e-Pulsar - Feb

e-Pulsar Jun - Jul 00

Astronomical Feature of the Month
T-Tauri Stars

    A question was raised on the list server about T Tauri stars, a perfect lead-in for a Feature of the Month... T Tauri stars (TTS) are young, solar-like stars seen near many molecular clouds in our galaxy. They are pre-main sequence stars that have temperatures and masses similar to the Sun, but they are brighter and have fast rotation rates (a few days compared to a month for the Sun). TTSs are active, variable stars.
    The first T Tauris were found in 1945 and were identified by their optical variability and strong chromospheric lines. There is evidence for large areas of starspot coverage plus some variable X-ray and radio emissions. Some have molecular outflows or strong stellar winds. About half of all T Tauri stars are surrounded by disks, though most TTSs are in binary systems. They tend to have more lithium than the Sun, an element easily destroyed at "low" core temperatures. The star’s energy is derived from gravitational collapse because the core is not hot enough for proton-proton fusion.
The clouds of gas which collapse are thought be rotating very slowly and to not have uniform density, being denser in their centers than near the edge. This leads to an inside-out collapse - meaning that the denser core collapses faster than the less dense outer regions of the cloud. The inside-out collapse leads to the formation of the forming star in the center of the cloud which then slowly builds up it’s mass by accreting the outer layers of the cloud.
    Another noteworthy aspect of this later stage of formation is that before the star actually get hot enough to ignite nuclear fusion, an intense stellar wind is generated. Often times because the cloud was slowly rotating, a disk of material forms around the star. The disk collimates the intense stellar wind into two oppositely directed beams - producing what is called a "bipolar flow."
    For the most part, the cloud has a chance to accrete onto the protostar before the violent states of evolution begin. Classical TTS are young, low mass pre-main sequence stars, with circumstellar accretion disks. Sounds like a subject that would make for an interesting painting or two....