Boron depletion in F and G dwarf stars and the beryllium-boron correlation

Boron provides a special probe below the stellar surface since it survives to greater depths than do Li and Be. To search for B depletions we have observed B in 13 F and G dwarfs with large Be depletions; for comparison we have also obtained spectra of five stars that are undepleted in Li and Be. We have used HST with STIS to obtain spectra of the B I resonance line at 2497 angstrom. The spectral resolution is 30,000 or 114,000, and the median signal-to-noise ratio is 70 pixel(-1). New Be and Li spectra have been obtained at Keck I with HIRES of four of the five standard stars at similar to 48,000 resolution. Abundances have been determined by the spectrum synthesis method with MOOG. A comparison between the standard stars and those with severe Be depletions shows a distinct difference in the B abundances between the two groups of 0.22 dex. We have discovered a correlation between the Be and B abundances. The slope between A( Be) and A( B)(NLTE) is 0.22 +/- 0.05 [ where A(element) = log N( element)/N(H) = 12.00], which, as expected, is shallower than the slope between A( Li) and A( Be) of 0.38. We have normalized the light-element abundances to account for the observation that the initial abundances are somewhat lower in lower metallicity stars by employing recently published empirical relations between Be and [Fe/H] and between B and [Fe/H]. The correlation between the normalized A( Be) and A( B)(NLTE) has a slope of 0.18 +/- 0.06. The star with the largest Be depletion, HR 107, a main-sequence Ba star, also has the largest B depletion, with the B abundance lower by a factor of 3.5 relative to the standard stars.