The Abundance of Interstellar Boron

Abstract

We use new Space Telescope Imaging Spectrograph (STIS) and archival Goddard High Resolution Spectrograph (GHRS) observations to study interstellar B II λ1362 and O I λ1355 absorption along seven sight lines. Our new column density measurements, combined with measurements of four sight lines from the literature, allow us to study the relative B/O abundances over a wide range of interstellar environments. We measure sight-line-integrated relative gas-phase abundances in the range [B/O]=-1.00 to -0.17, and our data show that the B/O abundances are anticorrelated with average sight line densities over the range log<n_H>~-1.3 to +0.7. Detailed comparisons of the B II and O I line shapes show that the B/O ratio is significantly higher in warm interstellar clouds than in cool clouds. These results are consistent with the incorporation of boron into dust grains in the diffuse ISM. Since boron is likely incorporated into grains, we derive a lower limit to the present-day total (gas+dust) interstellar boron abundance of B/H>~(2.5+/-0.9)×10^-10. The effects of dust depletion and ionization differences from element to element will make it very difficult to reliably determine ¹¹B/¹⁰B along most interstellar sight lines. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.