Aging, timescale

Studies on minerals and crystallization ages timescales of crystallization... 

U-Th mineral isochrons to date ancient volcanic rocks and thus deduce their ( Th/ Th)o ratios at the time of eruption (if isochron ages are close to eruption ages, see section 3.5 below), or alternatively rely on samples dated by another method (e g., K-Ar, Ar-" °Ar) to recalculate their initial Th isotope ratios. On the shorter timescale covered by Ra- Th disequilibria (0-8 ka), it can be even more difficult to find a suite of well dated samples, because eruption ages should be obtained from historical accounts... 

Abundance results for additional clusters are currently underway and include analyses of the neutron-capture elements (in order to trace the onset of contributions from low-mass Type II SNe as well as AGB stars). Combined with their ages, the nucleosynthetic histories of the outer halo clusters will better constrain the timescales of formation and construction of the Galaxy. 

In the next sections I will present the main results from these recent Li and Be datasets. In particular, I will address the following questions 1. What are the timescales of Li depletion 2. Is the Sun typical 3. What parameter drives Li depletion at old ages 4. Do stars deplete Be at the same rate as Li Whereas I will mostly focus on stars with masses close to solar, I mention in passing that light element depletion is also strongly dependent on stellar mass. 

Solar-type stars start destroying Li soon after their arrival on the ZAMS. The difference in the average Li between the Pleiades and the 2 Gyr clusters implies depletion timescales of the order of 1 Gyr. After 2 Gyr they become extremely long for part of the stars (Li depletion virtually stops) or very short for another fraction. The Sun belongs to the second category and, as such, it is not representative of all stars with the same mass, age, and metallicity. Li depletion is clearly not a monotonic function of age and at least one additional... 

What happens for cooler (i.e. less massive) stars on the red side of the Li dip As we shall see now, the stellar mass or the effective temperature of the dip is a transition point for stellar structure and evolution. First of all it is a transition as far as the rotation history of the stars is concerned. Indeed the physical processes responsible for surface velocity are different, or at least operate with different timescales on each side of the dip. At the age of the Hyades, the stars hotter than the dip still have their initial velocity while cooler stars have been efficiently spun down (Fig. 1). This behavior is linked to the variation of the thickness of the superficial H-He convection zone which gets rapidly deeper as Teff decreases from 7500 to 6000K (e.g. TC98). Below 6600 K, the stars have a sufficiently deep... 

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... 

The lifetime of the molecular cloud is considered to be a time line running from cloud formation, star evolution and finally dispersion in a period that is several tci. The chemistry of the TMC and, to a good approximation, all molecular clouds must then be propagated over a timescale of at most 20 million years. The model must then investigate the chemistry as a function of the age of the cloud, opening the possibility of early-time chemistry and hence species present in the cloud being diagnostic of the age of the cloud. The model should then expect to produce an estimated lifetime and the appropriate column densities for the known species in the cloud. For TMC-1 the species list and concentrations are shown in Table 5.4. 

The half-life is independent of the initial number of nuclei. For 14C decay the half-life is 5717 years, whereas the 238U decay half-life is 4.5 billion years. Carbon dating works well for timescales in the recent past and is used for dating objects such as the Turin shroud, but 238U is better for timescales of the age of the solar system. 

Fowler s argument essentially applies to the thin disk, for which his age estimate of 10 Gyr is in impressive agreement with independent stellar age estimates (see Fig. 8.41). Pagel (2001) extended the argument to include the thick disk by supposing that the initial spike (taking S to be 0.4 in accordance with Fig. 8.40, rather than 0.2) resulted from thick-disk (or bulge) formation on a short timescale at an... 

Volume percent of sedimentary rock as a function of age. Source-. From Ronov, A. B. (1964). Geochemistry International 1, 713-737. Note Eon timescales have since been redefined. 

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