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Introduction

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Part of the book series: Astrophysics and Space Science Library ((ASSL,volume 437))

Abstract

We start this introduction by presenting a brief history of appearance of accretion disks in astrophysical studies and by pointing out importance of diskoseismology. Then, we review astrophysical objects which have accretion disks, and present a brief survey of observational evidences of time variations in accretion disks, focusing on those which will be related to disk oscillations. The main oscillatory phenomena which we focus our attention are V/R variations in Be stars, positive and negative superhumps in dwarf novae, high-frequency quasi-periodic oscillations in neutron-star and black-hole X-ray binaries.

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Notes

  1. 1.

    In the case of the Schwarzschild metric, the circular particle orbit around a central object is dynamically stable till the radius 3r G (see, for example, Kato et al. 2008). This means that the gas can fall, by gradually losing angular momentum by viscosity, till the radius of 3r G with roughly keeping circular orbit.

  2. 2.

    Under certain situations, gravitational energy released is swallowed into central black holes as advection energy or outflows without being radiated away as thermal energy from disks. They are advetion-dominated accretion flows (ADAF) or radiatively inefficient accretion flows (RIAF) (see Sect. 1.6).

  3. 3.

    Ando, H. & Osaki, Y. (1975) proposed that the solar oscillations are excited by the κ-mechanism, which is widely known as the major excitation mechanism of radial pulsation of stars and of some non-radial oscillations in stars. In solar non-radial oscillations, however, many oscillation modes are observed simultaneously. Hence, it is difficult to consider that all of them are excited by the κ-mechanism alone, because the mechanism requires a proper phase relation between the change of κ (opacity) and oscillation motion. This is one of the reasons why the excitation processes of solar non-radial oscillations are considered to be due to stochastic processes of turbulent motions. The stochastic processes will be also one of prominent mechanisms of excitation of disk oscillations, which will be discussed in Sect. 13.2

  4. 4.

    The term of diskoseismology was introduced first perhaps by R.V. Wagoner and his groups in the middle of 1980s.

  5. 5.

    There is another type of disks, called excretion disks. In these disks, gases are ejected from central objects by getting angular momentum. The disks surrounding Be stars belong this types of disks (Lee et al. 1991).

  6. 6.

    For the meaning of “scaled down”, see the next footnote.

  7. 7.

    The frequencies observed at Sgr A are much lower than those in LMXBs, but they are short-term oscillations in the following sense. If the frequencies of QPOs are roughly equal to the Keplerian frequency of the innermost region of relativistic disks, they are on the order of (GMr G 3)1∕2, where M is the mass of the central source and r G is the Schwarzschild radius defined by r G = 2GMc 2. Hence, the frequency is proportional to 1∕M. If this scaling is applied, the frequencies of QPOs of Sgr A are roughly on the line of the 1∕M scaling from those of QPOs of LMXBs. In this sense, we called the QPOs in Sgr A short-term variations.

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© 2016 Springer Japan

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Kato, S. (2016). Introduction. In: Oscillations of Disks. Astrophysics and Space Science Library, vol 437. Springer, Tokyo. https://doi.org/10.1007/978-4-431-56208-5_1

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