T-Tauri Stars

In Chapter 21 we shall consider variable stars, which are stars that vary their light output over time. Variable stars may be classified according to whether they change brightness with a well-defined period (regular variables) or in a less predictable way (irregular variables). A particular class of red (spectral class F-M) irregular variable star called a T-Tauri Variable is found often in association with standard indicators of star birth such as young O and B stars and dust clouds. The class is named for the prototype, the variable star called T-Tauri, which is located in the constellation Taurus. T-Tauri variables have several unusual characteristics.

  • They exhibit emission lines with properties suggesting the presence of low-density material surrounding the stars.
  • They are more luminous than main sequence stars of the same spectral class, suggesting that they are larger than the corresponding main sequence stars.
  • They exhibit strong winds called T-Tauri winds, with bipolar jet outflows at velocities as high as 300-400 km/s. Unusual nebulosity called Herbig-Haro Objects is often found in the direction of the outflows. (Astronomers often used the noun nebulosity in loose reference to objects with a nebula-like appearance that may not be traditional nebulae.)

    • This information suggests that T-Tauri stars are stars in the process of formation that are still contracting to the main sequence. They are more luminous than their main-sequence counterparts because the protostar is larger than the main sequence star to which it will collapse, and the accretion and outflow expected for collapsing protostars explains the surrounding low density material and strong winds. The following HR diagram for a very young cluster supports this interpretation.

    The stars marked in yellow are stars that exhibit at least one observational characteristic associated with T-Tauri stars (horizontal bars on a symbol indicate the presence of emission lines and vertical bars indicate a variable star). Notice that only the most luminous and hottest stars of the cluster have made it to the main sequence. Most of the lower luminosity stars, especially most of the ones with T-Tauri character, are still above the main sequence and moving toward it.