T Tauri phase

Fig. 2.2 The state of the incipient solar system during the T Tauri phase of the young sun. The central region around the sun was blown free from the primeval dust cloud. Behind the shock front is the disc with the remaining solar nebula, which contained the matter formed by the influence of the solar wind on the primeval solar nebula. From Gaffey (1997)... 

Fig. 5. The T Tauri-phase fluxes (top) compared to present solar fluxes (Ackermann, 1971) for ultraviolet and visual wavelengths. The strong emission lines of Lyman-a (1215 A), Mgll (2800 A) and Balmer-a (6563 A) can be seen in the T Tauri spectrum. Fluxes have been reduced to those intercepted at a distance of 1 AU from the star.

Fig. 6. T Tauri-phase and present solar fluxes converted to photons per bin, as required by the photochemical code. The discontinuity near 6500 A represents a change in the bin size.

Fig. 7. The ratio of T Tauri-phase fluxes to the present solar fluxes, using a logarithmic scale. Although the enhancement of the T Tauri over the solar fluxes is seen at all wavelengths, it is the most dramatic in the far-ultraviolet.

The enhancement of the T Tauri-phase ultraviolet flux over the present sun s fluxes is clearly very wavelength-dependent (see also Table 1). Thus use of this average T Tauri spectrum should represent a significant improvement to our wavelength-independent treatment used previously as input for the photochemical models (Canuto et al., 1982). 

Table 1 - Solar energy flux averaged over the Earth during T-Tauri phase (Values in parentheses correspond to today).

The spectacular events associated with the T-Tauri phase are probably the result of nuclear reactions beginning in the star s core. 

Recall that the pattern just described applies to stars of about the same mass as our own Sun. Evolutionary patterns differ for stars with greater or lesser masses. For example, more massive stars may begin their evolution in much the same way as solar-size stars, hut they take less time and a somewhat different pathway during the Hayashi and T-Tauri phases of their lives. More massive stars travel more quickly across the H-R diagram, reaching the Main Sequence in a million years or less, while less massive stars may take up to a billion years before they "settle down" into the Main Sequence. 

Before stars like the Sun move onto the main sequence they go through the T Tauri phase with a circumstellar or protoplanetaiy disk. Mid-infrared imaging... 

After about one million years (for solar-mass stars, this process is much faster for higher masses), the combination of outflow and infall disperses the majority of the envelope and the star is optically revealed, although a circumstellar disk is still present. For solar-mass stars, this is the T Tauri phase, while for intermediate masses, these stars are referred to as Herbig Ae/Be stars (Hillenbrand et al. 1992). Several million years after the primordial disk has almost disappeared. 

The lunar soil is an ideal repository for implanted solar wind elements, as are certain gas-rich meteorites. Deuterium is depleted relative to the terrestrial standard in these materials, the D H ratio of <3 X 10 being consistent with the hypothesis that D is converted into He in the proto-Sun. Ion probe mass spectrometry has been used to study Mg, P, Ti, Cr and Fe which are present to enhanced levels in lunar minerals, indicating an exposure age of approximately 6 X 1Q4 y. The isotopic data indicate that the light isotopes of a number of elements have been preferentially lost from lunar material because of volatilization by micrometeorites or solar wind bombardment. There is some indication, from a study of Ne in gas-rich meteorites, of a large solar flare irradiation during the early history of the Solar System, perhaps related to the T-Tauri phase of the Sun. 

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