Upper limits for interstellar boron and beryllium abundances toward ZET Oph.

Observations of interstellar boron and beryllium in the ζ Oph spectrum were made using Copernicus. While neither element was detected, the boron limit can be used to derive [B/H] < 3 × 10^-11 in the main interstellar cloud seen toward ζ Oph. Since [B/H] =15 0.7 × 10^-10 in the interstellar gas between the Sun and κ Ori, and since in stars of Type A and B, [B/H] ∼2 × 10^-10, we conclude that boron is depleted in the main ζ Oph cloud and that the abundance of boron in the interstellar medium is variable, probably by at least a factor of 5. It is argued that future searches for B ii and Be II in interstellar gas should be at the 50 and 10 μÅ level, respectively.