Boron Abundances in Diffuse Interstellar Clouds

TL;DR

This study measures boron abundances in diffuse interstellar clouds across various Galactic environments, revealing trends in depletion and production mechanisms, and comparing gas-phase abundances with stellar and solar system values.

Contribution

It provides the first comprehensive survey of B abundances in diffuse clouds using archival HST data, highlighting variations in B/O ratios and potential recent B-11 production evidence.

Findings

Higher B/O ratio in Sagittarius-Carina spiral arm.

Gas-phase B abundance is depleted by 60% relative to solar.

Sight lines with enhanced B suggest recent B-11 production.

Abstract

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.