The origins of the stable isotopes of boron remain uncertain despite much theoretical and observational effort. Spallation reactions between relativistic Galactic cosmic rays (GCR) and interstellar nuclei can adequately account for the production of 10B and contribute to the cosmic abundance of 11B. However, an additional source of 11B synthesis is required to raise the isotopic ratio of 11B/10B from its GCR spallation value (2.4) to the value measured in carbonaceous chondrites (4.0). The ν-process, neutrino-induced spallation in Type II supernovae, is a potentially significant source of 11B production. Since neutrino-induced spallation does not result in substantial yields for 10B, this process could naturally explain the enhancement in 11B/10B over the predictions of standard GCR spallation. Without the ν-process, enhanced 11B production, relative to 10B, could be attributed to an increased flux of low-energy (5-40 MeV nucleon-1) cosmic rays, which are unobservable from Earth due to magnetic shielding by the solar wind.
In this thesis, I present a comprehensive survey of boron abundances in diffuse interstellar clouds from Space Telescope Imaging Spectrograph (STIS) observations made with the Hubble Space Telescope in an effort to identify the sources responsible for light element nucleosynthesis. The present sample of 56 Galactic sight lines is the result of a complete search of archival STIS data for the B II λ1362 resonance line. Each detection is confirmed by the presence of absorption due to O I λ1355, Cu II λ1358, and Ga II λ1414 (when available) at the same velocity. Like B+, these species represent the dominant ionization stage of their element in neutral diffuse clouds and therefore should coexist. Profile templates based on synthesized absorption profiles of O I, Cu II, and Ga II are fitted to the B II line, yielding the total boron column density along each line of sight. By synthesizing B II profiles with components seen in high-resolution ground-based observations of Ca II λ3933 and K I λ7698, I show that unresolved structure in the UV data has little influence on final boron abundances.
Many sight lines exhibit multiple complexes of absorption components well separated in velocity. I interpret such sight lines as tracing both local gas, with velocity components near 0 km s-1 relative to the local standard of rest (LSR), and gas associated with a distant spiral arm, with components at more negative velocities. In these cases, profile templates are created for each complex and fitted to that portion of the B II profile independently, thereby enabling a comparative analysis of boron abundances in distinct physical regions within the Galaxy. With this procedure, I find suggestive evidence for a higher B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of the Sun. Abundances of secondary elements increase relative to those of primary elements toward the Galactic center due to enhanced rates of star formation and stellar nucleosynthesis. The finding that the B/O ratio is elevated toward the inner Galaxy may thus indicate the secondary nature of boron, which in turn would cast doubt on the efficiency of the ν-process, a primary production mechanism.
Measured line-of-sight abundance ratios of B/H and B/O firmly establish that boron is depleted (relative to the Sun) in diffuse gas. A comparison of these ratios with measures of gas density, namely the average line-of-sight hydrogen density 〈nH〉 and the fraction of hydrogen in molecular form f(H2), further demonstrates that the depletion increases in high-density sight lines, a trend seen in the abundances of many other elements, including in the oxygen, copper, and gallium results presented here. Elemental depletion onto interstellar dust grains increases with average gas density due to the larger fraction of cold clouds, relative to warm gas, along the line of sight. In the case of boron, this amounts to an abundance difference of 0.8 dex between the warm and cold phases of the diffuse interstellar medium (ISM). The gas-phase abundance of boron in warm, diffuse gas, found by averaging the abundances derived for sight lines exhibiting the least depletion, is B/H = (2.4±0.6) × 10-10. This value is consistent with the boron abundances determined for a variety of Galactic disk stars, but is depleted by 60 percent relative to the solar system value of (6.0±0.6) × 10-10, obtained from meteoritic samples. By clearly defining the trend due to depletion, my survey reveals sight lines with enhanced B/H and B/O ratios that potentially trace recent production of 11B either by cosmic-ray or neutrino-induced spallation. These sight lines will be key to discerning the relative importance of the two production routes for 11B synthesis.