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 1362 and O I 1355 absorption along seven sightlines. Our new column density measurements, combined with measurements of four sightlines from the literature, allow us to study the relative B/O abundances over a wide range of interstellar environments. We measure sightline-integrated relative gas-phase abundances in the range [B/O] = -1.00 to -0.17, and our data show the B/O abundances are anticorrelated with average sightline 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)x10^-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 sightlines.

Accepted for publication in The Astrophysical Journal. 13 pages, including 2 tables and 4 figures. Also available at http://fuse.pha.jhu.edu/~howk/Papers/

References in corpus (3)

• Modeling the Warm Ionized Interstellar Medium and Its Impact on Elemental Abundance Studies
• Abundances and Physical Conditions in the Warm Neutral Medium Towards mu Columbae
• Background and Scattered Light Subtraction in the High-Resolution Echelle Modes of the Space Telescope Imaging Spectrograph

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