Astron. Astrophys. 343, 923-932 (1999)

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 <sup>3

1</sup> Observatoire de Paris, 61 avenue de l'Observatoire, Paris, France
² Observatoire de Paris-Meudon, F-92125 Meudon Cedex, France
³ Institut d'Astrophysique, 98bis bd Arago, Paris, France
⁴ DAPNIA, CEA-Saclay, F-91191 Gif-sur-Yvette Cedex, France

Received 21 July 1998 / Accepted 4 January 1999

Abstract

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

* Based on observations made at the Canada-France-Hawaii Telescope and at Observatoire de Haute Provence

Send offprint requests to: M. Spite

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Analysis
• 4. HD 84937
  • 4.1. Fitting the observations with a synthetic spectrum
  • 4.2. Could HD 84937 be a binary star?
  • 4.3. Why is ⁶Li found, among very metal-poor stars, only in HD 84937?
• 5. The two other stars
• 6. Discussion
• 7. Conclusions
• Acknowledgements
• Appendix
• References

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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