BORON IN VERY METAL-POOR STARS

We have observed the B I lambda 2497 line to derive the boron abundanc es of two very metal-poor stars selected to help in tracing the origin and evolution of this element in the early Galaxy: BD +23 degrees 313 0 and HD 84937. The observations were conducted using the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. A very d etailed 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 a nd 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 com bination of high effective temperature (T-eff greater than or similar to 6000 K, for which boron is mainly ionized) and low metallicity ([Fe /H] less than or similar to -1) makes it difficult to obtain accurate estimates of boron abundances from the B I lambda 2497 line. This is t he case of HD 84937 and three other published objects (including two s tars with [Fe/H] similar to -3), for which only upper limits can be es tablished. ED +23 degrees 3130, with [Fe/H] similar to -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 t he seven remaining stars with T-eff < 6000 K and [Fe/H] < -1, for whic h beryllium abundances were derived using the same stellar parameters, shows a linear increase with a slope of similar to 1. Furthermore, th e B/Be ratio found is constant at a value of similar to 20 for stars i n the range -3 < [Fe/H] < -1. These results point to spallation reacti ons of ambient protons and alpha-particles with energetic particles en riched in CNO as the origin of boron and beryllium in halo stars.