THE INTERSTELLAR BORON ABUNDANCE TOWARD ORION

We have used the Goddard High-Resolution Spectrograph to measure the a bundance of interstellar boron in several low-density sight lines towa rd Orion. In particular, we detected the B II 1362 Angstrom line towar d lambda Ori with an equivalent width (1.0 +/- 0.3 m Angstrom) that im plies a B/H abundance ratio (9.1 x 10(-11)) equal, within the errors, to that found toward zeta Oph and a factor of 4 below that found in th e Sun. Because the boron abundances toward lambda Ori and zeta 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 mai nly 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 int erstellar metallicity caused by a recent infall of metal-poor material .