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Big bang nucleosynthesis updated with the NACRE compilation
E. Vangioni-Flam 1,
A. Coc 2 and
M. Cassé 1,3
Received 27 March 2000 / Accepted 23 May 2000
We update the Big Bang Nucleosynthesis calculations on the basis of the recent NACRE compilation of reaction rates. The average values of the calculated abundances of light nuclei do not differ significantly from those obtained using the previous Fowler's compilation. However, is slightly depressed at high baryon to photon ratio . Concerning , its abundance is significantly lower than the one calculated with the Caughlan & Fowler (1988) rates as anticipated by Rauscher & Raimann (1997). We estimate the uncertainties related to the nuclear reaction rates on the abundances of D, , , , , , and of cosmological and astrophysical interest. The main uncertainty concerns the reaction rate affecting the synthesis of at rather high baryonic density and also the and reactions. On the left part of the lithium valley the uncertainty is reduced due to the improvement of the measurement of the reaction rate. The observed abundances of the nuclei of interest are compared to the predictions of the BBN model, taking into account both observational and theoretical uncertainties. Indeed, the abundance observed in halo stars (Spite plateau) is now determined with high precision since the thickness of this plateau appears, in the light of recent observations, exceptionnaly small ( 0.05 dex). The potential destruction/dilution of in the outer layers of halo stars which could mask the true value of the primordial abundance is in full debate, but the present trend is towards a drastic reduction of the depletion factor (about 0.10 dex). It is why we use this isotope as a preferred baryometer. Even though much efforts have been devoted to the determination of deuterium in absorbing clouds in the line of sight of remote quasars, the statistics is very poor compared to the long series of lithium measurements. Taking into account these lithium constraints, two possible baryonic density ranges emerge, and . In the first case, Li is in concordance with D from Webb et al. (1997) and from Fields & Olive (1998) and Peimbert & Peimbert (2000). In the second case, agreement is achieved with D from Tytler et al. (2000) and from Izotov & Thuan (1998).
Concerning the less abundant light isotopes, the theoretical BBN abundance of is affected by a large uncertainty due to the poor knowledge of the reaction rate. However, at high , its abundance is so low that there is little chance to determine observationally the true BBN abundance. But, at low , its abundance being one thousandth of that of primordial , 6/7 ratio measurements at very low metallicity are not totally hopeless in the future. Nevertheless, in the present situation, is cosmologically relevant, though indirectly, since its mere presence in a few halo stars, corroborates the fact that it is essentially intact in these stars together with and thus the Spite plateau can be used as such to infer the primordial abundance. The Be and B abundances produced in the Big Bang are orders of magnitudes lower, and spallation of fast carbon and oxygen is probably their unique source, in the early Galaxy.
Key words: cosmology: miscellaneous cosmology: early Universe
Send offprint requests to: E. Vangioni-Flam
Online publication: July 27, 2000