Accretion disk

Image taken by the Hubble Space Telescope of a growing disc surrounding the black hole of the nucleus of the elliptical galaxy NGC 4261.

An accretion disk or accretion disk is a disk-shaped structure made of gas and dust revolving around a massive central object. The disk material, due to loss of rotational energy, tends to decay towards the center, where the mass adds to that of the central object. The dynamics of these astrophysical objects is mainly governed by the law of conservation of angular momentum. The disc can be vertically extended giving rise to a toroidal type structure. Accretion disks can be found around black holes, active galactic nuclei (in English more called by its acronym: AGN Active Galactic Nuclei), or around very young stars in the process of formation. In the latter case, they are also called circumstellar disks and planetary systems usually originate from disks of this type. Later, the difference in matter densities in the accretion disk will cause clusters to form from where the rest of the elements of the system finish forming, such as planets and their satellites.

In these high energy density astrophysical systems, charged particles such as protons and atomic nuclei can be accelerated to relativistic speeds and generate ultra-high energy cosmic rays, which are a type of cosmic radiation with a broader energy spectrum. higher than usual observed in astronomy.

As an example, when honey is slowly dropped from a container, a kind of accretion disk is created between the Earth and the honey in the container, due to gravity. This is basically an accretion disk as it forms on similar gravitational principles. It could be said that, on Earth, the consistency of honey (and similar liquids) behaves similar to that of the stellar mass that constitutes an accretion disk, which is made of the plasma that makes up stars.

A star or other body located in a binary system can also form an accretion disk by stealing matter from its companion's outer layers. This matter forms a ring around the captor star, being able to fall on its surface after describing a spiral trajectory. Due to the enormous speeds that matter reaches in said fall, a strong emission of X-rays is observed, which has served to detect objects that do not emit radiation on their own, such as neutron stars or black holes. These binary systems are known as X-ray binaries.

Formation of accretionary disks

Artistic representation of a growing disc in a star fed by material from her binary companion.

The disk is a common structure in the universe. Both galaxies and stars have formed at the same time in accretion disks of very different dimensions. The reason that such common structures originate from formless gas clouds is simple. Almost every mass of gas has a certain angular momentum, a minimal amount of rotation. That is, the huge clouds that collapse to form these structures rotate initially, albeit very slowly. The rotating gas system is held in a delicate equilibrium that can be broken by a pressure wave from a supernova or by reaching a critical amount of mass, for example. When instability occurs and the cloud is compressed by the increasing effect of gravity, it begins to undergo certain changes that will lead it to form a disk.

As the cloud is compressed, it rotates faster due to conservation of angular momentum. But this spin only occurs along its plane of spin. In the areas of greatest rotation, the centrifugal force acquires greater and greater intensity. This increasingly pronounced asymmetry is what, little by little, ends up giving shape to the record. The regions overlying and underlying the plane of spin, ie the poles, fall free at high speed while the gas rotating along the plane is greatly slowed down by the increasing centrifugal force. Thus, the combined action of rotation and gravity is what, in the end, will give the characteristic disk shape.

The most active accretion disks present strong jets of material emission along the axis of rotation. This phenomenon is commonly called ambipolar diffusion. The structure and nature of the jet emission mechanisms are not precisely known, although it is believed that they have to do with the presence of a strong magnetic field. The strongly ionized central material spits out a part of itself through field lines that act as guides.

Accretion disks around young stars

Artistic vision of a protoplanetary disc.

Secondary dust disk around the AU Microscopii system. Image of Hubble Space Telescope.

The formation of a star from a cloud of molecular gas is a process that takes place on time scales between 10⁵ and 10⁶ years. Since angular momentum must be conserved, most of the material initially falls onto an accretion disk that slowly accumulates on the central star. Angular momentum is redistributed towards the outer regions of the disk, that is, most of the accretionary mass on the central star while a small part of the outer material spreads away and carries with it the angular momentum necessary to produce the inner accretion. These disks have lifetimes of 1-10 Myr. Young stars show signs of accretion through excess infrared (disk presence) and ultraviolet (material accretion) emission. The disk, illuminated and heated by the central star, can be seen in some astronomical images in the infrared and millimeter wave ranges. Discs that cannot be resolved optically (spatial extent less than instrument resolution) can be detected by means of the Spectral Energy Distribution (SED) Spectral Energy Distribution) which exhibits excess emission in the infrared.

In the case of multiple systems, it has been verified that two different configurations of growth disks can occur: either a disk is formed around each of the system components and a common disk around all of them, or directly a common disk is formed around the system components, without "individual" disks.

In young stars that are already within the main sequence and with ages around 100 million years, secondary disks of dust can be observed without significant remnants of gas orbiting the central star. These second-generation disks would form from destructive impacts between planetesimals left over from planetary formation capable of producing a large amount of dust.

Growth disks around compact objects

Often, in binary systems where one of the stars is a compact object, such as a pulsar or a black hole, observations show hints of material flowing from the bright star toward the compact object. This occurs when the star has its outer layers inside the Roche limit of the compact object. The uprooted material flows over said object forming an accretion disk around it. In the case of black holes, matter becomes so accelerated that radiation emissions from the vortex are in the X-ray wavelength. X-ray sources are often, in fact, a tell-tale clue. Your presence.