Boron in Very Metal-Poor Stars

We have observed the B I λ 2497 Å line to derive the boron abundances of two very metal-poor stars selected to help in tracing the origin and evolution of this element in the early Galaxy: BD +23◦3130 and HD 84937. The observations were conducted using the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. A very detailed abundance analysis via spectral synthesis has been carried out for these two stars, as well as for two other metal-poor objects with published spectra, using both Kurucz and OSMARCS model photospheres, and taking into account consistently the NLTE effects on the line formation. We have also re-assessed all published boron abundances of old disk and halo unevolved stars. Our analysis shows that the combination of high effective temperature (Teff ∼> 6000 K, for which boron is mainly ionized) and low metallicity ([Fe/H] ∼< −1) makes it difficult to obtain accurate estimates of boron abundances from the B I λ 2497 Å line. This is the case of HD 84937 and three other published objects (including two stars with [Fe/H] ∼ −3), for which only upper limits can be established. BD +23◦3130, with [Fe/H] ∼ −2.9 and log N(B)NLTE = 0.05± 0.30, appears then as the most metal-poor star for which a firm measurement of the boron abundance presently exists. The evolution of the boron abundance with metallicity that emerges from the seven remaining stars with Teff < 6000 K and [Fe/H] < −1, for which beryllium abundances were derived using the same stellar parameters, shows a linear increase with a slope ∼ 1. Furthermore, the B/Be ratio found is constant at a value ∼ 20 for stars in the range −3 < [Fe/H] < −1. These results point to spallation reactions of ambient protons and α particles with energetic particles enriched in CNO as the origin of boron and beryllium in halo stars. Subject headings: Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: individual (BD +23◦3130, HD 84937) — stars: late-type — stars: Population II