• 천문학회지
• /
• 제33권3호
• /
• pp.137-142
• /
• 2000

CN and CH band strengths for ten new bright giants in the globular cluster M15 have been measured from archival spectra obtained with the Multiple Mirror Telescope. Using published indices for other bright M15 giants, a CN-CH band strength anticorrelation is found for bright red giants. However, stars that do not follow the CN-CH anticorrelation are also found. They seem to show a positive correlation between the two indices. Among them, all the AGB and HB stars of the sample are included. Stars I-38 and X6, which are located near the RGB fiducial line in the CMD, have low measured CH(G) indices compared with other RGB stars. Stars IV-38, S4, and S1, which are all near the RGB tip, have strong measured CH(G) indices. Therefore, most of their evolutionary states are suspected to be different from those of a normal single RGB star.

• 천문학논총
• /
• 제15권spc1호
• /
• pp.15-30
• /
• 2000

A large scatter of the chemical abundances among globular cluster red giants has been observed. Especially the chemical elements C, N, O, Na, Mg, and Al vary form star to star within globular clusters. Except for $\omega$ Cen and M22, most globular clusters could be considered to be monometallic of their iron peak elements within error ranges. The variations in light elements among globuar cluster giants appear much more pronounced than in field halo giants of comparable Fe-peak metallicity. It has been found that in general the nitrogen abundance is anticorrelated with both carbon and oxygen, while it is correlated with Na and AI. These intracluster abundance inhomogeneities can be interpreted either by mixing of nucleosythesized material from the deep stellar interior during the red giant branch phase of evolution or by inhomogeneities of primordially processed material, from which the stars were formed. The simple way of distingushing between two senarios is to obtain the element abundances of main-sequence stars in globular clusters, which are too faint for high resolution spectroscopic studies until now. Both 'evolutionary' and 'primodial' origins are accepted for explanations of abundance variations among red giants and CN-CH anticorrelations among main-sequence stars in globular clusters. This paper reviews chemical abundances of light elements among globular cluster giants, with brief reviews of cannonical stellar evolution of low mass stars after main-sequence and deep mixing for abundance variations of cluster giants, and a possible connection between deep mixing and second parameter.

• 천문학회지
• /
• 제38권2호
• /
• pp.315-318
• /
• 2005

We have carried out a precise Doppler survey of G-type giants aiming to unveil the properties of planetary systems in intermediate-mass stars ($1.5-5M_{\bigodot}$). G-type giants are promising targets for Doppler planet searches around massive stars, because they are slow-rotators and have many sharp absorption lines in their spectra and their surface activities are relatively low in contrast to their younger counterparts on the main-sequence (B-A stars). We are now monitoring radial velocities of about 300 late G-type (including early K-type) giants using HIgh Dispersion Echelle Spectrograph (HIDES) at Okayama Astrophysical Observatory. We have achieved a Doppler precision of about 6-7 m/s over a time span of 3 years using an iodine absorption cell. We found that most of the targets have radial velocity scatters of ${\sigma}{\~} 10-20 m\;s^{-1}$ over 1-3 years, with the most stable reaching levels of 6-8 m $s^{-1}$. Up to now, we have succeeded in discovering the first extrasolar planet around a G-type giant star HD 104985, and also found several candidates showing significant radial velocity variations, suggesting the existence of stellar and substellar companions. Observations have continued to establish their variability.

• 천문학회보
• /
• 제35권2호
• /
• pp.79.2-79.2
• /
• 2010

We present the results of simultaneous observations of SiO v=1, 2, J=1-0, $^{29}SiO$ v=0, J=1-0, and $H_2O$ $6_{16}-5_{23}$ maser lines performed with the KVN Yonsei 21 m radio telescope from 2009 Nov. to 2010 Jan (ApJ, 719, 126, 2010). We searched for these masers in 47 symbiotic stars and detected maser emission from 21 stars, giving the first time detection from 19 stars. Both SiO and $H_2O$ masers were detected from seven stars of which six stars are D-type symbiotic and one is an S-type star, WRAY 15-1470. In the SiO maser emission, the $^{28}SiO$ v=1 maser was detected from 10 stars, while the v=2 maser detected from 15 stars. In particular, the $^{28}SiO$ v=2 maser emission without the v=1 maser detection was detected from nine stars with its detection rate of 60 %, which is much higher than that of isolated Miras/red giants. The $^{29}SiO$ v=0 maser emission was also detected from two stars, H 2-38 and BF Cyg, together with the $^{28}SiO$ v=2 maser. We conclude that these different observational results between isolated Miras/red giants and symbiotic stars may be related with the presence of hot companions in a symbiotic binary system.

• 천문학회지
• /
• 제38권2호
• /
• pp.23-31
• /
• 2005

CN and CH band strengths for fourteen bright giants in the globular cluster M71 have been measured from archival spectra obtained with the Multiple Mirror Telescope. Adding the collected. data from the literature we confirm a bimodality of CN distribution on the red giant branch and the honzontal branch, and CN-CH anti-correlations on the lower giant branch and horizontal branch. However a CN-CH anti-correlation on the upper red giant branch is not quite clear as those of other branches. The small number If statistics could not be excluded as a possible cause. To confirm this, a greater number of sample stars are needed. We also confirm that the ratio of CN-strong to CN-weak stars is quite different from that in 47 Tuc, although the anti-correlation between CN and CH bands, the bimodality of the CN distribution, and the spatial distribution of CN stars in M71 are found to be similar to those III 47 Tuc.

• 천문학회보
• /
• 제43권1호
• /
• pp.66.3-67
• /
• 2018

We present distinct chemical and kinematic properties associated with the inner and outer halos of the Milky Way, as identified by metal-poor stars from the Sloan Digital Sky Survey. In particular, using carbon-enhance metal-poor (CEMP) giants, we first map out the fractions of CEMP-no stars (without strongly enhanced neutron-capture elements) and CEMP-s stars (with a large enhancement of s-process elements) in the inner- and outer-halo populations, separated by their spatial distribution of carbonicity ([C/Fe]). The CEMP-no and CEMP-s objects are classified by their different levels of absolute carbon abundances, A(C). We investigate characteristics of rotational velocity and orbital eccentricity for these sub-classes within the halo populations. Distinct kinematic features and fractions between CEMP-no and CEMP-s stars identified in each halo region will provide important clues on the origin of the dichotomy of the Galactic halo.

• 천문학회보
• /
• 제41권1호
• /
• pp.59.3-59.3
• /
• 2016

We report the results of search for exoplanets by a precise radial velocity (RV) survey by using the high-resolution spectroscopy of the fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at Bohyunsan Optical Astronomy Observatory (BOAO). Since 2003, we have conducted a precise RV survey for ~500 stars, including 55 K giants, ~200 G giants, 10 M giants, 40 K dwarfs, and ~200 northern circumpolar stars. We present the detection of around 20 new exoplanets and brown dwarfs.

• 천문학회보
• /
• 제46권1호
• /
• pp.50.1-50.1
• /
• 2021

Gravitational instability (GI) can produce massive gas giants on wide orbits by fragmentation of protoplanetary disks (PPDs). While most previous works focus on PPDs around solar mass stars, gas giants have been observed in systems with a wide range of stellar masses including M dwarfs. We use the GIZMO code to perform global three-dimensional simulations of self-gravitating disks around low-mass stars. Our models consider heating by turbulent viscosity and stellar irradiation and the β cooling occurring over the dynamical time. We run various models with differing disk-to-star mass ratio q and disk temperature. We find that strongly gravitating disks either produce spirals or undergo fragmentation. The minimum q value for fragmentation is 0.2-0.7, with a smaller value corresponding to a more massive star and/or a smaller disk. The critical q value depends somewhat sensitively on the disk temperature, suggesting that the stellar irradiation is an important factor in determining GI. We discuss our results in comparison with previous work as well as recent ALMA observations.

• Journal of Astronomy and Space Sciences
• /
• 제38권3호
• /
• pp.175-183
• /
• 2021

The dependencies of the chemical element abundances in stellar atmospheres with respect to solar abundances on the second ionization potentials of the same elements were investigated using the published stellar abundance patterns for 1,149 G and K giants in the Local Region of the Galaxy. The correlations between the relative abundances of chemical elements and their second ionization potentials were calculated for groups of stars with effective temperatures between 3,764 and 7,725 K. Correlations were identified for chemical elements with second ionization potentials of 12.5 eV to 20 eV and for elements with second ionization potentials higher than 20 eV. For the first group of elements, the correlation coefficients were positive for stars with effective temperatures lower than 5,300 K and negative for stars with effective temperatures from 5,300 K to 7,725 K. The results of this study and the comparison with earlier results for hotter stars confirm the variations in these correlations with the effective temperature. A possible explanation for the observed effects is the accretion of hydrogen and helium atoms from the interstellar medium.

• 천문학논총
• /
• 제7권1호
• /
• pp.231-253
• /
• 1992

A model for the distribution of stars in the disk and the spheroid of our Galaxy is reexamined from an edge-on view of the Galaxy obtained by selecting infrared sources from the IRAS Point Source Catalog. The sources are counted as a function of galactic latitude. longitude and $12{\mu}m$ apparent magnitude. The source counts are reasonably separated into the disk component and the spheroid component contributions and each of the contributions is further interpreted as a convolution of a spatial density distribution and a luminosity function based on the least-square fit method. The spatial density of the disk component has an exponential radial scale length of $h_R{\sim}2.6\;kpc$ and the vertical distribution follows a canonical $sech^2$ law with a scale height $h_z{\sim}240\;pc$. The distribution of the spheroid component can be represented by an oblate spheriod with an axis ratio $k{\sim}0.61$ and a de Vaucouleurs' $r^{1/4}$ law with an effective radius of $R_e{\sim}120\;pc$. The steep density gradient of the spheroid component is consistent with that of late M giants in the central bulge. The luminosity functions of the disk and the spheroid component stars resemble respectively those of the K luminosity function of disk M giants (Garwood and Jones 1986) and the bolometric luminosity function of M giants in bulge fields (Frogel et al, 1990).