Astron. Astrophys. 362, 666-672 (2000)

1. Introduction

The rare light elements (Li, Be, and B) are probes of the early universe, Galactic evolution, and stellar structure. Beryllium has a special place in the general scheme of nucleosynthesis, being the lightest stable nuclide not synthesized in the Big Bang. Together with and , it is considered a pure product of cosmic-ray (CR) spallation nucleosynthesis, being generated only by the bombardment of and by protons and -particles (Reeves et al. 1970; Meneguzzi et al. 1971). This unique origin has made it a particularly useful monitor of time-integrated factors of Galactic evolution such as the product of particle fluxes and abundance of targets, since its production during the Galactic epoch appears to be limited to the interstellar medium (ISM). Recent studies of Be in halo stars (e.g. Molaro et al. 1997, Boesgaard et al. 1999) have suggested that the nucleosynthesis processes responsible for its formation may be more complex than previously supposed; the linearity observed in the trend [Be/H] vs. [Fe/H] ¹ cannot be easily reproduced by spallation reactions between -particles and protons hitting CNO in the ISM. Hence, the study of the evolution of Be in the Galaxy is an important constraint of Galactic cosmic-ray (GCR) theory. The above-mentioned linearity, in fact, seems to support the idea that Type II supernovae (SN) accelerate freshly synthesized C and O and subsequently fragment into Be and B (Vangioni-Flam et al. 1998). New data, especially at low metallicities (below [Fe/H] = -3.0), are essential to distinguish between different hypotheses, like, for instance, the mass interval of the SN progenitor.

Although this linearity strongly suggests a Galactic origin for Be, some inhomogeneous Big Bang Nucleosynthesis models (IBBN) have shown to be able to produce beryllium abundances as high as log (Be/H) = -13.00 (Kajino & Boyd 1990; cf. Orito et al. 1997 for a more recent review), i.e. potentially observable in very metal-deficient stars. Such Big Bang component may appear as a constant Be-plateau, independent of metallicity, similar to what is found for lithium (cf. Spite & Spite 1982), but beryllium has been analyzed in one star only (BD -13^o3442) at [Fe/H]3.0 (Boesgaard et al. 1999). Thus, this hypothesis has not been fully discarded yet.

Here, we report on our very recent attempt to measure beryllium in two of the most metal-poor stars ever observed in the spectral region near the atmospheric cut-off, where the Be lines fall (around 3130 Å). These two new measurements will be compared to the current observational picture and we will show that this type of observations and measurements are now well within reach of UVES at VLT.

© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000
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