• 천문학회지
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• 제34권4호
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• pp.265-269
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• 2001

We present preliminary numerical simulations of tilted-disk accretion around a rotating black hole. Our goal is to explore whether hydrodynamic instabilities near the Bardeen-Petterson radius could be responsible for generating moderate-frequency quasi-periodic oscillations in X-ray binaries. We review the relevant general relativistic hydrodynamic equations, and discuss preliminary results on the structure and dynamics of a thin, Keplerian disk.

• 천문학논총
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• 제8권1호
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• pp.163-168
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• 1993

At intermediate mass transfer rates, accretion disks in binary star systems undergo a thermally-driven limit cycle instability. This instability leads to outburst episodes when the disk is bright and the flow through the disk is rapid separated by long intervals when the disk is dim and the flow through it is low. This intrinsic outburst mechanism can help to understand a wide range of astrophysical phenomena from dwarf novae to soft X -ray transients involving white dwarf, neutron star, and black holes. and to a deeper understanding of the mechanism of angular transport and viscosity in the accretion disk.

• Journal of Astronomy and Space Sciences
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• 제2권1호
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• pp.7-18
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• 1985

Accretion rate가 $M=10^{16}g/sec$ 와 $M=10^{18}g/sec$인 경우의 정상상태의 Accretion disk의 모델들을 만들었다. 중심에 있는 별은 백색왜성으로 가정하고 질량은 $1M_\odot$이고 반경은 $10^9cm$로 보았다. 이 모델을 왜신성인 Z cha와 Z Cam에 응용하였다.

• 천문학회보
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• 제46권2호
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• pp.35.1-35.1
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• 2021

Variability in young stellar objects (YSOs) can be caused by various time-dependent phenomena associated with star formation, including accretion rates, geometric changes in the circumstellar disks, stochastic hydromagnetic interactions between stellar surfaces and inner disk edges, reconnections within the stellar magnetosphere, and hot/cold spots on stellar surfaces. Among these YSO variability phenomena, bursts of accretion, which are the most remarkable variability, usually occur sporadically, making it challenging to catch the bursting moments observationally. However, the burst accretion process significantly affects the chemical conditions of the disk and envelope of a YSO, which can be used as a prominent tracer of episodic accretion. I will introduce our ensemble studies of YSO variability at mid-IR and submillimeter and also cover the ALMA observations of several YSOs in the burst accretion phase, especially in the view of chemistry.

• Journal of Astronomy and Space Sciences
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• 제33권3호
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• pp.173-176
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• 2016

We report on a systematic analysis of the spin and superorbital modulations of the high-mass X-ray binary 4U 0114+650, which consists of the slowest spinning neutron star known. Utilizing dynamic power spectra, we found that the spin period varied dramatically during the RXTE ASM and Swift BAT observations. This variation consists of a long-term spin-up trend, and two ~1,000 day and one ~600 day random walk epochs previously, MJD 51,000, ~MJD 51,400-52,000, and ~MJD 55,100-56,100. We further found that the events appear together with depressions of superorbital modulation amplitude. This provides evidence of the existence of an accretion disk, although the physical mechanism of superorbital modulation remains unclear. Furthermore, the decrease of the superorbital modulation amplitude may be associated with the decrease of mass accretion rate from the disk, and may distribute the accretion torque of the neutron star randomly in time.

• Journal of Astronomy and Space Sciences
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• 제35권1호
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• pp.1-6
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• 2018

The reanalysis of the previously published abundance pattern of mild barium star HD202109 (${\zeta}$ Cyg) and the chemical compositions of 129 thin disk barium stars facilitated the search for possible correlations of different stellar parameters with second ionization potentials of chemical elements. Results show that three valuable correlations exist in the atmospheres of barium stars. The first is the relationship between relative abundances and second ionization potentials. The second is the age dependence of mean correlation coefficients of relative abundances vs. second ionization potentials, and the third one is the changes in correlation coefficients of relative abundances vs. second ionization potentials as a function of stellar spatial velocities and overabundances of s-process elements. These findings demonstrate the possibility of hydrogen and helium accretion from the interstellar medium on the atmospheres of barium stars.

• 천문학회지
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• 제29권spc1호
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• pp.229-230
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• 1996

• 천문학회지
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• 제29권spc1호
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• pp.223-225
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• 1996

We review current theoretical understanding of the spectral properties (low and high states, transition of states, quasi-periodic oscillations etc.) of the low mass as well as supermassive black hole candidates.

• 천문학회지
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• 제35권3호
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• pp.123-130
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• 2002

An elliptical accretion disk may be formed by tidally disrupted debris of a flying-by star in an active galactic nucleus (AGN) or by tidal perturbation due to a companion in a binary black hole system. We investigate the iron K$\alpha$ line profiles expecting from a geometrically thin, relativistic, elliptical disk in terms of model parameters, and find that a broad and skewed line profile can be reproduced well. Its shape is variable to the model parameters, such as, the emissivity power-law index, the ellipticity of the disk, and the major axis orientation of the elliptical accretion disk. We suggest that our results may be useful to search for such an elliptical disk and consequently the tidal disruption event.

• 천문학회지
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• 제29권spc1호
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• pp.227-228
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• 1996

The long-term light curves of the novalikes TT Ari, KR Aur, AM Her and MV Lyr, were compiled and analysed for solar-like cyclical behaviour. The frequency analysis showed long-term cyclical modulations of the brightness of the stars, which can be ascribed to changes of the radii of the late type secondaries in order of ${\Delta}R/R{\simeq} 10^{-4}-10^{-5}$.