A new set of HST boron observations I. Testing light elements stellar depletion

A sample of 7 new stars ranging in metallicity from [Fe/H]=−2.0 to [Fe/H]=−0.75 has been analyzed in the boron spectral region. The spectra were observed with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST). The targets were selected on the basis of their lithium and beryllium abundances in order to investigate in a more complete way light element depletion and internal mixing by comparing all 3 elements Li, Be, and B at once in the same objects. Four stars out of 7 are characterized by strongly depleted Li and Be abundances, compared to stars of similar characteristics. We find that 2 of them (HD 2665 and HD 3795) are also significantly B-depleted. Two others (HD 106516 and HD 221377) have normal or near normal B abundances despite being depleted by a factor ≥10 in both Li and Be abundances. These stars place strong constraints on the nature and depth of the mixing processes responsible for their light element abundances. Two other stars, HD 94028 and HD 194598, are found to have normal B abundances, despite a 0.3 dex difference in their Be abundances claimed by Thorburn & Hobbs (1996). We consider the reported Be difference unconfirmed. The 7th star belonging to this cycle of HST observations is HD 160617. Although subjected to a separate analysis (Primas et al. 1998a), it has also been included here because of the remarkable aspect of its low B, probably low Be, and completely normal Li. No stellar destruction mechanism can explain this. Rather, chemical inhomogeneities in the halo could be the cause.