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Lithium isotopic abundances in metal-poor halo stars

Martin Asplund, David L. Lambert, Poul Erik Nissen, Francesca Primas, Verne V. Smith

TL;DR

This study presents high-fidelity Li isotopic measurements in 24 metal-poor halo stars, using 1D LTE and selective 3D tests to derive ^6Li/^7Li from the Li 670.8 nm line and Li abundances from the subordinate Li 610.4 nm line. It finds a pronounced metallicity trend in 7Li with minimal scatter, implying a near-flat Spite plateau with a primordial abundance of log ε(^7Li) ≈ 2.04–2.15 (for [Fe/H] < −2.2), significantly below the WMAP/BBN prediction of log ε(^7Li) ≈ 2.65. Importantly, 6Li is detected in nine stars, including LP 815-43, suggesting a pre-Galactic origin and presenting a 6Li plateau that becomes tilted after accounting for pre-main-sequence depletion. The results imply that standard BBN cannot fully explain Li in these stars and point toward non-standard nucleosynthesis or stellar depletion scenarios, with consequences for cosmology and Galactic chemical evolution. The work underscores the need for further high-S/N observations, refined 3D non-LTE modeling, and broader metallicity coverage to resolve the Li enigmas and their implications for early-universe physics.

Abstract

Very high-quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESO's VLT/UVES for the purpose of determining Li isotopic abundances. The derived 1D, non-LTE 7Li abundances from the LiI 670.8nm line reveal a pronounced dependence on metallicity but with negligible scatter around this trend. Very good agreement is found between the abundances from the LiI 670.8nm line and the LiI 610.4nm line. The estimated primordial 7Li abundance is $7Li/H = 1.1-1.5 x 10^-10, which is a factor of three to four lower than predicted from standard Big Bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, 6Li is detected in nine of our 24 stars at the >2sigma significance level. Our observations suggest the existence of a 6Li plateau at the level of log 6Li = 0.8; however, taking into account predictions for 6Li destruction during the pre-main sequence evolution tilts the plateau such that the 6Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of 6Li in the very metal-poor star LP815-43. Such a high 6Li abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic ray spallation and alpha-fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible 6Li production channels include proto-galactic shocks and late-decaying or annihilating supersymmetric particles during the era of Big Bang nucleosynthesis. The presence of 6Li limits the possible degree of stellar 7Li depletion and thus sharpens the discrepancy with standard Big Bang nucleosynthesis.

Lithium isotopic abundances in metal-poor halo stars

TL;DR

This study presents high-fidelity Li isotopic measurements in 24 metal-poor halo stars, using 1D LTE and selective 3D tests to derive ^6Li/^7Li from the Li 670.8 nm line and Li abundances from the subordinate Li 610.4 nm line. It finds a pronounced metallicity trend in 7Li with minimal scatter, implying a near-flat Spite plateau with a primordial abundance of log ε(^7Li) ≈ 2.04–2.15 (for [Fe/H] < −2.2), significantly below the WMAP/BBN prediction of log ε(^7Li) ≈ 2.65. Importantly, 6Li is detected in nine stars, including LP 815-43, suggesting a pre-Galactic origin and presenting a 6Li plateau that becomes tilted after accounting for pre-main-sequence depletion. The results imply that standard BBN cannot fully explain Li in these stars and point toward non-standard nucleosynthesis or stellar depletion scenarios, with consequences for cosmology and Galactic chemical evolution. The work underscores the need for further high-S/N observations, refined 3D non-LTE modeling, and broader metallicity coverage to resolve the Li enigmas and their implications for early-universe physics.

Abstract

Very high-quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESO's VLT/UVES for the purpose of determining Li isotopic abundances. The derived 1D, non-LTE 7Li abundances from the LiI 670.8nm line reveal a pronounced dependence on metallicity but with negligible scatter around this trend. Very good agreement is found between the abundances from the LiI 670.8nm line and the LiI 610.4nm line. The estimated primordial 7Li abundance is $7Li/H = 1.1-1.5 x 10^-10, which is a factor of three to four lower than predicted from standard Big Bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, 6Li is detected in nine of our 24 stars at the >2sigma significance level. Our observations suggest the existence of a 6Li plateau at the level of log 6Li = 0.8; however, taking into account predictions for 6Li destruction during the pre-main sequence evolution tilts the plateau such that the 6Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of 6Li in the very metal-poor star LP815-43. Such a high 6Li abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic ray spallation and alpha-fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible 6Li production channels include proto-galactic shocks and late-decaying or annihilating supersymmetric particles during the era of Big Bang nucleosynthesis. The presence of 6Li limits the possible degree of stellar 7Li depletion and thus sharpens the discrepancy with standard Big Bang nucleosynthesis.

Paper Structure

This paper contains 34 sections, 3 equations, 23 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. Selection of Stars
  4. Spectroscopic Observations
  5. Data Reduction
  6. Radial Velocities and Binary Stars
  7. Analysis: Methods, Models and Input Data
  8. Stellar Model Atmospheres
  9. Spectral Line Formation
  10. Atomic Data
  11. Stellar Parameters
  12. Effective Temperature
  13. Surface Gravity and Absolute Magnitude
  14. Metallicity
  15. Oxygen abundance
  16. ...and 19 more sections

Figures (23)

  • Figure 1: Measured resolving power of UVES based on the FWHM of thorium lines in the echelle order containing the Li i 670.8 nm line. Filled circles: July 2000 (EEV CCD); Open circles: February 2002 (EEV CCD); open squares: August 2004 (MIT CCD). The curves are 3rd order fits to each set of data.
  • Figure 2: Comparison of the Li i region in the July 2000 spectrum of HD 140283 with the February 2002 spectrum (shifted by 0.05). The lower panel shows the difference between the two spectra.
  • Figure 3: Sample spectra around the Li i 670.8 nm line with the spectra of CD $-48\hbox{$^\circ$}2445$ and CD $-33\hbox{$^\circ$}3337$ shifted 0.10 and 0.20, respectively. Note, that the region between the Li i and the Ca i lines in the spectrum of CD $-33\hbox{$^\circ$}3337$ is affected by several faint lines, which are not identified on the figure.
  • Figure 4: Observed H$\alpha$ profile for BD $+03\hbox{$^\circ$}0740$ (dots) compared to theoretical profiles calculated for atmospheric models with the gravity and metallicity of the star (log $g$$= 4.04$, [Fe/H] $= -2.65$) and three values of the effective temperature: $T_{\rm eff}$ = 6170 K, 6270 K (best fit), and 6370 K. The spectral bands used in the definition of the H$\alpha$ index is marked.
  • Figure 5: Difference in $T_{\rm eff}$ as estimated from H$\alpha$ and from $(b-y)$ and $(V-K)$ photometry calibrated on the infrared flux method as a function of $T_{\rm eff}$ ( upper panel) and [Fe/H] ( lower panel).
  • ...and 18 more figures