Since Li is not formed by standard big bang nucleosynthesis, its detection in metal-poor stars has far-reaching consequences. A team of astronomers, led by a scientist from Paris Observatory, puts into question all measurements of the isotopic ratio Li/Li obtained so far, since the asymmetry of the lithium lines has not been taken into account. Using a high-quality spectrum of HD 74000 (S/N ratio of 600 per pixel and spectral resolution of 120,000) obtained with HARPS at ESO, the authors demonstrate that line asymmetries generated by convective Doppler shifts in the atmosphere of metal-poor stars result in an excess absorption in the red wing of the Li absorption feature that mimics the presence of Li. A reappraisal of Li abundance determinations in halo stars is thus in order.
Several groups have tried to explain this unexpected result. The ideas range from a pregalactic alpha+alpha fusion process in accelerated, metal-free supernova ejecta from primordial massive stars, to scenarios involving supersymmetric particles. However, severe energetic difficulties in these attempts have been pointed out, especially in view of the discrepancy between the cosmological abundance of Li derived from WMAP and the observed value of the Spite plateau, implying that much more Li has been formed than what is observed.
The present exploratory study is based on a 20 hour exposure of the metal-poor halo TO star HD 74000, taken with the spectrograph HARPS, mounted at the ESO La Silla 3.6m telescope. The S/N ratio per pixel is 600, typical of what is needed for the Li measurement. The amount of convective line asymmetry in this star was determined from five unblended iron lines, selected to have a similar strength and ionization stratification as the the Li resonance line at 670.8 nm. The average of their profiles, rescaled to a common central depth, and convolved with an additional thermal broadening to correct for the lower mass of Li with respect to Fe, is shown in Fig. 1 (red open circles). The convective line asymmetry is clearly visible. Moreover, it is shown to be large enough to mimic a Li blend at the level found by Asplund et al. (2006).
In addition, this purely observational approach was checked by numerical simulations. For that purpose, a 3D non-LTE code was developed by M. Steffen (AIP) and R. Cayrel (GEPI) and applied to a hydrodynamical model of the 3-dimensional convective atmosphere of HD 74000 (Fig. 2), computed with the CO5BOLD code. The resulting synthetic line profiles exhibit a convective asymmetry that is very similar in shape to the asymmetry derived from the five Fe I lines, only slightly less in amplitude. A great merit of the theoretical approach is that it has supplied, on top of a justification of the observed line asymmetry, a substantial line shift (to the red), observed but never explained before, now clearly connected with 3D hydrodynamical effects.
Roger Cayrel (GEPI, Obs-Paris), Matthias Steffen (AIP, Germany), Hum Chand (IAP, Paris), Piercarlo Bonifacio (GEPI, CIFIST, INAF-OAT), Monique Spite, François Spite (GEPI, Obs-Paris), Patrick Petitjean (IAP, Paris), Hans-Günter Ludwig (GEPI, CIFIST), Elisabetta Caffau (GEPI, Obs-Paris):
Line shift, line asymmetry and the Li/Li isotopic ratio determination.
Astron. Astrophys. 473 (2007), L37.
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Dr Matthias Steffen
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