Thermal timescale

The uncertainty in the age of pre main sequence stars is therefore of the order of the thermal timescale at the luminosity of D-burning smaller than a few times 105 yr for normal T Tauri, and larger than 106 yr for very low mass stars and brown dwarfs (BD). In fact, comparing observations spanning a wide range of masses we could even constrain the models, for example we can ascertain whether the Stahler et al. (1986) picture of collapse is valid also in the BD regime, or... 

In the absence of nuclear energy sources, a star contracts on a thermal timescale and radiates energy at the expense of gravitational potential energy. Since, by the Virial Theorem, the total energy... 

When the core hydrogen abundance drops to Xc < 0.5, any further increases in Tc fail to compensate for the drop in energy generation and the whole star starts to contract. As nuclear reactions are extinguished, the convective core vanishes, but only when Xc gravitational energy, with the core contracting on a thermal timescale tx 2 x 106 years. 

When the mass of the helium-layer accreted into the low-entropy core is sufficiently high, helium ignition will occur at the boundary between the non-degenerate and degenerate material. The star will expand on the short thermal timescale of the envelope ( 103 y) to become a yellow supergiant -a stage which will be described in more detail in Sect. 15.7. 

Once the star has fully expanded to become a yellow or red supergiant, a deep surface convection zone will develop which will dredge helium- and carbon-rich material to the surface, and will completely dilute what is left of the hydrogen envelope. As the star contracts for a second time, its surface will be helium- and carbon-rich. Any planetary nebula ejected at the tip of the AGB phase may still be ionized, since the nebula recombination time is long compared with the thermal timescale of the envelope. Additional helium-rich material may also have been ejected, or be visible as a stellar wind. 

For high enough values of Ga the oscillatory mode is the capillary-gravity wave. The time scales tgr, and cap associated with this twofold wave are much smaller than the viscous and thermal timescales (at least for Pr l, Bo l). Then dissipative effects are relatively weak and the dispersion relation is... 

I decays to the ground state by fluorescence emission with a lifetime of 3.3 ns (see above), then I returns to A in less than a nanosecond, a process that is slightly thermally activated, strongly sensitive to deuteration, and apparently involves several intermediates [118, 132, 144]. Because no production of the B state is observed during I deexcitation and return to A, states I and B must exchange on timescales at least slower than the nanosecond. 

Values of the thermal and dose shift rate factors are found using the equations at the temperature (T) and dose rates of interest. These factors are then used to determine the behaviour in service by shifting the service timescale (ts) by ... 

It is important to note that the reciprocal of the shear rate is the time taken for unit strain to occur in the material and is the characteristic timescale for our experiment. As long as the microstructure can reorganise by thermal motion in a shorter timescale, the value of De is less than 1 and the structure will remain in a configuration that is close to its... 

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