FRACTIONATION OF CO IN THE DIFFUSE CLOUDS TOWARD ZETA OPHIUCHI

Ultraviolet spectra of interstellar CO lines from the diffuse clouds toward xi Oph were obtained with the Goddard High Resolution Spectrometer on the Hubble Space Telescope. The ratios a(CO) = N((CO)-C-12-O-16)/N((CO)-C-13-O-16) = 150 +/- 27 (1 sigma) and N((CO)-C-12-O-16)/N((CO)-C-12-O-17) congruent-to N((CO)-C-12-O-16)/N((CO)-C-12-O-18) less-than-or-similar-to 800 (3 sigma) are derived from the A 1II-X 1SIGMA+5-0 and 6-0 bands at 1392 and 1367 angstrom, respectively, and published observations from Copernicus of weaker (CO)-C-12-O-16 A-X bands. Since a(CO) exceeds (by 3 sigma) the intrinsic C-12/C-13 ratio of 70 as given by the CH+ optical lines, CO is evidently fractionated on the smallest scale explored to date. The absence of (CO)-C-12-O-18 lines also indicates fractionation because the intrinsic ratio O-16/O-18 is likely to be close to the solar value of 500. The A-X bands and the B 1SIGMA+-X 1SIGMA+0-0 band, also observed with GHRS, are well matched with a total column density N(CO) = 2.2 x 10(15) cm-2 spread between two clouds separated by 1.2 km s-1 and at an excitation temperature T(ex) = 4.2 K. The observed fractionation and excitation temperature suggest that the CO molecules reside in low-density gas [n(H) = n(H) + 2n(H-2) congruent-to 100-200 cm-3].