Precise boron abundance in a sample of metal-poor stars from far-ultraviolet lines

Context . The light elements beryllium (Be; Z = 4) and boron (B; Z = 5) are mainly produced by spallation reactions between cosmic rays and carbon (C; Z = 6), nitrogen (N; Z = 7), and oxygen (O; Z = 8) nuclei. Only traces of Be or B would have been produced in the Big Bang, but there could be a contribution from the ν-process in type II supernovae. Their abundances at very low metallicities have been debated in the literature, with the aim of understanding their origin. Aims . Our aim is to derive the boron abundance in a sample of metal-poor stars based for the first time on observations with the STIS spectrograph on board the Hubble Space Telescope, using clean B lines measured in space ultraviolet. Methods . We identified a measurable line of B I at 2089.6 Å. In our sample of metal-poor warm stars, this line is practically free from blending lines, and for this reason the precision of the presently derived boron abundances is unprecedented. Results . We find that in the interval −2.6 < [Fe/H] < −1.0, the slope of the relation A(B) versus [Fe/H] is significantly larger than 1, and thus steeper than that obtained with Be abundances. As a consequence, we find in this interval of metallicity a B/Be ratio that slightly increases with [Fe/H]. Since at [Fe/H] = −1 the abundance of B is already close to the solar abundance, there should be a break in the B enrichment at a metallicity of about [Fe/H] = −1.