• Journal of Astronomy and Space Sciences
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• v.29 no.2
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• pp.181-189
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• 2012

To broaden the understanding of classical Cepheid structure, evolution and atmospheres, we have extended our continuing secret lives of Cepheids program by obtaining XMM/Chandra X-ray observations, and Hubble space telescope (HST) / cosmic origins spectrograph (COS) FUV-UV spectra of the bright, nearby Cepheids Polaris, ${\delta}$ Cep and ${\beta}$ Dor. Previous studies made with the international ultraviolet explorer (IUE) showed a limited number of UV emission lines in Cepheids. The well-known problem presented by scattered light contamination in IUE spectra for bright stars, along with the excellent sensitivity & resolution combination offered by HST/COS, motivated this study, and the spectra obtained were much more rich and complex than we had ever anticipated. Numerous emission lines, indicating $10^4$ K up to ${\sim}3{\times}10^5$ K plasmas, have been observed, showing Cepheids to have complex, dynamic outer atmospheres that also vary with the photospheric pulsation period. The FUV line emissions peak in the phase range ${\varphi}{\approx}0.8-1.0$ and vary by factors as large as $10{\times}$. A more complete picture of Cepheid outer atmospheres is accomplished when the HST/COS results are combined with X-ray observations that we have obtained of the same stars with XMM-Newton & Chandra. The Cepheids detected to date have X-ray luminosities of log $L_X{\approx}28.5-29.1$ ergs/sec, and plasma temperatures in the $2-8{\times}106$ K range. Given the phase-timing of the enhanced emissions, the most plausible explanation is the formation of a pulsation-induced shocks that excite (and heat) the atmospheric plasmas surrounding the photosphere. A pulsation-driven ${\alpha}^2$ equivalent dynamo mechanism is also a viable and interesting alternative. However, the tight phase-space of enhanced emission (peaking near 0.8-1.0 ${\varphi}$) favor the shock heating mechanism hypothesis.

• The Bulletin of The Korean Astronomical Society
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• v.13
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• pp.9.3-10
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• 1988

• The Bulletin of The Korean Astronomical Society
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• v.9
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• pp.6.1-6.1
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• 1984

• The Bulletin of The Korean Astronomical Society
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• v.11
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• pp.4.2-5
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• 1986

• Publications of The Korean Astronomical Society
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• v.4 no.1
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• pp.31-52
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• 1989

The rotational properties of late-type main sequence stars in the solar neighborhood have been investigated. So rotation periods and stellar radii are determined for 104 field stars, 8 Ursa Major Group stars, and 20 Hyades cluster stars. Most of the rotation periods are derived using the Noyes et al. (1984)'s relation between chromospheric activity and rotation period. Stellar radii are calculated by the Stefan law for the nearby stars within 25 pc from the sun. Rotational velocities at equator are determined by the above rotation periods and stellar radii. Their distribution along the (B-V) color shows an upper boundary and an abrupt drop for the stars in the range of 0.4<(B-V)<0.8, as found from the apparent rotational velocity data. Furthermore, it is apparent that there is an lower boundary of rotational velocity. The inclination of rotation axis to line-of-sight is obtained by comparing the rotational velocity at equator with the apparent rotational velocity given by the analysis of the line profiles. For the field stars, it is found that the inclination has no correlation with the galactic lattitude and follows random distribution.

• Journal of The Korean Astronomical Society
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• v.44 no.3
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• pp.97-108
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• 2011

The photometric light curves of the W-type W UMa eclipsing contact binary system BB Pegasi have been found to be extremely asymmetric over all the observed 63 years in all wavelengths UBVR. The light curves have been characterized by occultation primary minima. Hence, the morphology of these light curves has been studied in view of these different asymmetric degrees. The system shows a distinct O'Connell effect, as well as depth variation. A 22.96 years of stellar dark spots cycle has been determined for the system. Almost the same cycle (22.78 yr) has been found for the depth variation of MinI and MinII. We also present an analysis of mid-eclipse time measurements of BB Peg. The analysis indicates a period decrement of $5.62{\times}10^{-8}$ day/yr, which can be interpreted in terms of mass transfer at a rate of $-4.38{\times}10^{-8}M_{\odot}$/yr, from the more to the less massive component. The O - C diagram shows a damping sine wave covering two different cycles of 17.0 yr and 12.87 yr with amplitudes equal to 0.0071 and 0.0013 day, respectively. These unequal durations show a non-periodicity which may be explained as a result of magnetic activity cycling variations due to star spots. The obtained characteristics are consistent with similar chromospherically active stars, when applying the Applegate's (1992) mechanism.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.49.1-49.1
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• 2010

The optical lightcurves of flare events can be regarded as a direct indicator about the existence of magnetic activity in low-mass stars. Stellar flares are generated by magnetodynamic processes in the stellar interiors as on the Sun and indicate that the locally intensified active regions still exist on the photosphere. However previous photometric observations are limited to a few selected active objects because of their faintness and randomness of the flare occurrence. Based on dedicated deep (r~23), long-term (24 night) time-series monitoring of the open cluster M37 from MMT 6.5m transit survey program, we searched for flare-like transient phenomena in the 3,052 M-dwarf lightcurves with relatively high-temporal resolution (30s-90s). In order to collect all statistical significant events, we applied the change-point analysis with filtering algorithm using local statistics. We found a number of flares from 412 M-dwarf stars that are probable cluster members. Nearly half of them have periodic brightness variations with a near or distorted sinusoidal shape. With a small exception of binary cases, most of these variations appear to reflect the presence of large starspots resulting in rotational brightness modulations. We will discuss the relationship among magnetic activity indicators and dependence on spectral type.

• Publications of The Korean Astronomical Society
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• v.1 no.1
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• pp.13-20
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• 1984

New empirical relations between stellar CaII emission and rotation or age are derived by analyzing Wilson's CaII flux measurements (1968, 1978) of lower main sequence stars, and then we correlate them with their age and rotation rate. It is found that stellar chromospheric emission decays smoothly with age as a star slows down rotationally, establishing that both the emission level and rotation rate decrease with the square root of age.

• Journal of Astronomy and Space Sciences
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• v.24 no.1
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• pp.1-30
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• 2007

We present the Pre-Main Sequence (PMS) evolutionary tracks of stars with $0.065{\sim}5.0M_{\odot}$. The models were evolved from the PMS stellar birthline to the onset of hydrogen burning in the core. The convective turnover timescales which enables an observational test of theoretical model, particulary in the stellar dynamic activity, are also calculated. All models have Sun-like metal abundance, typically considered as the stars in the Galactic disk and the star formation region of Population I star. The convection phenomenon is treated by the usual mixing length approximation. All evolutionary tracks are available upon request.

• Journal of The Korean Astronomical Society
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• v.43 no.6
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• pp.201-211
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• 2010

We present an analysis of the measurements of mid-eclipse times of V839 Oph, collected from literature sources. Our analysis indicates a period increase of $3.2{\times}10^{-7}$ day/yr. This period increase of V839 Oph can be interpreted in terms of mass transfer of rate $1.76{\times}10^{-7}M_{\odot}/yr$, from the less to the more massive component. The O - C diagram shows a damping sine wave covering two different complete cycles of 36.73 yr and 19.93 yr with amplitudes approximately equal to 0.0080 and 0.0043 day, respectively. The third cycle has to be expected to cover about 13.5 years with lower amplitude than those of the former two cycles. These unequal duration cycles show a non periodicity which may be explained as resulting from either the presence of a tertiary component to the system or cyclic magnetic activity variations due to star spots. For the later mechanism, the obtained characteristics are consistent when applying Applegate (1992) mechanism.