If the accreted material is not enough relative to the central celestial bodyangular momentumThe matter will flow radially to the central celestial body, forming spherically symmetric accretion.However, generally speaking, the accreted materials have large angular momentum. They do not fall directly onto the central object along the radial direction, but rotate around the central object to form a disk with poor rotation, called an accretion disk.The material at the inner edge of the disk falls into the central celestial body along a spiral orbit.A lot of energy is released in the process of accretion.This process is often accompanied by ejection phenomena, such asJet。Compact celestial bodyWith a strong gravitational field, accretion is very important.The accretion of compact celestial bodies is the most effective way to release energy, and its efficiency is even higher than that of nuclear reactions.Accretion theory is widely used in astrophysics.X-rayClose binary、Cataclysmic variable starMany observational phenomena of AGN needAccretion diskModel.
There are two types of accretion that are important in the universe.The first is the process of small particles colliding with each other and sticking together to form larger objects.Collisions must be "just right" for this to happen - if the collision is too violent, it will break objects (tear) rather than make them stick together.When the sun is born from a gas and dust cloud in spaceCollapseWhen the sun was young, a disk of matter was formed around the sun, which subsided toward the equatorial plane.This is very similar to the larger scale replica of Saturn's ring we see today.Planets and other celestial bodies in the solar system are formed by accretion in the rotating disk of matter composed of small particles no more than 1mm in size at the beginning.
The second type of accretion isMassive celestial bodyThrough itsgravitational fieldThe process of obtaining material from the surroundings.Ordinary like our sunfixed starIt is continuously accreting matter from interstellar space, but on a small scale.Celestial objects with strong gravitational field, such asneutron starandblack holeAnd its accretion is much stronger.Therefore, the material falling towards the celestial body (mostly from the neighboring companion stars in the binary system) forms aAccretion disk。Because matter gets energy when it falls in the gravitational field, atoms in the disk collide with each otheratomThe temperature of can become so high that X rays can be radiated.Such processes, which take place on a very large scale in the center of some galaxies containing black holes millions of times the mass of the sun, may providea quasarEnergy.
Accretion classification
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There are two types of accretion that are important in the universe.
The first is the process of small particles colliding with each other and sticking together to form larger objects.The collision must be just right for this to happen - if the collision is too violent, it will break objects rather than make them stick together.When the sun is born from a gas and dust cloud in space and collapses under its own gravity, a matter disk that settles toward the equatorial plane is formed around the young sun.This is very similar to the larger scale replica of Saturn's ring we see today.Planets and other celestial bodies in the solar system are formed by accretion in the rotating disk of matter composed of small particles no more than 1mm in size at the beginning.
The second type of accretion is the process that a massive object acquires material from its surroundings through the attraction of its gravitational field.Ordinary stars like the sun are constantly absorbing material from interstellar space, but on a small scale.The accretion of celestial bodies with strong gravitational fields, such as neutron stars and black holes, is much stronger.As a result, the material falling towards the celestial body (mostly from the nearby companion star in the binary system) forms an accretion disk.Because matter gets energy when it falls in the gravitational field, and atoms in the disk collide with each other, the temperature of atoms can become so high that they can radiate X-rays.Such processes take place at the center of some galaxies containing black holes with millions of times the mass of the sun on a very large scale, which may provide energy for quasars.
Spherically symmetric accretion
Spherically symmetric accretion is the simplest accretion process.Assuming that the density isρ, temperature isTThere is a static medium with mass ofMAnd the mass of medium particles ism, kinetic energy iskBT, define the accretion radius with the central celestial body as the centerRa：
amongcS isIsothermal sound velocity。The sum of kinetic energy and gravitational potential energy of particles located at the accretion radius is zero. The thermal motion of particles within the accretion radius is not enough to overcome the gravitational effect and are accreted by the central celestial body. Particles outside the accretion radius will not be accreted.Under the influence of diffusion of mediumAccretion rateThe total angular momentum of the accretion material is zero.
The accretion process of a point mass object moving in a medium with uniform density and low temperature is calledBondi Hoyle Littleton accretion(Bondi Hoyle Lyttleton Accretion), or Bondi accretion.If the central object has a velocity relative to the mediumVMotion, the kinetic energy of the particle is approximately, and the accretion radius at this time is called Bondi accretion radius:
The motion speed of the celestial body is generally much higher than the sound speed of the medium. The diffusion effect can be ignored. The accretion rate is about. If the accretion material does not have strict cylindrical symmetry, the total angular momentum is not zero, and an accretion disk can be formed.
Axisymmetric accretionIf the accretion material has high enough angular momentum, it is possible to form an accretion disk.The loss of angular momentum of the accretion material flow is generally very slow, but the energy is dissipated continuously, and finally it is located in the orbit with the smallest energy under the condition of a certain angular momentum, that is, the circular orbit, and it rotates around the central celestial body at almost Kepler speed.The radius of this track is called the rounding radius:
amonglIs the angular momentum of the accreted material per unit mass.The necessary condition for the formation of an accretion disk is that the radius of the celestial body is far less than the radius of rounding, otherwise the accretion material flow will fall directly onto the surface of the celestial body, and the accretion disk cannot be formed.
Accretion disk
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An accretion disk is a ring of material around a star or other celestial body, and the material in the ring spirals down to the celestial body in the disk.
Accretion materialCompact celestial bodyA disk formed around it.For compact stars without magnetic field, or in regions far away from strong magnetic field, the accretion movement is mainly controlled by the gravitational field of compact stars.At this time, if the accreted material does not have enough angular momentum, the incoming jet is radial, forming spherically symmetric accretion.If the accretion materials have large angular momentum, they will not fall directly onto the compact star along the radial orbit, but move around the compact star to form a disk that rotates around the compact star in angular difference, which is called an accretion disk.The matter on the accretion disk, affected by viscosity, rotates along a spiral path towards the surface of the star.Near the surface of a star, the density of matter increases rapidly and releases energy outward.The specific properties of accretion disks depend on the specific conditions of compact stars and the original physical properties of accretion materials.At present, the accretion disk model is often used to explain the energy mechanism of X-ray close binary stars.
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Scientists' observations on accretion disks
According to the US Science Daily, a group of international research teams observed an accretion disk around a black hole for the first time, and verified the real electromagnetic spectrum characteristics of the accretion disk predicted by astronomers for a long time.
Black holes and their bright accretion disks are thought to form quasars, which emit strong light at the center of some distant galaxies.The team used a polarizing filter to isolate the light emitted by the accretion disk, which is formed by other materials near the black hole.The members of the research team include Robert?Antonussi and Omai?Breyers.Antonussi said, "This research work is convincing and strong evidence to explain quasars."
Quasars are brilliant and bright cores of distant galaxies, located at the center of supermassive black holes, and can form brightness 1 trillion times brighter than the sun.Quasars may be a powerful energy source fueled by interstellar gas, which is believed to be sucked into black holes from the surrounding accretion disks.This latest study confirmed a long-standing prediction that accretion disks eject strong luminous radiation lines.
Antonussi pointed out that astronomers found that many physical processes used to explain the energy source and light formation of quasars were accompanied by some cosmic matter falling intoSupermassive black holeLike hovering around an accretion disk, the surface of a black hole is a spherical plane, marking the boundary of the black hole.In this process, friction heats cosmic matter to form spectra of all wavelengths, including infrared, visible and ultraviolet light.Finally, these cosmic matter fall into the black hole, thus increasing the mass of the black hole.He said, "If this is true, we can predict the physical rules that the electromagnetic spectrum of quasars should follow." But it is impossible to test this prediction at present, because astronomers cannot distinguish the difference between the light emitted by accretion disks and the dust particles and ionized gas clouds in black holes.
Through the United Kingdom in the Mauna Kea Mountains on the Hawaiian IslandInfrared telescope(UKIRT)?Kishmoto evaluated these extraneous rays and measured the spectrum of the accretion disk.The results show that the spectra match with the previous predictions.The researchers also used data from the European Space AgencyVery large telescopeWider range of data obtained by polarization analyzer.It is reported that Kishmoto is an astronomer of the Max Planck Radio Astronomy Association, and alsoUniversity of California, Santa Barbarapost-doctoral.
Polarization filter analysis shows that, in fact, direct light does not appear polarization, and it does not choose the direction of its electric field.Accretion disks emit direct light, just like dust particles and ionized gases.However, a small amount of light released from the accretion disk is exactly the kind of light that researchers expect to study, which is reflected by the gas very close to the black hole, thus polarized.
Antonussi said, "So our research plan is only forpolarizationLight, as if there is no other light here, we can see the true spectrum released by the accretion disk.On this basis, we can better understand how black holes consume phagocytic substances and expand. "
Antonussi said that through the spectral research and analysis of light-emitting objects such as quasars, a large amount of incredible and precious information about characteristics and processes can be obtained.Our understanding of the physical process of accretion disks is still very limited, but at least we are sure that we have a comprehensive understanding of this.
On September 1, 2023, Science published the latest research results of black hole accretion magnetic field mainly based on the observation results of Huiyan satellite in the form of a long article.The study found direct observational evidence for the formation of magnetic trapping accretion disks around black holes using the observation data of China's first space X-ray astronomical satellite, Huiyan, in combination with ground radio and optical telescopes.[1]
