The Interstellar Boron Abundance toward Orion

Abstract

We have used the Goddard High-Resolution Spectrograph to measure the abundance of interstellar boron in several low-density sight lines toward Orion. In particular, we detected the B II 1362 Å line toward λ Ori with an equivalent width (1.0±0.3 mÅ) that implies a B/H abundance ratio (9.1 × 10^-11) equal, within the errors, to that found toward ζ Oph and a factor of 4 below that found in the Sun. Because the boron abundances toward λ Ori and ζ Oph are so close, even though these sight lines differ in <n_H> by nearly a factor of 10, our data support the view that interstellar boron is mainly found in the gas phase. However, we cannot exclude the possibility that at least half the interstellar boron is locked up within grains. The true abundance of boron in the local interstellar medium could be lower than in stars like the Sun because of dilution of the local interstellar metallicity caused by a recent infall of metal-poor material.