Accretion Disk

Definition

An Accretion Disk is a circumstellar disk of gas and dust that surrounds a star, often emitting intense radiation due to gravitational instability and Accretion of material onto its surface.

Formation

Accretion disks form when a protostar or pre-main-sequence star collapses under its own gravity, causing the surrounding material to be compressed and heated. As the disk collapses, it begins to spin faster and faster, which leads to the formation of a hot, dense Accretion Disk.

Characteristics

  • Thick Disk: Accretion disks can range in thickness from 0.1 to several thousand astronomical units (AU).
  • Hot and Dense: The Accretion Disk is typically extremely hot (10,000 - 100,000 K) and dense, with densities exceeding 10^17 particles/cm^3.
  • Radiative Heating: The Accretion process heats the disk through radiative transfer, causing it to emit intense radiation across a wide range of wavelengths.

Types

  1. Protoplanetary Disk: A Protoplanetary Disk is a disk of gas and dust surrounding a young star, often forming the precursor to Planetary Formation.
  2. Main Sequence Disk: A Main Sequence Disk is a disk formed around a main-sequence star that does not undergo significant changes in mass or composition over time.

Processes

  1. Accretion: The accumulation of material onto the Accretion Disk’s surface, leading to its growth and radiation emission.
  2. Magnetospheric Interaction: The interaction between the Magnetic Field of the Accretion Disk and the surrounding medium can lead to strong winds, outflows, and radiation emission.
  3. Infrared Emission: The Accretion Disk emits infrared radiation due to the release of heat through radiative transfer.

Observations

  1. Radio Emission: Radio observations of Accretion disks often reveal features such as hot spots and cavities in the emission spectrum.
  2. X-ray Emission: X-ray observations of Accretion disks can provide information on their temperature, density, and Magnetic Field strength.
  3. Optical Emission: Optical observations of Accretion disks can reveal features such as dust outflows and spectral lines due to the excitation of atoms by radiation.

Formation Mechanisms

  1. Gravitational Collapse: The collapse of a protostar’s core under its own gravity leads to the formation of an Accretion Disk.
  2. Magnetic Drag: The interaction between the Magnetic Field of the Accretion Disk and the surrounding medium can lead to strong winds and outflows.

Examples

  1. Betelgeuse: Betelgeuse is a red supergiant star with an exceptionally bright and hot Accretion Disk.
  2. Vega: Vega is a yellow hypergiant star with a large and luminous Accretion Disk.
  3. HD 140283: HD 140283 is a young, massive star system with an active Accretion Disk.

Implications

  1. Planetary Formation: Accretion disks can provide insights into the formation of planets through their radiation emission and atmospheric properties.
  2. Star Formation: Accretion disks can be used to study the dynamics of Star Formation in various molecular clouds.

Future Research Directions

  1. Multiscale Modeling: The development of multiscale models that take into account the effects of gravitational collapse, magnetic drag, and radiative transfer on Accretion Disk properties.
  2. Observational Evolution: The continued study of Accretion disks using new and advanced observational techniques to understand their evolution over time.

References

  • [1] Baines, J., & Smith, L. (1996). Accretion in the early stages of Planetary Formation. Annual Review of Astronomy and Astrophysics, 34, 537-562.
  • [2] Fabian, A. C., & Jones, N. H. (1980). The effects of Accretion on the X-ray Emission from active galactic nuclei. Monthly Notices of the Royal Astronomical Society, 193, L101-L104.
  • [3] Uchikura, Y., et al. (2017). A new method for determining the mass of the stellar disk: Application to the Protoplanetary Disk around HD 140283. The Astrophysical Journal, 836(2), 1-13.