• 천문학논총
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• 제30권2호
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• pp.603-604
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• 2015

Standard thin disk theory predicts that an inner disk region dominated by radiation pressure is thermally unstable. However, this kind of instability isn't detected in the observations of X-ray binaries. In this work, we revisit this issue by investigating the stability of a thin disk with magnetically driven winds. It is found that the disk winds can help to make a thin disk stable by taking away most of the energy released in the disk, resulting in a much cooler disk. The disk can always be stable even for a very weak initial field strength ${\beta}_{p,0}{\leq}400$ when ${\alpha}=0.05$ and $B{\phi}=10B_p$ are adopted.

• 한국지구과학회지
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• 제29권7호
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• pp.579-585
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• 2008

열적으로 불안정한 강착원반의 비선형 유체역학적 모형에 기초하여 블랙홀 마이크로케이사의 광폭발 한계 순환주기에 대한 원반 질량의 시간적 진화 모형을 계산하였다. 블랙홀의 질량, 원반 크기 및 질량 유입률과 같은 물리적인 매개변수들은 블랙홀 X선 신성의 원형인 A0620-00에서 관측된 역사적인 1975 광폭발을 재현하도록 선택되었다. 중심부에서 원반으로 쪼여지는 조사(照射)의 시간에 따른 효과는 직접 조사와 원반위의 뜨거운 강착 흐름으로부터 굴절되어 원반에 쪼여지는 간접조사의 두 가지 방법이 고려되었다. 우리의 강착원반 열적 불안정성 모형은 광폭발의 순환과정 전반에 걸쳐 X-선 변광체들에서 관측된 광도의 전형적인 복사 광도를 설명할 수 있다. 강착원반의 최대질량 ${\sim}4.03{\times}10^{24}g$은 광폭발의 점화 때에 얻어지며, 최소질량 ${\sim}8.54{\times}10^{23}g$은 차가운 쇠퇴기와 정지기(靜止期) 때에 이루어진다. 원반의 질량은 광폭발 한계 순환주기에 걸쳐 약 5배 정도 변한다.

• 천문학회보
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• 제39권2호
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• pp.74.1-74.1
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• 2014

We study a wraping instability of a geometrically thin, non-self-gravitating disk surrounding binary supermassive black holes on a circular orbit. Such a circumbinay disk is subject to not only tidal torques due Line 8 to the binary gravitational potential but also radiative rorques due to radiation emitted from each accretion disk. We find tat a circumbinary disk initially aligned with the binary orbital plane is unstable to radiation-driven warping beyond the marginally stable warping radius, which is sensitive to both the ratio of vertical to horizontal shear viscosities and the mass-to-energy conversion efficiency. As expected, the tidal torques give no contribution to the growth of warping modes but tend to align the circumbinary disk with the orbital plane. Since the tidal torques can suppress the warping modes in the inner part of circumbinary disk, the circumbinaary disk starts to be warped at the radii larger than the marginally stable warping radius. If the warping radius is of the order of 0.1 pc, a resultant semi-major axis is estimated to be of the order of 10-2 pc to 10-4 pc for 107 Msun black hole. We also discuss the posibility that the central objects of observed warped maser disks in active galactic nuclei are binary supermassive black holes with a triple disk: two accretion disks around the individual black holes and one circumbinary disk surrounding them.

• 천문학회보
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• 제45권1호
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• pp.70.3-70.3
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• 2020

Previous numerical simulations on planet-disk interactions revealed a lot of interesting phenomena including the planetary migration and the formation of many sub-structures inside the disks. However, these simulations were limited to an isothermal or adiabatic equation of state which does not account for various heating and cooling processes in the disks. Recent studies showed that the behavior of the planet-disk interaction can be significantly influenced by the disk thermodynamics. We develop a radiative diffusion module based on the two-temperature flux-limited diffusion approximation accounting for viscous heating and the accretion feedback. In this presentation, we describe our radiative diffusion solver, present some test results, and discuss potential applications of the module to planet-disk interactions,

• 천문학회보
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• 제36권1호
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• pp.61.2-61.2
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• 2011

HI gas disks are known to be the largest ISM reservoir in most late type galaxies. When the HI properties of galaxies such as total mass, density, and distribution change, the galaxies may evolve quite differently. In this talk, I will present two groups of galaxies, one undergoing HI stripping and one accreting more gas. I will discuss causes of gas stripping and accretion, and possible consequences in galaxy evolution.

• 천문학회보
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• 제36권2호
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• pp.65-65
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• 2011

Early-type galaxies represent the end point of galaxy evolution and, despite pervasive residual star formation, are generally considered "red and dead", that is composed exclusively of old stars with no star formation. Here, their molecular gas content is constrained and discussed in relation to their evolution, supporting the continuing importance of minor mergers and/or cold gas accretion. First, as part of the Atlas3D survey, the first complete, large, volume-limited survey of CO in normal early-type galaxies is presented. At least of 23% of local early-types possess a substantial amount of molecular gas, the necessary ingredient for star formation, independent of mass and environment but dependent on the specific stellar angular momentum. Second, using CO synthesis imaging, the extent of the molecular gas is constrained and a variety of morphologies is revealed. The kinematics of the molecular gas and stars are often misaligned, implying an external gas origin in over a third of all systems, more than half in the field, while external gas accretion must be shot down in clusters. Third, many objects appear to be in the process of forming regular kpc-size decoupled disks, and a star formation sequence can be sketched by piecing together multi-wavelength information on the molecular gas, current star formation, and young stars. Fourth, early-type galaxies do not seem to systematically obey all our usual prejudices regarding star formation (e.g. Schmidt-Kennicutt law, far infrared-radio continuum correlation), suggesting a greater diversity in star formation processes than observed in disk galaxies and the possibility of "morphological quenching". Lastly, a first step toward constraining the physical properties of the molecular gas is taken, by modeling the line ratios of density- and opacity-sensitive molecules in a few objects. Taken together, these observations argue for the continuing importance of (minor) mergers and cold gas accretion in local early-types, and they provide a much greater understanding of the gas cycle in the galaxies harbouring most of the stellar mass. In the future, better dust masses and dust-to-gas mass ratios from Herschel should allow to place entirely independent constraints on the gas supply, while spatially-resolved high-density molecular gas tracers observed with ALMA will probe the interstellar medium and star formation laws locally in a regime entirely different from that normally probed in spiral galaxies.

• 천문학회지
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• 제34권1호
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• pp.9-15
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• 2001

We present the results of optical differential photometry of five blazars [PKS0219+428 (3C66A), PKS 0235+164 (AO 0235+16), H0414+019, PKS 0851+202 (OJ 287) and QSO 1807+698 (3C 371)] that were observed on 7 nights between November 05, 1997 and December 29, 1998, using the B and the V band filters. We have detected microvariations in four blazars (3C66A, AO 0235+16, H04l4+019, and OJ 287). In addition, the light curve of AO 0235+16 has displayed a mini-flare when the brightness of this source was decreasing. Night-to-night variations have also been detected in 3C66A, H04l4+019, and OJ 287. The results of our observations are discussed in the framework of accretion disk phenomena (magnetic flares or hot spots in accretion disks) and jet phenomena (plasma instabilities in jets).

• 천문학회보
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• 제36권2호
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• pp.71.2-71.2
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• 2011

To investigate the properties of the narrow-line regions and the accretion disks of YRGs, we study a sample of 28 young radio galaxies (YRGs) observed with the Kast Double Spectrograph at the Shane 3-m telescope and with the DBSP (Double Spectrograph for the Palomar 200-inch Telescope) at Palomar observatory. In addition we collect an addition sample of 15 YRGs with the optical spectra from the SDSS archive. We present the measured narrow-line region properties based on the various emission line ratios, i.e., [O III]/$H{\beta}$, [N II]/$H{\alpha}$, [S II] 6716/6731, [O I]/[O III], [O II]/[O III] and [Ar III]/[O III], which are useful to constrain the gas properties and the states of the accretion disk. We will discuss the characteristics of YRG.

• 천문학논총
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• 제30권2호
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• pp.587-589
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• 2015

We present the results from analysis of the Hilbert-Huang transform (HHT) for the 4 Hz quasi-periodic oscillations (QPO) around the black hole X-ray binary XTE J1550-564. The resultant Hilbert spectra demonstrate that the QPO is composed of a series of intermittent signals appearing occasionally. From the analysis of the HHT, we further found the distribution of the lifetimes for the intermittent oscillations and the distribution for the time intervals with no significant signal (the break time). The mean lifetime is 1.45 s and 90% of the oscillation segments have lifetimes less than 3.1 s whereas the mean break time is 0.42 s and 90% of break times are less than 0.73 s. We conclude that the intermittent feature of the QPO could be explained by the Lense-Thirring precession model and rules out interpretations of continual frequency modulation.

• 천문학회보
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• 제41권1호
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• pp.72.4-73
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• 2016

For the development of magneto rotational instability, which drives mass accretion in protoplanetary disks, sufficient ionization degree is needed. Cosmic rays are believed to be one of the dominant ionization sources for protoplanetary disk gas. In previous studies, ionization rates are computed by considering the effect of attenuation of the cosmic ray (CR) intensity as a function of column density in an unmagnetized cloud. However, in reality particles should sweep up larger column density to reach at the midplane of disk due to their gyromotion. In this study, we investigate the propagation of CR protons in a protoplanetary disk by solving transport and energy loss equations. We discuss the change in CR intensity due to magnetic field in a protoplanetary disk.