Institute in General
   Events / Jobs
   Public Relations
   Research Groups
   Internal Pages
Search Telephone list Feedback Impressum Links
last change 2007 October 26, R. Arlt
Stellar physics
The asymmetry of the Li lines casts doubt
on previous determinations of the isotopic ratio 6Li/7Li

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.

In a recent paper, Asplund, Lambert, Nissen et al. (2006) reported Li/Li values for 24 metal-poor halo stars. The result was far from expectations. As no Li is produced by the primordial nucleosynthesis, it was expected to see a rise of Li/Li ratio with stellar metallicity, corresponding to the Li production due to spallation by cosmic rays. Instead, the Li abundance was found to be independent of metallicity (Li plateau). The inferred mean value of Li/Li is 0.044, with a standard deviation of 0.012.

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 authors of the present study have taken a completely different approach to resolve this problem: to question the reality of the measurements indicating the Li plateau. The motivation is that the Li blend occurs in the red wing of the much stronger Li feature. Now, it is well known that line asymmetries are generated by Doppler shifts due to convective flows in stellar atmospheres, especially if the convective motions affect the line-forming photospheric layers, which is indeed the case for the metal-poor turn-off (TO) stars where Li was found. Analyses with 1D models ignore these line asymmetries, and therefore must attribute the extra absorption in the red wing to the presence of Li, resulting in spurious isotopic ratios.

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.

The conclusion is that it is absolutely mandatory to take the convective line asymmetry into account for the determination of the Li/Li isotopic ratio in metal-poor halo stars. Doing so, the result for HD 74000 is Li/Li = 0. ± 0.02. This work suggests that the abundance of Li derived in previous studies must very likely be revised towards lower values, potentially beyond the limit of significance. More detailed investigations must clarify the role of the convective line asymmetry as a function of stellar effective temperature, gravity and metallicity.


The amount of line asymmetry measured in the halo TO star HD 74000, for the mean of five Fe I lines having a stratification and a strength similar to the components of the doublet of the Li resonance line. The red open circles represent the observed asymmetric profile. The green open circles represent the mirrored (with respect to the vertical at line center) blue wing of the asymmetric profile. The difference between the green and the red points defines the size of the asymmetry. This difference is shown in red at flux level 1.04. The effect of a Li blend corresponding to a Li/Li of 4.4 per cent (the value of the Asplund et al. Li plateau) is shown as the green signal at flux level 1.02. The two features are strikingly similar. Clearly, these two signals are highly degenerate. The black solid line shows the sum of the green symmetric wing and the depression due to 0.044 Li signal. It is always within ±1 sigma of the asymmetric (red circles) wing, demonstrating that the two effects (convection/line blend) are observationally indistinguishable. The red dots indicate the bisector of the asymmetric profile.

3D hydrodynamical simulation of convection in the surface layers of a metal-poor star (Teff=6300 K, log g=4.0, [M/H]=-2, representative of HD 74000)). The top face of the rectangular box shows the stellar granulation pattern, as seen in continuum intensity, the side faces show the specific entropy. The time evolution of the flow is illustrated in this animation.




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.

Highlight at Astron. & Astrophysics



Dr Matthias Steffen
Astrophysikalisches Institut Potsdam
An der Sternwarte 16
D-14482 Potsdam
(0331) 7499 371


[Stellar physics]

[AIP home page]

[Previous highlights]      [Press releases]     [Image archive]