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

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. 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. We identified a measurable line of B I at 2089.6 A. 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. 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.