Mini-Problem Set IX: Dating Stars Using Lithium: Burning Due 13 Feb

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Mini-Problem Set IX: Dating stars using Lithium: Burning due 13 Feb 2015 We use the results from the previous two problem sets to calculate actual times at which Lithium is depleted. We use numbers from, and compare with results of, Bildsten et al. (1997, ApJ 482, 442 [as before, you do not need the reference, but feel free to look it up if you want to]). 1. We start with getting numbers for the power-law approximation to the reaction rate. Bildsten −2/3 −1/3 6 et al. give that NAhσvi = S0fscrT6 exp(−aT6 ), where NA ≡ 1/mH, T6 ≡ T/10 K and 7 3 −1 −1 S0 = 7.2 × 10 m s kg , fscr ≃ 1, and a = 84.72 (we will discuss the meaning of these numbers – and how to calculate a – when we discuss nuclear fusion). For a reference temperature 6 Tr =3 × 10 K, show that, rounded to the nearest integer value, ν ≡ d lnhσvi/d ln T = 19. What 6 is NAhσvir? Check the power-law approximation at 2.5 and 3.5 × 10 K. 2. For ν = 19 and n =1.5, the integral over ξ in the previous problem set equals approximately 0.05. Use this, X =0.7, the numbers from item 1 and the results from mini problem set VIII for R and Tc to write dln XLi/dt in terms of M, Teff , µ, and t (scaling to 0.1 M⊙, 3000K, 0.6, and 1Myr, respectively). Integrate this equation, and determine the time required to reduce XLi by a factor 2. Compare both your numerical value and your exponents on M, Teff , and µ with the numerical −0.663 −3.50 −2.09 result of Bildsten et al., tdepl = 50.7 Myr(M/0.1 M⊙) (Teff /3000 K) (µ/0.6) . How much longer is the time it takes for Lithium to be reduced by a factor 20 (i.e., an additional factor 10)? 3. Jeffries and Oliviera (2005, MNRAS 358, 13) find that in NGC 2457, stars with mass less than 0.17 M⊙ still have Lithium, and they infer an age of ∼34Myr. Does this match what you would infer? (Note: if you were unsure of your own time scale, use that of Bildsten et al.). For presentation (and further thought): What fraction of the total luminosity will be produced by lithium burning? If you take the proton-proton cycle instead of lithium burning, what luminosity do you get for a 0.3 M⊙ zero-age main-sequence star? (Which is completely convective – see mini-PS V; see there and Fig. 3.3 for basic properties.).

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