The Abundance of Boron in Diffuse Interstellar Clouds

Abstract

We present a comprehensive survey of boron abundances in diffuse interstellar clouds from observations made with the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope. Our sample of 56 Galactic sight lines is the result of a complete search of archival STIS data for the B II 1362 resonance line, with each detection confirmed by the presence of absorption from O I 1355, Cu II 1358, and Ga II 1414 (when available) at the same velocity. Five previous measurements of interstellar B II from Goddard High Resolution Spectrograph observations are incorporated in our analysis, yielding a combined sample that more than quadruples the number of sight lines with significant boron detections. Our survey also constitutes the first extensive analysis of interstellar gallium from STIS spectra and expands on previously published results for oxygen and copper. The observations probe both high and low-density diffuse environments, allowing the density-dependent effects of interstellar depletion to be clearly identified in the gas-phase abundance data for each element. In the case of boron, the increase in relative depletion with line-of-sight density amounts to an abundance difference of 0.8 dex between the warm and cold phases of the diffuse interstellar medium. The abundance of boron in warm, low-density gas is found to be B/H = (2.4+/-0.6)*10^-10, which represents a depletion of 60% relative to the meteoritic boron abundance. Beyond the effects of depletion, our survey reveals sight lines with enhanced boron abundances that potentially trace the recent production of 11B, resulting from spallation reactions involving either cosmic rays or neutrinos. Future observations will help to disentangle the relative contributions from the two spallation channels for 11B synthesis.