by a single planet and the complexity of multiple planet lensing models. In this paper, we propose a new channel to search for multiple planets via microlensing. The method is based on the fact that the lensing light curve anomalies induced by multiple planets are well approximated by the superposition of those of the single planet systems where the individual planet-primary pairs act as independent lens systems. Then, if the source trajectory passes both of the outer deviation regions induced by the individual planets, one can unambiguously identify the existence of the multiple planets. We illustrate that the probability of successively detecting light curve anomalies induced by two Jovian-mass planets located in the lensing zone through this channel will be substantial. Since the individual anomalies can be well described by much simpler single planet lensing models, the proposed method has an important advantage of allowing one to accurately determine the parameters of the individual planets.
675
- YANG YUJIN
- Journal of the Korean astronomical society = 천문학회지
- 35, n.3
- pp.131-141
- 2002
- 원문 바로보기
We present Near-IR photometry of the Arches cluster, a young and massive stellar cluster near the Galactic center. We have analyzed the high resolution (FWHM <TEX>$\~$</TEX> 0.2') Hand K' band images in the Galactic Center Demonstration Science Data Set, which were obtained with the Gemini/Hokupa's adaptive optics (AO) system. We present the color-magnitude diagram, the luminosity function and the initial mass function (IMF) of the stars in the Arches cluster in comparison with the HST/NICMOS data. The IMF slope for the range of 1.0 < log (M/M<TEX>$\bigodot$</TEX>) < 2.1 is estimated to be <TEX>${\Gamma} = -0.79 {\pm} 0.16$</TEX>, in good agreements with the earlier result based on the HST/NICMOS data [Figer et al. 1999, ApJ, 525, 750]. These results strengthen the evidence that the IMF of the bright. stars close to the Galactic center is much flatter than that for the solar neighborhood. This is also consistent with a recent finding that the IMFs of the bright stars in young clusters in M33 get flatter as the galactocentric distance decreases [Lee et al. 2001, astro-ph 0109258]. It is found that the power of the Gemini/ AO system is comparable, with some limits, to that of the HST/NICMOS.
676
- CHAE JONGCHUL
- Journal of the Korean astronomical society = 천문학회지
- 35, n.1
- pp.59-65
- 2002
- 원문 바로보기
Solar observations support that magnetic reconnect ion ubiquitously occurs in the chromosphere as well as in the corona. It is now widely accepted that coronal magnetic reconnect ion is fast reconnect ion of the Petschek type, and is the main driver of solar flares. On the other hand, it has been thought that the traditional Sweet-Parker model may describe chromospheric reconnect ion without difficulty, since the electric conductivity in the chromoshphere is much lower than that in the corona. However, recent observations of cancelling magnetic features have suggested that chromospheric reconnect ion might proceed at a faster rate than the Sweet-Parker model predicts. We have applied the Sweet-Parker model and Petschek model to a well-observed cancelling magnetic feature. As a result, we found that the inflow speed of the Sweet-Parker reconnect ion is too small to explain the observed converging speed of the feature. On the other hand, the inflow speeds and outflow speeds of the Petschek reconnect ion are well compatible with observations. Moreover, we found that the Sweet-Parker type current sheet is subject to the ion-acoustic instability in the chromosphere, implying the Petschek mechanism may operate there. Our results strongly suggest that chromospheric reconnect ion is of the Petschek type.
677
- HONG S. S.
- Journal of the Korean astronomical society = 천문학회지
- 35, n.1
- pp.41-57
- 2002
- 원문 바로보기
We have written a code called QDM_sca, which numerically solves the problem of radiative transfer in an anisotropically scattering, spherical atmosphere. First we formulate the problem as a second order differential equation of a quasi-diffusion type. We then apply a three-point finite differencing to the resulting differential equation and transform it to a tri-diagonal system of simultaneous linear equations. After boundary conditions are implemented in the tri-diagonal system, the QDM_sca radiative code fixes the field of specific intensity at every point in the atmosphere. As an application example, we used the code to calculate the brightness of atmospheric diffuse light(ADL) as a function of zenith distance, which plays a pivotal role in reducing the zodiacal light brightness from night sky observations. On the basis of this ADL calculation, frequent uses of effective extinction optical depth have been fully justified in correcting the atmospheric extinction for such extended sources as zodiacal light, integrated starlight and diffuse galactic light. The code will be available on request.
678
- LEE CHANG WON
- Journal of the Korean astronomical society = 천문학회지
- 35, n.4
- pp.187-196
- 2002
- 원문 바로보기
A spectral line survey is performed from 159.7 to 164.7 GHz toward Orion-KL, as an extension of our previous line survey from 138.3 to 150.7 GHz with the same 14 m radio telescope of Taeduk Radio Astronomy Observatory. Typical system temperatures were 260 - 1000 K to achieve a sensitivity of about 0.02 - 0.04 K in TA unit. A total of 63 line spectra are detected in this survey. Among them, 54 lines lines are found to be the first detections towards an astronomical source and only 9 spectral lines have been previously identified from other observations. Forty-eight of 54 lines are believed to be from the known transitions of the known molecules, while 6 lines are 'unidentified'. All detected lines are found to be from a total of 10 molecular species and their isotopic variants. The molecular species with most numerous detected transitions are <TEX>$HCOOCH_3$</TEX> (22), followed by <TEX>$CH_3OCH_3$</TEX> (7), <TEX>$C_2H_5CN$</TEX> (7), and <TEX>$SO_2$</TEX> (6). The LTE rotation diagram analysis using all homogeneous data with those from previous survey gives more reliable determination of physical quantities. The derived values of the rotation temperatures and column densities for <TEX>$HCOOCH_3$</TEX>, <TEX>$CH_3OCH_3$</TEX>, and <TEX>$SO_2$</TEX> are are 75 <TEX>$\~$</TEX> 197 K and <TEX>$1.5 {\~}18 {\times} 10^{15}\;cm^{-2}$</TEX>, respectively.
679
- KIM SANG CHUL
- Journal of the Korean astronomical society = 천문학회지
- 35, n.1
- pp.9-28
- 2002
- 원문 바로보기
We present UBVI CCD photometry of the stellar contents and globular cluster(GC) candidates in the spiral galaxy NGC 300 in the Sculptor group. Color-magnitude diagrams for 18 OB associations having more than 30 member stars are presented. The slope of the initial mass function for the bright stars in NGC 300 is estimated to be <TEX>${\Gamma}= -2.6{\pm} 0.3$</TEX>. Assuming the distance to NGC 300 of (m - M)o = 26.53 <TEX>$\pm$</TEX> 0.07, the mean absolute magnitude of three brightest blue stars is obtained to be < <TEX>$M_v^{BSG}$</TEX> (3) > = -8.95 mag. We have performed search for GCs in NGC 300 and have found 17 GC candidates in this galaxy. Some characteristics of these GC candidates are discussed.
680
- OH SU YEON
- Journal of the Korean astronomical society = 천문학회지
- 35, n.3
- pp.151-157
- 2002
- 원문 바로보기
From the data of solar wind observation by ACE spacecraft orbiting the Earth-Sun Lagrangian point, we selected 48 forward interplanetary shocks(IPSs) occurred in 2000, maximum solar activity period. Examining the profiles of solar wind parameters, the IPSs are classified by their shock drivers. The significant shock drivers are the interplanetary coronal mass ejection(ICME) and the high speed stream(HSS). The IPSs driven by the ICMEs are classified into shocks driven by magnetic clouds and by ejectas based on the existence of magnetic flux rope structure and magnetic field strength. Some IPSs could be formed as the blast wave by the smaller energy and shorter duration of shock drivers such as type II radio burst. Out of selected 48 forward IPSs, <TEX>$56.2\%$</TEX> of the IPSs are driven by ICME, <TEX>$16.7\%$</TEX> by HSS, and <TEX>$16.7\%$</TEX> of the shocks are classified into blast-wave type shocks. However, the shock drivers of remaining <TEX>$10\%$</TEX> of the IPSs are unidentified. The classification of the IPSs by their driver is a first step toward investigating the critical magnitudes of the IPS drivers commencing the magnetic storms in each class.