Abundance in halo stars

A weak but useful carbon line [Cl] 8727.13 A disappears in halo dwarfs with metallicities below —1. To measure carbon abundance in halo stars one can use four Cl high excitation lines near 9100 A and the CH band at 4300 A. The Cl lines at 9100 A together with the OI triplet at 7771 A have been used by Tomkin et al. (1992) and Akerman et al. (2004) to study the behaviour of C/O versus metallicity. However, Cl and OI lines employed in these papers are sensitive to a non-LTE effects and one has to bare in mind that this sensitivity is different for C and O. The CH band at 3145 A used by Israelian et al. (1999) is almost saturated in disk stars and several blends makes the abundance analysis less accurate. To ensure a homogeneous analysis of the C/O and N/O ratio from NH,CH and OH lines in the near-UV, we used the same model atmospheres and tools as in our previous studies. The oxygen abundances were compiled from Israelian et al. (1998, 2001) and Boesgaard et al. (1999). 

Spite and Spite (1982) and Spite et al. (1984) have determined the Li abundance in Halo stars (see also Hobbs and Duncan 1987). It is about 8 times smaller than the current... 

Boesgaard, A.M., Deliyannis, C.P., King, J.R., Ryan, S.G., Vogt, S.S. Beers, T.C. 1999 Beryllium abundances in halo stars from Keck/HIRES observations. A J 117, 1549. 

Ba/Eu] is low in halo stars, indicating that the s-process (which is the main source of Ba) has not had sufficient time to contribute to the abundances in halo stars, while the r-process (the main source of Eu) dominates in metal-poor stars. Indeed, very metal-poor halo giants show evidence for a purely r-process contribution to the abundances of heavy neutron capture elements (Sneden et al. 2000). 

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