T Tauri star
Encyclopedia
T Tauri stars are a class of variable star
s named after their prototype – T Tauri
. They are found near molecular cloud
s and identified by their optical variability
and strong chromospheric
lines.
). Their surface temperatures are similar to those of main sequence
stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low for hydrogen fusion
. Instead, they are powered by gravitational energy released as the stars contract towards the main sequence
, which they reach after about 100 million years. They typically rotate with a period between one and twelve days, compared to a month for the Sun, and are very active and variable.
There is evidence of large areas of starspot
coverage, and they have intense and variable X-ray
and radio
emissions (approximately 1000 times that of the Sun). Many have extremely powerful stellar wind
s. Another source of brightness variability are clumps (protoplanet
s and planetesimal
s) in the disk surrounding T Tauri stars.
Their spectra show a higher lithium
abundance than the Sun and other main sequence stars because lithium is destroyed at temperatures above 2,500,000 K. From a study of lithium abundances in 53 T Tauri stars, it has been found that lithium depletion varies strongly with size, suggesting that "lithium burning
" by the P-P chain
, during the last highly convective and unstable stages during the later pre–main sequence phase of the Hayashi contraction
may be one of the main sources of energy for T Tauri stars. Rapid rotation tends to improve mixing and increase the transport of lithium into deeper layers where it is destroyed. T Tauri stars generally increase their rotation rates as they age, through contraction and spin-up, as they conserve angular momentum. This causes an increased rate of lithium loss with age. Lithium burning will also increase with higher temperatures and mass, and will last for at most a little over 100 million years.
The P-P chain for Lithium burning is as follows
|- style="height:2em;"
| ||+ || ||→ |||| ||
|- style="height:2em;"
| ||+ || ||→ || ||+
|- style="height:2em;"
| ||+ || ||→ || || ||(unstable)
|- style="height:2em;"
| || || ||→ ||2 ||+ energy
|}
It will not occur in stars with less than sixty times the mass of Jupiter. In this way, the rate of lithium depletion can be used to calculate the age of the star.
Roughly half of T Tauri stars have circumstellar disk
s, which in this case are called protoplanetary discs because they are probably the progenitors of planetary system
s like the solar system. Circumstellar discs are estimated to dissipate on timescales of up to 10 million years. Most T Tauri stars are in binary star
systems. In various stages of their life, they are called Young Stellar Objects
(YSOs). It is thought that the active magnetic field
s and strong solar wind
of Alfvén waves of T Tauri stars are one means by which angular momentum
gets transferred from the star to the protoplanetary disc. A hypothesised T Tauri stage for our Solar System
would be one means by which the angular momentum
of the contracting Sun
was transferred to the protoplanetary disc and hence, eventually to the planets, resulting in the theory that before our own Sun matured, it was once a T Tauri star.
Analogs of T Tauri stars in the higher mass range (2–8 solar mass
es)—A and B spectral type pre–main sequence stars, are called Herbig Ae/Be stars. More massive (>8 Solar mass) star
s in pre–main sequence stage are not observed, because they evolve very quickly: when they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are main sequence
objects.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s named after their prototype – T Tauri
T Tauri
T Tauri is a variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind...
. They are found near molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
s and identified by their optical variability
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
and strong chromospheric
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
lines.
Characteristics
T Tauri stars are pre–main sequence stars – the youngest visible F, G, K, M spectral type stars (<2 Solar massSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
). Their surface temperatures are similar to those of main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low for hydrogen fusion
Proton-proton chain reaction
The proton–proton chain reaction is one of several fusion reactions by which stars convert hydrogen to helium, the primary alternative being the CNO cycle. The proton–proton chain dominates in stars the size of the Sun or smaller....
. Instead, they are powered by gravitational energy released as the stars contract towards the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
, which they reach after about 100 million years. They typically rotate with a period between one and twelve days, compared to a month for the Sun, and are very active and variable.
There is evidence of large areas of starspot
Starspot
Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...
coverage, and they have intense and variable 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...
and radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
emissions (approximately 1000 times that of the Sun). Many have extremely powerful stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
s. Another source of brightness variability are clumps (protoplanet
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...
s and planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...
s) in the disk surrounding T Tauri stars.
Their spectra show a higher lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
abundance than the Sun and other main sequence stars because lithium is destroyed at temperatures above 2,500,000 K. From a study of lithium abundances in 53 T Tauri stars, it has been found that lithium depletion varies strongly with size, suggesting that "lithium burning
Lithium burning
Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium-7 and a proton producing two helium-4 nuclei. The temperature necessary for this...
" by the P-P chain
Proton-proton chain reaction
The proton–proton chain reaction is one of several fusion reactions by which stars convert hydrogen to helium, the primary alternative being the CNO cycle. The proton–proton chain dominates in stars the size of the Sun or smaller....
, during the last highly convective and unstable stages during the later pre–main sequence phase of the Hayashi contraction
Hayashi track
The Hayashi track is a path taken by protostars in the Hertzsprung–Russell diagram after the protostellar cloud has reached approximate hydrostatic equilibrium...
may be one of the main sources of energy for T Tauri stars. Rapid rotation tends to improve mixing and increase the transport of lithium into deeper layers where it is destroyed. T Tauri stars generally increase their rotation rates as they age, through contraction and spin-up, as they conserve angular momentum. This causes an increased rate of lithium loss with age. Lithium burning will also increase with higher temperatures and mass, and will last for at most a little over 100 million years.
The P-P chain for Lithium burning is as follows
- {| border="0"
|- style="height:2em;"
| ||+ || ||→ |||| ||
|- style="height:2em;"
| ||+ || ||→ || ||+
|- style="height:2em;"
| ||+ || ||→ || || ||(unstable)
|- style="height:2em;"
| || || ||→ ||2 ||+ energy
|}
It will not occur in stars with less than sixty times the mass of Jupiter. In this way, the rate of lithium depletion can be used to calculate the age of the star.
Circumstellar disk
A 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...
s, which in this case are called protoplanetary discs because they are probably the progenitors of planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...
s like the solar system. Circumstellar discs are estimated to dissipate on timescales of up to 10 million years. Most T Tauri stars are in binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
systems. In various stages of their life, they are called Young Stellar Objects
Young stellar object
Young stellar object denotes a star in its early stage of evolution.This class consists of two groups of objects: protostars and pre–main sequence stars. Sometimes they are divided by mass - massive YSO , intermediate mass YSO and brown dwarfs....
(YSOs). It is thought that the active 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 and strong solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
of Alfvén waves of T Tauri stars are one means by which angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
gets transferred from the star to the protoplanetary disc. A hypothesised T Tauri stage for our Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
would be one means by which the angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
of the contracting Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
was transferred to the protoplanetary disc and hence, eventually to the planets, resulting in the theory that before our own Sun matured, it was once a T Tauri star.
Analogs of T Tauri stars in the higher mass range (2–8 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es)—A and B spectral type pre–main sequence stars, are called Herbig Ae/Be stars. More massive (>8 Solar mass) star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s in pre–main sequence stage are not observed, because they evolve very quickly: when they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
objects.