Hubble Space Telescope Observations of C 2 Molecules in Diffuse Interstellar Clouds

Abstract

Interstellar C2 F-X (1342 A) and D-X (2313 A) bands in the spectrum of zeta Oph were detected using the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST). The total C2 column density is (1.79 +/- 0.06) 10¹³/sq cm for an adopted f-value of 0.0545 for the 2313 A 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: f₀₀^FX/f₀₀^DX = 1.83 +/- 0.18 and f₂₀^AX/f₀₀^DX = 0.0226 +/- 0.0029. For the Mulliken system, lines are detected up to a rotational level J double prime = 24. The relative populations along the rotational ladder are shown to be consistent with the physical and environmental conditions suggested by other diagnostics. Interstellar C2 molecules were detected towards zeta Per (N(C2) = (0.80 +/- 0.23) 10¹³) but not towards Beta¹, pi, and omega¹ Sco(N(C2) less than or equal to 0.17 x 10¹³/sq cm.