• 천문학회지
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• 제55권2호
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• pp.59-66
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• 2022

Stellar magnetic activity is important for formulating the evolution of the star. To represent the stellar magnetic activity, the S index is defined using the Ca II H+K flux measure from the Mount Wilson Observatory. Mg II lines are generated in a manner similar to the formation of Ca II lines, which are more sensitive to weak chromospheric activity. Mg II flux data are available from the International Ultraviolet Explorer (IUE). Thus, the main purpose of this study was to analyze the magnetic activity of stars. We used 343 high-resolution IUE spectra of 14 main-sequence G stars to obtain the Mg II continuum surface flux and Mg II line-core flux around 2,800 Å. We calculated S index using the IUE spectra and compared it with the conventional Mount Wilson S index. We found a color (B - V ) dependent association between the S index and the Mg II emission line-core flux. Furthermore, we attempted to obtain the magnetic activity cycles of these stars based on the new S index. Unfortunately, this was not successful because the IUE observation interval of approximately 17 years is too short to estimate the magnetic activity cycles of G-type stars, whose cycles may be longer than the 11 year mean activity cycle of the sun.

• 천문학논총
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• 제15권spc1호
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• pp.93-102
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• 2000

We have reviewed the magnetic activity in close binaries. Solar like magnetic activity indicators such as photometric spots, chromo spheric emission, coronal X-ray and radio emission, and flare activity are commonplace in many cool stars with convective envelopes. Using the UV spectra we confirmed the strength of stellar activity increases with more rapid rotation and later spectral types which corresponds to the increasing depth of the star's convective envelope. Apart from very young stellar objects such as T Tauri stars, the stars with the highest levels of activity are close binary systems composed of cool stars, i.e., the chromospherically active binaries such as RS CVn, BY Dra, W UMa and related systems. The IUE low and high dispersion spectra of V711 Tau, VW Cep and SW Lac are used for ultraviolet photometry and for a variation study of chromospheric activity. Evidence of chromospherically activity is indicated by the intensity variation of the Mg II emission line with orbital phase.

• 천문학회지
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• 제19권2호
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• pp.91-107
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• 1986

Making use of our extended version of $\ddot{O}pik's$ convection theory, we have calculated magnetic cycle periods of the sun and late type stars by using Parker's dynamo theory, where we have included the non-linear effect. We presented a relationship between the computed cycle period and spectral type to analyze observed magnetic activities of the late type stars and long-term luminosity variations. It is found that (1) the stellar magentic-cycle period increases towards the later spectral type, (2) the rapid rotation facilitates the activity-related luminosity variation of stars later than about K5, (3) differential rotation plays a critical role in determining the magnetic activity-cycle period, and (4) the non-local effect should be taken into account in order to understand the observed long-term luminosity variations.

• 천문학회보
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• 제39권2호
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• pp.83.2-83.2
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• 2014

Flare and rotational variabilities induced by stellar activity are important for studying the effect of magnetic fields on the evolution of red dwarf stars. The level and frequency of magnetic activity in these stars have a different aspect at every moment of the observations due to the effect of age-rotation relation. The use of both tracers is thus essential to have a relatively homogeneous set of stellar activity data for statistical studies. The archival light curves and imaging data of the open cluster M37 taken by MMT 6.5m telescope were used for this work. In order to achieve much more accurate photometric precisions and also to make the most efficient use of the data, the entire imaging database were re-analyzed with our new time-series photometry technique and carefully calibration procedures. Based on the new light curves, we study, for the first time, a variety of aspects of those two variabilities in red dwarfs and their relation to magnetic activity. In this talk, we present all observational evidences that support the idea that the strength of magnetic activity is closely connected with the rotation rate of a star and its evolutionary status (age-activity-rotation paradigm). In conclusion, we suggest future directions to improve our understanding of stellar activity in cool stars with photometric time-series data.

• 천문학회지
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• 제54권4호
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• pp.129-137
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• 2021

It has been established that the acoustic mode parameters of the Sun and Sun-like stars vary over activity cycles. Since the observed variations are not consistent with an activity-related origin, even Sun-like stars showing out-of-phase changes of mode frequencies and amplitudes need to be carefully studied using other observational quantities. In order to test whether the presumed relations between the global seismic parameters are a signature of the stellar activity cycle, we analyze the photometric light curve of HD 49933 for which the first direct detection of an asteroseismic signature for activity-induced variations in a Sun-like star was made, using observations by the CoRoT space telescope. We find that the amplitude of the envelope significantly anti-correlates with both the maximum frequency of the envelope and the width of the envelope unless superflare-like events completely contaminate the light curve. However, even though the photometric proxy for stellar magnetic activity appears to show relations with the global asteroseismic parameters, they are statistically insignificant. Therefore, we conclude that the global asteroseismic parameters can be utilized in cross-checking asteroseismic detections of activity-related variations in Sun-like stars, and that it is probably less secure and effective to construct a photometric magnetic activity proxy to indirectly correlate the global asteroseismic parameters. Finally, we seismically estimate the mass of HD 49933 based on our determination of the large separation of HD 49933 with evolutionary tracks computed by the MESA code and find a value of about 1.2M_☉ and a sub-solar metallicity of Z = 0.008, which agrees with the current consensus and with asteroseismic and non-asteroseismic data.

• Journal of Astronomy and Space Sciences
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• 제17권1호
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• pp.39-44
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• 2000

lUE 저분산 자로플 가지고 44i Boo, W UMa, AW UMa및 VW Cep에서 나오는 자외선 방출선들의 특징을 조사하였다 단파장영역과 장파장영역에서의 방출선 선세기를 구하여, 이를 자$\boxUl$선 등급으로 변환하여 광도곡선을 얻었다 단파장 영ff에서 나오는 방출선인 C I, C II, C IV, SiIV, N V에 대한 선세기를 구한 결파, C IV가 모든 별에서 가장 센 방출을 보였으며. Si IV와 NV는 비교적 작은 양의 선세기를 나타내었다. 4개의 W UMa 별 쿵에근 44i Boo가 가장 높은 선세기를 보였으며, 단과장 영역에서는 위상 0.2와 0 8부근에서 자외선 팡도가 maximup을 보이는 변광을 확인하였다 또한, 전이영역에서 나오는 C IV, Si IV, N V 방출선을 합하여 이를 극소기의 태양값과 비교하여 태양보다 최대40배의 높은 활동성을 가지고 있음을 알았다. 짧은 주기를 갖는 별,즉 44i Boo, W UMa, AW UMa순으로 전이영역의 헐동성이 감소하는 것으로 나타났다. 장 파장 영역에서는 AW UMa와 VW Cep의 Mg ll 방출선을 조사하였는데, 단파장의 방출선 보다 뚜렷한 주기적 변광을 보였으며, 이를 이응하여 두 별의 유효온도를 추정하였다..

• 천문학논총
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• 제30권2호
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• pp.175-178
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• 2015

Due to the lack of an accretion disk in a polar (magnetic cataclysmic variable, MCV), the material transferred from the secondary is directly accreted onto the white dwarf, forming an accretion stream and a hot spot on the white-dwarf component. During the eclipses, different light components can be isolated. Therefore, the monitoring of eclipsing polars could provide valuable information on several modern astrophysical problems, e.g., CVs as planetary hosting stars, mass transfer and mass accretion in CVs, and the magnetic activity of the most rapidly rotating cool dwarfs. In the past five years, we have monitored about 10 eclipsing polars (e.g., DP Leo and HU Aqr) using several 2-m class telescopes and about 100 eclipse profiles were obtained. In this paper, we will introduce the progress of our research group at YNOs. The first direct evidence of variable mass transfer in a CV is obtained and we show that it is the dark-spot activity that causes the mass transfer in CVs. Magnetic activity cycles of the cool secondary were detected and we show that the variable mass transfer is not caused by magnetic activity cycles. These results will shed light on the structure and evolution of close binary stars (e.g., CVs and Algols).

• 천문학회지
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• 제41권1호
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• pp.1-6
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• 2008

The X-ray spectra of late type stars can generally be well fitted by a two temperature component model of the corona. We find that the temperatures of both components are strong functions of stellar age, although the temperature of the hotter plasma in the corona shows a larger scatter and is probably affected by the activity of stars, such as flares. We confirm the power-law decay of the temperature of the hot plasma, but the temperature of the cool plasma component decays linearly with log(age).

• 천문학회보
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• 제41권2호
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• pp.43.1-43.1
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• 2016

The duration of star formation activity is a key to understanding the formation process of star clusters. Although a number of astronomers have attempted to derive the underlying age spread in photometric diagrams with a variety of stellar evolutionary models, the resultant findings are subject to uncertainties due to intrinsic variability of pre-main sequence (PMS) stars, observational errors, difficulties in reddening correction, and systematic differences in adopted stellar evolutionary models. The distribution of Li abundance for PMS stars in a cluster could, on the other hand, provide an alternative way to estimate the age spread. In this study, a total of 134 PMS stars in NGC 2264 are observed with the high resolution multi-object spectrogragh Hectochelle attached to the 6.5m Multi Mirror Telescope. We have successfully detected Li ${\lambda}6708$ resonance doublet for 86 low-mass PMS stars. The Li abundance of the stars is derived from their equivalent width using a curves of growth method. After correction for non-LTE effects, the underlying age spread of 3 - 4 Myr is inferred from the Li abundance distribution of low-mass PMS stars. We suggest that NGC 2264 formed on a timescale shorter than 5 Myr given the presence of embedded populations.

• 천문학회지
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• 제21권1호
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• pp.79-95
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• 1988

To examine relations between stellar activity and rotation we estimated parameters of stellar activity such as $R'_{HK}$, $R'_{MgII}$, $R'_{CII}$, $R'_{CIV}$ and $R'_{X-ray}$ from the published data which measure the activity levels of stellar chromospheres, transition regions and coronae. In the present study we considered only the main sequence stars in an attempt to minimize the influence of other stellar parameters such as radius, age and stellar convection on stellar activity since they are also known to affect the magnetic field generation. In the present analysis we selected only those stars that satisfy the following conditions: (1) flux measurements are available together with Ca II fluxes and (2) rotation periods are determined by Ca II observations. We derived relations between the ${\bar{R}}ossby$ number $R_o$ and stellar activity $R'_{HK}$, $R'_{MgII}$, $R'_{CII}$, $R'_{CIV}$ and $R'_{X-ray}$ and assessed the relations by plotting $R'_{HK}$, $R'_{MgII}$ and $R'_{X-ray}$ against rotation period $P_{rot}$ for comparison with observations. From the comparison it is found that as far as the rotation-activity relation is concerned, (1) normalized surface flux $R'_{HK}$ is better than the surface flux $F'_{HK}$, in the sense that $R'_{HK}$ differentiates the color dependence better and (2) $R'_{HK}$ defined by Rutten (1984) describes the observations notably better than $R'_{HK}$ of Noyes et al. (1984).