Polar jet
Encyclopedia
A polar jet is a phenomenon often seen in astronomy
, where streams of matter are emitted along the axis of rotation
of a compact object. It is usually caused by the dynamic interactions within an accretion disc
. When matter is emitted at speeds approaching the speed of light
, these jets are called relativistic jet
s. The largest polar jets are those seen in active galaxies
such as quasar
s. Other systems which often contain polar jets include cataclysmic variable star
s, X-ray binaries
and T Tauri star
s. Herbig–Haro objects are caused by the interaction of polar jets with the interstellar medium
. Bipolar outflow
s or jets may also be associated with protostar
s (young, forming stars), or with evolved post-AGB
stars (often in the form of bipolar nebula
e)
While it is still mostly a mystery to physicists how polar jets are formed and powered, the two most often proposed origins of this power are the central object (such as a black hole
), and the accretion disc
. Accretion disc
s around many stellar objects are able to produce jets, although those around a black hole are the fastest and most active. This is because the speed of the jet is around the same speed as the escape velocity
of the central object. This makes the speed of a jet from an accreting black hole near the speed of light, while protostar
jets are much slower. While it is not known exactly how accretion disc
s manage to produce jets, they are thought to generate tangled magnetic field
s that cause the jets to collimate
. The hydrodynamics
of a de Laval nozzle
may also give a hint to the mechanisms involved.
One of the best ways of exploring how polar jets are produced is to determine the composition of the jets at a radius where they can be directly observed. For example, if a jet originates in the accretion disc
, its plasma
is likely to have electron-ion composition, whereas if it originates in the black hole
it will likely be electron-positron in nature. Also, the plasma emits various forms of radiation such as X-ray
s and radio waves
, which aid diagnosis.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, where streams of matter are emitted along the axis of rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
of a compact object. It is usually caused by the dynamic interactions within an accretion disc
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...
. When matter is emitted at speeds approaching the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, these jets are called relativistic jet
Relativistic jet
Relativistic jets are extremely powerful jets of plasma which emerge from presumed massive objects at the centers of some active galaxies, notably radio galaxies and quasars. Their lengths can reach several thousand or even hundreds of thousands of light years...
s. The largest polar jets are those seen in active galaxies
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
such as quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s. Other systems which often contain polar jets include cataclysmic variable star
Cataclysmic variable star
Cataclysmic variable stars are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state...
s, X-ray binaries
X-ray binary
X-ray binaries are a class of binary stars that are luminous in X-rays.The X-rays are produced by matter falling from one component, called the donor to the other component, called the accretor, which is compact: a white dwarf, neutron star, or black hole.The infalling matter releases...
and T Tauri star
T Tauri star
T Tauri stars are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.-Characteristics:...
s. Herbig–Haro objects are caused by the interaction of polar jets with the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
. Bipolar outflow
Bipolar outflow
A bipolar outflow represents two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with protostars , or with evolved post-AGB stars ....
s or jets may also be associated with protostar
Protostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
s (young, forming stars), or with evolved post-AGB
Protoplanetary nebula
A protoplanetary nebula or preplanetary nebula is an astronomical object which is at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch phase and the subsequent planetary nebula phase. A PPN emits strongly in infrared radiation, and is a...
stars (often in the form of bipolar nebula
Bipolar nebula
A bipolar nebula is a distinctive nebular formation characterized by an axially symmetric bi-lobed appearance.Many, but not all, planetary nebulae exhibit an observed bipolar structure...
e)
While it is still mostly a mystery to physicists how polar jets are formed and powered, the two most often proposed origins of this power are the central object (such as a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
), and the accretion disc
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...
. Accretion disc
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...
s around many stellar objects are able to produce jets, although those around a black hole are the fastest and most active. This is because the speed of the jet is around the same speed as the escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
of the central object. This makes the speed of a jet from an accreting black hole near the speed of light, while protostar
Protostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
jets are much slower. While it is not known exactly how accretion disc
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...
s manage to produce jets, they are thought to generate tangled magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s that cause the jets to collimate
Collimated light
Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...
. The hydrodynamics
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
of a de Laval nozzle
De Laval nozzle
A de Laval nozzle is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass-shape...
may also give a hint to the mechanisms involved.
One of the best ways of exploring how polar jets are produced is to determine the composition of the jets at a radius where they can be directly observed. For example, if a jet originates in the accretion disc
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...
, its plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
is likely to have electron-ion composition, whereas if it originates in the black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
it will likely be electron-positron in nature. Also, the plasma emits various forms of radiation such as X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s and radio waves
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...
, which aid diagnosis.