The Abundance of Interstellar Boron1

We use new Space Telescope Imaging Spectrograph (STIS) and archival Goddard High Resolution Spectrograph (GHRS) observations to study interstellar B II j1362 and O I j1355 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-lineÈintegrated 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 to ]0.7. Detailed comparisons of the B II and O I log SnHT B [1.3 line shapes show that the B/O ratio is signiÐcantly higher in warm interstellar clouds than in cool clouds. These results are consistent with the incorporation of boron into dust grains in the di†use ISM. Since boron is likely incorporated into grains, we derive a lower limit to the present-day total (gas ] dust) interstellar boron abundance of The e†ects of dust depletion and B/H Z (2.5 ^ 0.9)] 10~10. ionization di†erences from element to element will make it very difficult to reliably determine 11B/10B along most interstellar sight lines. Subject headings : ISM: abundances È ISM: atoms È ultraviolet : ISM