Astron. Astrophys. 348, 211-221 (1999)

1. Introduction

Measurements of the abundance of the ⁶Li isotope in stellar atmospheres are of considerable interest and have attracted much attention since the first detection of ⁶Li in the metal-poor turnoff star HD 84937 by Smith et al. (1993). The reason for this interest is threefold:

i) Detection of ⁶Li in halo turnoff stars puts strong limits on the possible depletion of ⁷Li, and thus allows better determination of the primordial ⁷Li abundance from the observed Li abundance of stars on the `Spite plateau'. (Copi et al. 1997, Pinsonneault et al. 1998)

ii) ⁶Li abundances as a function of [Fe/H] provide an additional test of theories for the production of the light elements Li, Be and B by interactions between fast nuclei and ambient ones (Ramaty et al. 1996, Yoshii et al. 1997, Fields & Olive 1999a,b, Vangioni-Flam et al. 1999).

iii) Information on depletion of ⁶Li as a function of stellar mass and metallicity puts new constraints on stellar models in addition to those set by ⁷Li depletion. This is so because the proton capture cross section of ⁶Li is much larger than that of ⁷Li. Hence, at a given metallicity there will be a mass interval, where ⁶Li but not ⁷Li is being destroyed according to standard stellar models (Chaboyer 1994).

Altogether, ⁶Li abundances may contribute to the study of such different fields as Big Bang nucleosynthesis, cosmic ray physics and stellar structure. It will, however, require a rather large data set of ⁶Li abundances to get information in all these areas. The most metal-poor stars around the turnoff are of particular interest in connection with the determination of the primordial ⁷Li abundance, whereas more metal-rich halo stars and disk stars are of interest for the study of the formation and astration of the light elements.

Recent studies of ⁶Li abundances have concentrated on halo stars. Following the first detection of ⁶Li in HD 84937 by Smith et al. (1993) at a level corresponding to an isotopic ratio of ⁶Li/⁷Li 0.05, Hobbs & Thorburn (1994, 1997) have confirmed the detection, and found upper limits of ⁶Li/⁷Li for 10 stars. More recently, Smith et al. (1998) report the probable detection of ⁶Li in another halo star BD +26 3578 with about the same metal abundance, mass and evolutionary stage as HD 84937, and give tight upper limits of ⁶Li/⁷Li for 7 additional stars. Finally, Cayrel et al. (1999a) have observed the LiI line in HD 84937 with very high S/N and confirmed the presence of ⁶Li with a high degree of confidence.

In the case of disk stars there has not been any systematic search for ⁶Li since the studies of Andersen et al. (1984) and Maurice et al. (1984). In these papers an upper limit of ⁶Li/⁷Li of about 0.10 is set for about 10 disk stars ranging in metallicity from -1.0 to +0.3. The meteoritic ⁶Li/⁷Li ratio is close to 0.08 (Anders & Grevesse 1989) and the interstellar ratio is similar - possibly with significant variations (Lemoine et al. 1995). For metal-poor disk stars the ratio may be considerably higher than in the solar system. According to recent models for the galactic evolution of the light elements (Vangioni-Flam et al. 1999, Fields & Olive 1999a,b) the ⁶Li/⁷Li ratio reaches a maximum of about 0.3 at a metallicity of . At higher metallicities the ratio decreases due to the production of ⁷Li in AGB stars, novae and supernovae of type II by the -process (Matteucci et al. 1995, Woosley & Weaver 1995, Vangioni-Flam et al. 1996). Hence, it seems well justified to look for ⁶Li in the metal-poor disk stars. Any detection will provide important constraints of the chemical evolutionary models, and with a large set of data it may also be possible to constrain the degree of ⁶Li depletion as a function of stellar mass and metallicity.

In the present paper we present results for the ⁶Li/⁷Li ratio for five metal-poor disk stars ranging in metallicity from -0.8 to -0.6. Very high S/N spectra of the LiI 6708 Å resonance line are presented in Sect. 3 and analyzed with model atmosphere techniques in Sect. 4. This has led to a rather clear detection of ⁶Li in the two stars with the highest masses and tight upper limits for ⁶Li in the other stars. The consequences of these results are discussed in Sect. 5.

© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999
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