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