T 4 2 Tauri al (APOD 2009 August 3)

[neufer]

http://antwrp.gsfc.nasa.gov/apod/ap090803.html
http://antwrp.gsfc.nasa.gov/apod/ap010604.html
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<<John Russell Hind FRS (May 12, 1823 – December 23, 1895) was an English astronomer born in Nottingham. At age 17 he went to London to serve an apprenticeship as a civil engineer, but through the help of Charles Wheatstone he left engineering to accept a position at the Royal Greenwich Observatory under George Biddell Airy. Hind remained there from 1840 to 1844, at which time he succeeded W. R. Dawes as director of the private observatory of George Bishop. In 1853 Hind became Superintendent of the Nautical Almanac, a position he held until 1891.

Hind is notable for being one of the early discoverers of asteroids. He also discovered and observed the variable stars R Leporis (also known as Hind's Crimson Star), U Geminorum, and T Tauri (also called Hind's Variable Nebula), and discovered the variability of μ Cephei. Hind discovered Nova Ophiuchi 1848 (V841 Ophiuchi), the first nova of modern times (since the supernova SN 1604).>>

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<<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. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri; but it is actually 420 light years behind it and was not formed with the rest of them. Like all T Tauri stars, it is very young, being only a million years old. Its distance from Earth is about 580 light years, and its apparent magnitude varies unpredictably from about 9.3 to 14.

The T Tauri system consists of at least three stars, only one of which is visible at optical wavelengths; the other two shine in the infrared and one of them also emits radio waves. Through VLA radio observations, it was found that the young star (the "T Tauri star" itself) dramatically changed its orbit after a close encounter with one of its companions and may have been ejected from the system.

Physically nearby is NGC 1555, a reflection nebula known as Hind's Nebula or Hind's Variable Nebula. It is illuminated by T Tauri, and thus also varies in brightness. The nebula NGC 1554 was likewise associated with T Tauri and was observed in 1868 by Otto Wilhelm von Struve, but soon disappeared or perhaps never existed, and is known as "Struve's Lost Nebula".

A Herbig-Haro object also appears to be associated with Hind's nebula, or perhaps with the T Tauri system itself.

The T Tauri wind, so named because this young star is currently in this stage, is a phase of stellar development between the accretion of material from the slowing rotating material of a solar nebula and the ignition of the Hydrogen that has agglomerated into the protostar. A protostar is the denser parts of a cloud core, typically with a mass around 104 solar masses in the form of gas and dust, that collapses under its own weight/gravity, and continues to attract matter.

The protostar, at first, only has about 1% of its final mass. But the envelope of the star continues to grow as infalling material is accreted. After a few million years, thermonuclear fusion begins in its core, then a strong stellar wind is produced which stops the infall of new mass. The protostar is now considered a young star since its mass is fixed, and its future evolution is now set.>>

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<<T Tauri stars (TTS) 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. T Tauri stars are pre-main sequence stars – the youngest visible F, G, K, M spectral type stars (<2 Solar mass). 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 winds. Another source of brightness variability are clumps (protoplanets and planetesimals) 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 pre-main sequence later 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. 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 disks, which in this case are called protoplanetary discs because they are probably the progenitors of planetary systems 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 fields 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 masses)—A and B spectral type pre-main sequence stars, are called Herbig Ae/Be stars. More massive (>8 Solar mass) stars 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.>>

[rigelan]

I usually get the impression that the skies are unchanging. Except for the planets, nothing seems to ever be different. But on the time-scale of astronomy these stars ARE mere young children. And these things do change.

T Tauri stars are now generally recognized as young -- less than a few million years old -- sun-like stars still in the early stages of formation.

[Chris Peterson]

I usually get the impression that the skies are unchanging. Except for the planets, nothing seems to ever be different. But on the time-scale of astronomy these stars ARE mere young children. And these things do change.

And many things change even on a human time scale, although you have to look carefully to see them. Over years (sometimes even months) nebulas change shape and individual stars change positions with respect to others. Stars vary in brightness- sometimes over hours.