The isotopic 6Li/7Li ratio in Centaurus X-4 and the origin of Li in X-ray binaries

Free access article

Issue		A&A Volume 470, Number 3, August II 2007


Page(s)		1033 - 1041
Section		Stellar structure and evolution
DOI		http://dx.doi.org/10.1051/0004-6361:20066875

A&A 470, 1033-1041 (2007)
DOI: 10.1051/0004-6361:20066875

The isotopic ⁶Li/⁷Li ratio in Centaurus X-4 and the origin of Li in X-ray binaries

J. Casares¹, P. Bonifacio^{2, 3, 4}, J. I. González Hernández^{1, 2, 3}, P. Molaro^{3, 4}, and M. Zoccali⁵

¹ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
    e-mail: jcv@iac.es
² CIFIST Marie Curie Excellence Team
³ Observatoire de Paris, GEPI, 92195 Meudon Cedex, France
    e-mail: [Piercarlo.Bonifacio;Jonay.Gonzalez-Hernandez]@obspm.fr
⁴ Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
    e-mail: molaro@ts.astro.it
⁵ Pontificia Universitad Catolica de Chile, Chile
    e-mail: mzoccali@puc.cl

(Received 5 December 2006 / Accepted 4 May 2007)

Abstract
Context.Cool stars, companions to compact objects, are known to show Li abundances which are high compared to field stars of the same spectral type, which are heavily Li depleted. This may be due either to Li production or Li preservation in these systems.
Aims.Our aim is to measure the lithium isotopic ratio in the companion star of the neutron star X-ray binary Cen X-4.
Methods.We use UVES spectra obtained in years 2000 and 2004 around the orbital quadratures. The spectra are analysed with spectrum synthesis techniques and the errors estimated with Monte Carlo simulations.
Results. We measure A(Li) = 2.87 $\pm$ 0.20 and ⁶Li/⁷Li = 0.12 $^{\rm +0.08}_{\rm -0.05}$ at 68% confidence level. We also present updated system parameters with a refined determination of the orbital period and component masses i.e. 1.14 $\pm$ 0.45 ${M}_\odot$ and 0.23 $\pm$ 0.10 ${M}_\odot$ for the neutron star and companion, respectively.
Conclusions. In our view the low level of ⁶Li favours Li preservation scenarios, although Li production mechanisms cannot be ruled out. In the case of preservation, no Li is freshly created in the binary, but the tidally-locked companion has preserved its original Li by some mechanism, possibly inhibited destruction due to its fast rotation.

Key words: accretion, accretion disks -- stars: binaries: close -- stars: individual: Cen X-4 (=V822 Cen) -- X-rays: binaries

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