Boron in Very Metal-poor Stars

We have observed the B I λ2497 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 non-LTE (NLTE) effects on the line formation. We have also reassessed 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 λ2497 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 of ~1. Furthermore, the B/Be ratio found is constant at a value of ~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.