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New high S/N observations of the 6Li/7Li blend in HD 84937 and two other metal-poor stars *
R. Cayrel 1,
M. Spite 2,
F. Spite 2,
E. Vangioni-Flam 3,
M. Cassé 3,4 and
J. Audouze 3
Received 21 July 1998 / Accepted 4 January 1999
High signal to noise ratio spectra have been obtained with the GECKO spectrograph at CFHT, at a spectral resolution of 100 000, for three metal-poor stars in order to obtain more accurate abundances of the very fragile element 6Li. For two newly observed stars, BD +42 2667 and BD +36 2165 it appears that the first may have a detectable amount of 6Li, whereas no 6Li is found in the second one. The S/N ratio of only a few hundreds obtained for these two faint stars preclude however a firm conclusion. For the third star, the well known object HD84937, a very high S/N of 650 per pixel (over 1000 per resolved spectral element) was obtained, yielding greatly improved accuracy over previous determinations. A value of 6Li / 7Li = (one sigma) is obtained. We also conclude that the no- 6Li assumption is ruled out at the 95 per cent level, even in the most permissive case, when a variation of all the other free parameters (wavelength zero-point, continuum location, macroturbulent broadening, abundance of 7Li) is allowed.
The possibility that the 6Li feature is an artifact due to a once suspected binarity of HD 84937 is discussed, with the conclusion that this assumption is ruled out by the extant data on the radial velocity of the object. The 6Li abundance is compared with recent models of formation of the light elements Li, Be and B. This comparison shows that 6Li is either undepleted, or only moderately depleted in HD 84937, from its initial value. Under the assumption that the atmospheric depletion of 6Li and 7Li in stars is by slow mixing with hot layers (underneath the convective zone), in which these elements can burn, we conclude that the depletion of 7Li by this mechanism in HD 84937 is less than 0.1 dex.
This new upper limit to the efficiency of the depletion of 7Li by slow mixing burning, in a star located on the Spite plateau, leads to a more secure estimation of the primordial abundance of 7Li. However, the effect of temperature inhomogeneities in the convective zone, on the derived abundance of lithium still remains to be accurately determined.
Key words: stars: abundances stars: individual: BD +362165 stars: individual: BD +422667 stars: individual: HD 84937 stars: Population II cosmology: observations
Send offprint requests to: M. Spite
Online publication: March 1, 1999