Boron Depletion in F and G Dwarf Stars and the Beryllium-Boron Correlation

Abstract

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 Å. 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 ~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)=logN(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.

Based on observations obtained with the NASA/ESA Hubble Space Telescope through the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.