The Abundance of Interstellar Boron

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 11 B/ 10 B along most interstellar sight lines.