Accretion disk

Accretion disk refers to a structure (often a disk) formed by diffused material in orbital motion around a massive central body. This central body can be a star, black hole, white dwarf, or neutron star. The material in the disk originates from the outer atmosphere of a star or from interstellar medium and is pulled towards the central body by its gravitational force. As the material spirals inwards, it accelerates and heats up, emitting electromagnetic radiation in a wide range of wavelengths, including visible light, ultraviolet light, and X-rays.

Formation

Accretion disks are formed when a cloud of interstellar gas and dust collapses under the influence of gravity, but possesses enough angular momentum to prevent all the material from falling directly onto the central body. Instead, the material orbits the body and gradually spirals inward due to processes that transfer angular momentum outward and mass inward, such as viscous stress and magnetic braking.

Structure

The structure of an accretion disk can be broadly divided into three parts: the outer disk, the inner disk, and the boundary layer near the central object. The outer disk is relatively cool and composed mostly of molecules and dust. As one moves inward, the disk's temperature increases, and the material transitions from molecular to atomic and then to ionized plasma in the innermost regions. The boundary layer is where the disk material makes its final plunge onto the central object, reaching its highest temperatures.

Physics

The physics of accretion disks involves complex interactions between gravity, fluid dynamics, and radiation. Key processes include:

- Angular momentum transfer: This is crucial for the accretion process, as it allows material to move inward. It can occur through viscous shear, magnetic fields, or gravitational interactions in the case of binary systems. - Radiative cooling: As the disk material heats up due to viscous dissipation, it emits radiation, which cools the disk and allows the material to spiral inward. - Magneto-hydrodynamic (MHD) instabilities: These can lead to turbulence within the disk, enhancing the transfer of angular momentum.

Observational Evidence

Accretion disks are observed in a variety of astronomical systems. In young stellar objects (YSOs), they are a key component of star formation, feeding the growing star with material. Around compact objects, such as black holes and neutron stars in X-ray binary systems, accretion disks are responsible for high-energy phenomena, including X-ray emissions and relativistic jets. In active galactic nuclei (AGN), supermassive black holes accrete material from their surroundings, producing some of the most luminous objects in the universe.

Importance in Astronomy

Accretion disks are fundamental to our understanding of a wide range of astronomical phenomena. They play a critical role in the formation and evolution of stars, the growth of black holes, and the dynamics of binary star systems. The study of accretion disks also provides insights into the processes of mass transfer and angular momentum distribution in the universe.