The Lithium Isotope Ratio In Old Stars

Extract

The lithium isotope ratio in stars can be determined from high resolution observations of the profile of the Li I 6707 Å absorption line. Earlier studies of old F and G stars (Andersen et al. 1984, Maurice et al. 1984, Pilachowski et al. 1989) have led to upper limits of ⁶Li/⁷Li ranging from 0.05 to 0.10. Recently, Smith, Lambert & Nissen (1993) seem to have detected ⁶Li in HD 84937 - a metal-poor turnoff star with T_eff ⋍ 6200 K and [Fe/H] ⋍ —2.4. An isotope ratio ⁶Li/⁷Li = 0.05 ± 0.02 was determined (see Fig. 1) The detection has been confirmed by Hobbs & Thorburn (1994), who derived ⁶Li/⁷Li = 0.07 ± 0.03. The main contribution to the quoted (1σ) errors comes from the noise in the spectrum (S/N = 400) and possible errors in the Doppler broadening of the Li line (Nissen 1994). This broadening is due to stellar rotation and macro-turbulent motions in the stellar atmosphere and can be determined from the profiles of unblended metallic absorption lines.

As discussed in detail by Steigman et al. (1993) the presence of ⁶Li in the atmosphere of HD 84937 is consistent with the measured Be abundance (Boesgaard & King 1993) within the context of i) Standard Big Bang nucleosynthesis, ii) Pop. II cosmic ray nucleosynthesis and iii) standard (non-rotating) models for Li depletion. In particular, Steigman et al. derive D₆ > 0.2, where D₆ is the depletion factor for ⁶Li. As shown by Chaboyer (1994) standard stellar evolution models with new opacities predict D₆≃ 0.4 for turnoff stars and subgiants with T_eff > 5900 K. The same models predict D₇ ≃ 1.0, i.e. no ⁷Li depletion for main sequence stars as well as subgiants with T_eff ≥ 5800 K.