Boron in Lithium‐ and Beryllium‐deficient F Stars

Abstract

The Goddard High Resolution Spectrograph (GHRS) has been used with the Hubble Space Telescope (HST) to observe the B I region at 2497 A in nine F and G dwarfs of approximately solar metallicity. The stars were selected because they have a variety of Li and Be deficiencies. Most of the nine stars were newly observed at high spectral resolution and high signal-to-noise ratios at the Keck I 10 m telescope, the Canada-France-Hawaii 3.6 m telescope, and the University of Hawaii 2.2 m telescope at 3131 A for Be II and 6708 A for Li I. With spectrum synthesis we have determined the abundances of B in our nine program stars and in five other stars from the HST archive. The stellar parameters we have used have been determined in a self-consistent way for the program stars and the archive stars. Spectrum synthesis has also been used to determine the Li and Be abundances or upper limits. Corrections to the B and Li abundances due to non-LTE effects have been applied. The stars originate from the region on the ZAMS of the Li (and Be) dip. In spite of large deficiencies in Li and Be, we find a striking uniformity in the B abundances, i.e., there is no B dip. In all cases the Li deficiency is greater than the Be deficiency. For the coolest and most evolved star in our sample, ζ Her A, the B abundance is 0.6 dex lower than the mean for the other stars. This star also has the largest Be deficiency (more than a factor of 80) and the largest Li deficiency (more than a factor of 600). These data, together with other studies of the Li dip, argue strongly against diffusion and mass loss and in favor of slow mixing as the cause of the Li and Be dip and the absence of a B dip. Six stars with [Fe/H] from -0.75 to +0.15 have Be abundances ranging from the maximum of the sample to a factor of 4 below the maximum, yet these stars have a B/Be ratio that is constant to within ±0.10 dex and that is close to the predictions of Galactic cosmic-ray spallation of 10-15. The Be range for four stars with solar metallicity is still a factor of 2, and yet the B/Be ratio is constant to within ±0.03 dex. These results imply that the Galactic cosmic-ray production of B and Be is not uniform relative to the production of elements such as Fe by stellar nucleosynthesis.