T Tauri Stars (Variable Stars)
The stars that vary irregularly in brightness are called T TAURI VARIABLES, after their prototype in the dark clouds of Taurus. The light curve of one such variable. T Tauri stars have absorption-line spectra typical of F, G or K giants, consistent with their location above the main sequence. The lines are broadened, probably due to fairly rapid rotation, or possibly due to some other large-scale atmospheric motion. Superimposed on the absorption-line spectra are strong emission lines due to hydrogen, calcium and other elements in the star. There is also an emission continuum in the violet portion of the spectrum, which may also be due to hydrogen. The emission lines and continuum are formed in the chromosphere – the middle and upper portion of the atmosphere of the star. The spectrum also contains FORBIDDEN LINES, which cannot arise in a dense stellar atmosphere, but must arise in a region of much lower density. These lines, and the strong infrared radiation from these stars, presumably come from the cloud of gas and dust which still surrounds the star.
Many T Tauri stars are observed to have their spectral lines systematically shifted to shorter wavelengths. This, according to the Doppler principle, means that the material in the atmosphere of the star is moving outward towards the observer. The amount of material leaving the star can be calculated from the velocity of motion (up to 300kms”1) and from the strength of the lines; it amounts to about one ten-millionth of a solar mass per year. Since a star spends several million years in contracting to the main sequence, it may lose a significant amount of mass while it is a T Tauri star.
There is a subclass of the T Tauri stars, called the YY ORIONIS stars, in which the spectral lines are systematically shifted to the red, indicating that material is falling onto the star. These may be very young T Tauri stars, in which material is still contracting onto the star. In normal T Tauri stars, this inflow has been halted and reversed by some process that drives material away from the star. There are two subclasses of T Tauri stars called RW AURIGAE stars and T ORIONIS stars. Their observed light curves differ somewhat from those of normal T Tauri stars, and there is some evidence that they represent different evolutionary phases, but the evidence is rather weak.
What processes cause the emission lines, the light variations and the outflow of material in T Tauri stars ? Our own Sun may provide a clue, because these phenomena all occur, to a minor extent, during a solar flare. A solar flare is a localized brightening of the Sun’s surface, followed by the heating of the Sun’s chromosphere and the ejection of matter. Solar flares are caused by a complex interaction between rotation, convection and the Sun’s magnetic field. If T Tauri stars rotate fairly rapidly, and if convection is important in their outer layers (which it is in stars of F, G and K type) and if magnetic fields are strong in young stars, then it is not surprising that chromospheric activity is great in young stars.