Light-element abundance patterns in the orion association. I. Hubble space telescope observations of boron in G dwarfs

The boron abundances for two young solar-type members of the Orion association, BD -6°1250 and HD 294297, are derived from Hubble Space Telescope STIS spectra of the B I transition at 2496.771 Å. The best-fit boron abundances for the target stars are 0.13 and 0.44 dex lower than the solar meteoritic value of log ∈(B) = 2.78. An anticorrelation of boron and oxygen is found for Orion when these results are added to previous abundances obtained for four B-type stars and the G-type star BD -5°1317. An analysis of the uncertainties in the abundance calculations indicates that the observed anticorrelation is probably real. The B versus O relation observed in the Orion association does not follow the positive correlation of boron versus oxygen that is observed for the field stars with roughly solar metallicity. The observed anticorrelation can be accounted for by a simple model in which two poorly mixed components of gas (supernova ejecta and boron-enriched ambient medium) contribute to the new stars that form within the lifetime of the association. This model predicts an anticorrelation for Be as well, at least as strong as for boron.