The T-Tauri Star and T-Tauri Star Evolution
Main Sequence Stars
A main sequence star, like the Sun, forms from a protostar. Protostars form from nebulae composed of dust and gas. The process takes millions of years. A process to reach equilibrium between inward gravity and outward gas pressure will create a star. But a T-Tauri star is the last step.
Jet Disks From Stars
Star Formation and Protostars
Stars begin to form in a nebula, this is a vast region of gas and dust; and can be light years in size. The gas and dust starts to coalesce due to gravitational forces. This is the beginning of a protostar, and it is a process astronomers call accretion.
A protostar will eventually become a star. But, In order to become a star the protostar must achieve and maintain equilibrium. It is abalance between gravity pulling atoms toward the center and gas pressure, caused by thermal energy released, as the particles collide with each other, pushing away from the center. Achieving and keeping these forces balanced is difficult.
The Steps Toward Equilibrium in a Protostar.
- The core pulls gas and dust inward through gravity.
- At the center of the core, temperatures will rise as collisions between gas and dust particles increase.
- This leads to increased density of the core, as additional atoms try to share the same space.
- The increase in density will cause thermal pressures to increase.
- Gravity pulls atoms inward, but there is push outward due to gas pressure, which increases with temperature.
- When the forces due to gas pressure equal gravity, the protostar has reached equilibrium.
Pleiades in the Taurus Constellation
The Lithium Signature
T-Tauri Stage of Evolution
Between Steps 5 and 6 - the T-Tauri stage.
As the protostar heats by gravitational energy, it will be surrounded by a large irregular mass of gas and dust. When the cloud contracts it is the result of gravity, step 5. The energy is converted to kinetic energy. and causes the protostar to spin.This material is pushed away making a protostellar wind. A disk is left over from the collapse and pushes the wind out into two narrow jets. One jet emerges from each side of the disk at a right angle to the plane of the disk. Now the protostar has become a T-Tauri star.
Size and Brightness
The typical mass of a T-Tauri Star has less mass that the Sun by about half; it is also varies in brightness. They are unstable and will remain that way until their interior temperatures become high enough to support nuclear fusion.
The Lithium Signature
A telltale signature of a T-Tauri star is that they have a high lithium concentration. Lithium is found in nebulae or in very young clusters. They have a low temperature with strong emission lines and broad absorption lines. Each atomic element has a different structure, causing it to produce (or absorb) a different set of wavelengths. This identifies the atom. This means that a substance will emit spectral lines (at a particular wavelength) when it is heated and absorb light at the same wavelength when it is cool.
Source: Spectrum of Rigel
Categories of T-Tauri Stars and Their Location
Categories of T-Tauri Stars
The astronomy of the T-Tauri stars now include two categories, Classical, and Weak-lined. Classical T-Tauri stars are noted for their extensive dust-gas cloud disks producing strong emission lines. The weak-lined T-Tauri stars, however, are surrounded by a dust-gas cloud disk that is very low density or may not even exist.
Where are they located?
They closest ones are about 460 light years (140 parsecs) away. The nearest T-Tauri stars occur in the Taurus Cloud and the Rho Ophiuchus Cloud, both are about the same distance. They are named after the prototype T-Tauri in that molecular cloud.
Main Sequence Stars
After several million years the T-Tauri star accumulates enough mass, and the beginnings of nuclear reactions are now occurring; the T-Tauri star explodes into a high profile-main sequence star with various amounts of brightness and mass. Stage 6 is achieved and the star is in equilibrium. It enters anywhere in the main sequence as shown on the graph: it is the long curve in the graph, stretching from upper left down to the lower right.
The appearance of a main sequence star on the graph depends on the star's luminosity and temperature. Over time, and depending on the star's evolution, it may become a super giant or a dwarf star, and the star will move out of the main sequence. The star will then move into one of several distinctive areas as indicated below in the diagram.
Summary: The T-Tauri Star
Star formation takes several steps and over millions of years to develop. Stars begin to form in nebulae where gas and dust clouds exist. Over a long period of time the gas-dust clouds begin to coalesce due to the gravitational pull from the accumulated mass. The result is that the stars enter into the protostar stage where there is some luminosity due to kinetic energy.
Over time gravitational reactions begin and the protostar enters a final stage called the T-Tauri star stage of evolution, which has a notable lithium signature. The star is not in a stage of equilibrium yet. Again, after a long period of time the star finally achieves equilibrium and it breaks into the main sequence with nuclear reactions occurring all the time.
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