# Circumstellar disc

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The star SAO 206462 has an unusual circumstellar disc

A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place and around white dwarfs, they indicate that planetary material survived the whole of stellar evolution. Such a disc can manifest itself in various ways.

## Young star

Circumstellar discs HD 141943 and HD 191089.[1]
Main article: Protoplanetary disk

According to the currently accepted model of star formation, sometimes referred to as the nebular hypothesis, a star is formed by the gravitational collapse of a pocket of matter within a giant molecular cloud. The infalling material possesses some amount of angular momentum, which results in the formation of a gaseous protoplanetary disc around the young, rotating star. The former is a rotating circumstellar disc of dense gas and dust that continues to feed the central star. It may contain a few percent of the mass of the central star, mainly in the form of gas which is itself mainly hydrogen. The accretion disc phase lasts a few to 10 million years. Accretion rates are typically 10−7 to 10−9 solar masses per year but can vary.

The disc gradually cools in what is known as the T Tauri star stage. Within this disc, the formation of small dust grains made of rocks and ices can occur, and these can coagulate into planetesimals. If the disc is sufficiently massive, the runaway accretions begin, resulting in the appearance of planetary embryos. The formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form.

## Circumstellar discs around the Solar System

Artist's impression of a transitional disc around a young star.[2]

## Binary system

• Circumprimary disc is where a disc orbits the primary (i.e. more massive) star of the binary star system[3]
• Circumsecondary disc is one around the secondary (i.e. less massive) star of the binary star system
• Circumbinary disc is where a disc orbits both the primary and the secondary of the binary system

## Dust

• Debris discs consist of planetesimals along with fine dust and small amounts of gas generated through their collisions and evaporation. The original gas and small dust particles have been dispersed or accumulated into planets.[4]
• Zodiacal cloud or interplanetary dust is the material in the Solar System created by collisions of asteroids and evaporation of comet seen to observers on Earth as a band of scattered light along the ecliptic before sunrise or after sunset.
• Exozodiacal dust is dust around another star than the Sun in a location analogous to that of the Zodiacal Light in the Solar System.

## Disc evolution

Circumstellar discs are not equilibrium objects, but instead are constantly evolving. The evolution of the surface density $\Sigma$ of the disc, which is the amount of mass per unit area so after the volume density at a particular location in the disc has been integrated over the vertical structure, is given by: $\frac{\partial \Sigma}{\partial t} = \frac{3}{r} \frac{\partial}{\partial r} \left[ r^{1/2} \frac{\partial}{\partial r} \nu \Sigma r^{1/2} \right]$ where $r$ is the radial location in the disc and $\nu$ is the viscosity at location $r$.[5] This equation assumes axisymmetric symmetry in the disc, but is compatible with any vertical disc structure.

Viscosity in the disc, whether molecular, turbulent or other, transports angular momentum outwards in the disc and most of the mass inwards, eventually accreting onto the central object.[5] The mass accretion onto the star $\dot{M}$ in terms of the disc viscosity $\nu$ is expressed: $\dot{M} = 3 \pi \nu \Sigma \left[ 1 - \sqrt{\frac{r_\text{in}}{r}} \right]^{-1}$ where $r_\text{in}$ is the inner radius.