# Presentation: 702475

## Lithium isotopic abundance ratios in metal-poor star: 

- 4. Chi-square analysis of the Li I 6708 Å resonance line: 6Li/ 7Li
- Our best estimate of the lithium isotopic abundance corresponds to the minimum of the curves in the left panels of Fig. 3. Each 6Li/ 7Li point there has associated a wavelength shift () and a total lithium abundance (A(Li)) such that minimises the chi-square. The contour plots in Fig.3 shows r2  for our best  estimate. In the right panels, synthesis and observations are compared. Three different synthetic spectra are plotted: the fits for 6Li/ 7Li=0.00 and 0.10 and the best fits, 6Li/ 7Li=0.04 and 6Li/ 7Li=0.05 for BD-043208  and BD+263578, respectively._x0018__x0013_
-  Fig. 3 *Left*: Reduced chi-square r2 for  BD-043208 (top) and BD+263578 (bottom). *Right*: The observed Li I 6708 Å profile together with the computed lines for three different values for 6Li/ 7Li (0.00, 0.04 or 0.05, and 0.10).
**5. Results and conclusions**
- Our best fit for BD-043208 suggests 6Li/ 7Li values similar to values found in the literature, while for BD+263578, the fit suggests higher values (0.05) than the most recently published (0.01).
- We found that for some spectra, the result is sensitive to the parameters used in the analysis, such as the wavelength range adopted for the chi-square computation and also to exitexitchanges in the continuum level. Consequently, we refrain from claiming any 6Li detection before a more precise signal-to-noise and error analysis has been completed.
**References**
- Asplund 2006, ApJ, 644, 2_x0018__x0013_29
- Burbidge et al. 1957, *Reviews of Modern Physics*, 29, 547
- Coc et al. 2004, ApJ, 600, 544
- Spite F., & Spite, M. 1982, A&A,115
- _x0018__x0013_

**1. Stellar atmospheres of halo stars**
- Most of the chemical elements in the Universe are synthesised in stellar interiors (Burbidge et al. 1954). According to Big Bang calculations, only the light nuclei of hydrogen, deuterium, helium-3 and helium-4, and lithium-7 were synthesised in the epoch of nucleosynthesis. Hence, we are interested to study the chemical composition of the light elements in non-evolved stars of very low metal content whose composition resembles the primordial one_x0018__x0013__x0018__x0013_.
- The two stable lithium isotopes are 6Li and 7Li. 6Li is not produced in the Big Bang, and furthermore has a larger cross-section for destruction by proton fusion, and therefore is less abundant and difficult to detect in the spectra of metal-poor stars. There are published detections for only a handful of metal-poor stars (Smith et al 1998, Asplund et al. 2006 etc).
- The isotopic ratio 6Li /7Li has something to say about the discrepancy which exists between the 7Li abundance observed in metal-poor stars (Spite & Spite 1982) and the higher predicted value based on recent determinations of baryon-to-photon ratio  (see Fig. 1). If the explanation for the discrepancy is depletion in stars, why is 6Li, which is more fragile than 7Li, detected in some metal-poor_x0018__x0013_ stars?
- These detections of 6Li are challenging. It is important to confirm them and to add new detections to the picture. This may also help to discriminate between different models of 6Li production: cosmic-ray spallation processes associated to supernovae, to cosmological structure-formation shocks etc.
- 2. Measurement of lithium isotope ratio
- In the spectra of the Li I resonance line, the presence of the lithium-6 isotope is seen as a slightly asymmetry in the profile of the red wing. Detections of this isotope are based on fitting the observed spectra with synthesised spectra, and requires high quality data, both, observed and synthesised.
- _Observational data_
- High quality Subaru/HDS spectra, resolving power (R) ~ 100 000 and signal-to-noise ratios (*S/N*) of 620 and 480 for BD-043208  and BD+263578,  respectively.
- _Synthetic spectra_
- 1D-LTE synthesis was done using the spectral synthesis code BSYN and MARCS plane-parallel model atmospheres. The stellar parameters values effective temperature (Teff) and surface gravity (log g) were taken from the literature. We calculated the metalliciy ([Fe/H])) after measuring the equivalent widths of several Fe II lines from our spectra. The values of (Teff / logg / [Fe/H] / microturbulence) were: BD-043208  (6338/4.00/-2.21/1.5) and BD+263578  (6239/3.87/-2.25/1.5).
- _Method_
- Fitting was based on minimising the differences between the observations and the spectral synthesis. The minimum chi-square (2) procedure.
**3. Chi-square analysis of Ca ****I**** and Fe ****I**** lines: broadening parameter**
- We used a set of Ca I and Fe I lines (see Fig. 2) to fix the free parameter associated with the line broadening. This method has another free parameter, the element abundance  (A(Ca*)* or A(Fe)). The broadening parameter () is the combination of the macroscopic stellar and instrumental broadening. The left panel in Fig. 2 shows the variation of minimum r2 with  for the Ca I 6122 Å line and BD-043208. The right panels show the observations and the spectral synthesis associated to the minimum r2 value.
-  Fig.2 *Left*: Reduced chi-square r2 for BD-043208. *Right*: The observed Ca I 6122 Å profile of BD-043208, together with the computed lines for three different values for  (6.30, 6.6  and 6.90). Red bars indicate the wavelength range for the r2 calculations.

- Lithium isotopic abundance ratios in metal-poor star:
- a chi-square analysis of Subaru/HDS stellar spectra

- Ana Elia García Pérez1, Wako Aoki2, Susumu Inoue2 and Sean Ryan1
- 1 University of Hertfordshire, Hatfield, UK  2 National Astronomical Observatory of Japan, Mitaka, Tokyo, Japan

- Abstract
- Lithium is an element of great importance from a cosmological point of view. Observations of its light isotope lithium-6 (6Li) are very delicate. Detections for only a handful of metal-poor stars are found in the literature. Using the Subaru 8.2-m optical telescope and the High Dispersion Spectrograph (HDS), we have taken high quality spectra (*S/N* ~ 360-600, *R* ~ 100 000) of the Li I 6708 Å resonance line for a sample of several metal-poor stars. We present here the 1D-LTE analysis of the metal-poor stars BD-043208 and BD+263578. The isotopic contribution of 6Li, due to the possible presence of this isotope in the stellar atmospheres, is included in the analysis. The best fit to the observed spectra of these two stars suggest lithium isotopic abundance ratio (6Li/ 7Li) values of the order of 0.04 and 0.05 respectively. The fits are based on a minimum chi-square fitting procedure with three free parameters: the wavelength shift, the total lithium abundance and the lithium isotopic abundance ratio.

- 2= ∑(Oi – Si)2/2 , r2 = 2 /(n-np)
- (Oi observed data , Si synthetic data,  data noise,
-  n number of points, np number of free parameters)

- G64-37

- Fig. 1. Big Bang nucleosynthesis predictions  based on a standard cosmological model (Coc et al. 2004).