A WEAK DIFFUSE INTERSTELLAR BAND IN THE FAR-ULTRAVIOLET SPECTRUM OF ZETA-OPHIUCHI

Goddard High Resolution Spectrograph (GHRS) observations at 3.5 km s(-1) resolution reveal several new weak unidentified interstellar absorption lines in the ultraviolet spectrum of zeta Ophiuchi. The unidentified line at 1369.13 Angstrom has the appearance and characteristics of a weak diffuse interstellar band (DIB). The line has a smooth profile similar to many optical diffuse interstellar bands (i.e., a shallow asymmetric profile), it is clearly broader than identified interstellar lines near it in wavelength, and its full width at half maximum in ergs is comparable to the widths of the weak optical DIBs. The asymmetric profile cannot be attributed to blended absorption from diffuse clouds at different velocities; at this resolution the two principal cloud complexes on the sight line at heliocentric velocities of -27 and -15 km s(-1) are clearly separated. We compare this unidentified absorption feature to identified interstellar atomic and molecular absorption lines and optical DIBs observed on the zeta Oph and xi Per sight lines, and we conclude that it is reasonable to suggest that this absorption feature might be a DIB. This is not a unique interpretation however; the unidentified line could alternatively be due to gas in the zeta Oph H II region or a blend of unknown neutral atomic or molecular absorption lines.