A search for weak ultraviolet interstellar absorption features in IUE spectra of Rho Ophiuchi and Zeta Ophiuchi

We have applied procedures designed to reduce substantially the nonrandom, so-called 'fixed-pattern' noise present in IUE spectra to archival long-wavelength high-dispersion spectra of Rho Ophiuchi and Zeta Ophiuchi. Substantial elimination of the fixed-pattern noise via flat fielding can yield 2sigma equivalent width limits of 5-10 mA from the sum of a small number (about less than 5) of well-exposed archival spectra, and increases confidence in the reality of any weak features found. Examination of complete long-wavelength (about 2200-3250 A) spectra of these two stars has revealed, in addition to many known strong absorption lines, several lines of Fe I and Si I which had not previously been reported, as well as a small number of possible unidentified lines. We also present substantially improved upper limits to the equivalent widths of a number of other weak lines; limits an order of magnitude smaller, now achievable with the HST GHRS, should produce detections of some of these.