Boron Abundances in Diffuse Interstellar Clouds
  We present a comprehensive survey of B abundances in diffuse interstellar
clouds from HST/STIS observations along 56 Galactic sight lines. Our sample is
the result of a complete search of archival STIS data for the B II resonance
line at 1362 angstroms, with each detection confirmed by the presence of
absorption from other dominant ions at the same velocity. The data probe a
range of astrophysical environments including both high-density regions of
massive star formation as well as low-density paths through the Galactic halo,
allowing us to clearly define the trend of B depletion onto interstellar grains
as a function of gas density. Many extended sight lines exhibit complex
absorption profiles that trace both local gas and gas associated with either
the Sagittarius-Carina or Perseus spiral arm. Our analysis indicates a higher
B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of
the Sun, which may suggest that B production in the current epoch is dominated
by a secondary process. The average gas-phase B abundance in the warm diffuse
ISM is consistent with the abundances determined for a variety of Galactic disk
stars, but is depleted by 60 percent relative to the solar system value. Our
survey also reveals sight lines with enhanced B abundances that potentially
trace recent production of B-11 either by cosmic-ray or neutrino-induced
spallation. Such sight lines will be key to discerning the relative importance
of the two production routes for B-11 synthesis.
