HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF C-2 MOLECULES IN DIFFUSE INTERSTELLAR CLOUDS

Interstellar C-2 F-X (1342 Angstrom) and D-X (2313 Angstrom) bands in the spectrum of zeta Oph were detected using the Goddard High-Resolution Spectrograph on the Hubble Space Telescope. The total C-2 column density is (179 +/- 0.06) 10(13) cm(-2) for an adopted f-value of 0.0545 for the 2313 Angstrom band of the Mulliken (D-X) system. Relative f-values for the 0-0 F-X, 0-0 D-X, and 2-0 A-X (Phillips) bands are derived by combining ultraviolet and near-infrared spectra: f(00)(FX)/f(00)(DX) = 1.83 +/- 0.18 and f(20)(AX)/f(00)(DX) = 0.0226 +/- 0.0029. For the Mulliken system, lines are detected up to a rotational level J(n) = 24. The relative populations along the rotational ladder are shown to be consistent with the physical and environmental conditions suggested by other diagnostics. Interstellar C-2 molecules were detected towards xi Per [N(C-2) = (0.80 +/- 0.23) 10(13) cm(-2)] but not towards beta(1), pi, and omega(1) Sco [N(C-2)less than or equal to 0.17 x 10(13) cm(-2)].