⁶Li/⁷Li estimates for metal-poor stars

Abstract

Context. The presence of the lithium-6 isotope in some metal-poor stars is a matter of surprise because of the high values observed. Non-standard models of Big Bang nucleosynthesis and pre-Galactic cosmic ray fusion and spallation have been proposed to explain these values. However, the observations of this light isotope are challenging which may make some detections disputable. Aims. The goal was to determine 6 Li/ 7 Li for a sample of metal-poor stars; three of them have been previously studied and the remaining two are new for this type of study. The purpose was to increase, if possible, the number of lithium-6 detections and to confirm previously published results.Methods. Spectra of the resonance doublet line of neutral lithium Li i 670.78 nm were taken with the high dispersion spectrograph at the Subaru 8.2 m-telescope for a sample of five metal-poor stars (−3.12 ≤ [Fe/H] ≤ −2.19). The contribution of lithium-6 to the total observed line profile was estimated from the 1D-LTE analysis of the line asymmetry. Results. Observed asymmetries could be reproduced assuming isotopic abundance ratios 6 Li/ 7 Li of the order of: 0.004 for BD +26 • 3578, ∼0.010 for BD +02 • 3375 and G 64-37, 0.025 for BD +20 • 3603 and 0.047 for BD −04 • 3208. We found that these results were very sensitive to several of the assumptions made in the analysis, in particular, the treatment of the residual structure in the analysed spectra. Our final estimates for the errors are respectively Δ 6 Li/ 7 Li = ±0.028, 0.029, 0.039, 0.025 and 0.039. Conclusions. The 6 Li/ 7 Li ratios for the sample are comparable to or even lower than these error values, so that detections of lithium-6 can not safely be claimed despite of the high resolving power (R ∼ 95 000) and S /N (400−600).

A three-dimensional hydrodynamical line profile analysis of iron lines and barium isotopes in HD 140283

“…Synthetic barium profiles were, like the iron profiles, the result of an interpolation of profiles synthesised The isotope ratio was determined with a χ 2 routine modified from that of García Pérez et al (2009), which fits synthetic barium profiles with varying barium abundance and isotope configuration. The wavelength shift is also measured by shifting the observed profile so that the best fit can be attained, through the χ 2 minimum.…”

The barium isotopic fractions in five metal-poor stars

“…To recap on Gallagher et al (2010), we use a χ 2 code (derived from that of García Pérez et al 2009) to compare the observed and synthetic spectra of a number of Fe i and Fe ii lines, computed for different abundances A(Fe) 3 , wavelength shifts Δλ, and line broadening parameters. Two different broadening approaches are used, as described below.…”

Discussion D: Observational problems with Li, Be and B

“…In addition, one has to be very careful with the flat-fielding and rectification of the spectra to ensure that the continuum near the Li i 6708Å line is accurate to better than 0.2 %. In this connection, it was mentioned that the non-detection of 6 Li in G 64-37 ( 6 Li/ 7 Li = 0.01 ± 0.04) by García Pérez et al (2009) (in contrast to the 3-sigma detection by Asplund & Meléndez quoted above) may be due to problems with residual fringes after flat-fielding of the Subaru/HDS spectra, as also discussed by García Pérez et al themselves.…”