Hubble Space Telescope Measurements of Vacuum Ultraviolet Lines of Interstellar CH

Three interstellar absorption lines near 1370 A seen toward zeta Oph have been assigned by Watson to Rydberg transitions in the G-X (or 3d-X) band of CH. Our survey of a dozen diffuse interstellar lines of sight shows that the three absorption lines are consistent with the known column densities of CH, by deriving the following oscillator strengths: f(1368.74) = 0.019 +/- 0.003, f(1369.13) = 0.030 +/- 0.005, and f(1370.87) = 0.009 +/- 0.001. We also determined intrinsic line widths that correspond to decay rates of (1.5 +/- 0.6) x 10^11 s^-1, (3.8 +/- 0.7) x 10^11 s^-1, and (1.1 +/- 0.6) x 10^10 s^-1 for lambdalambda 1368, 1369, and 1370, respectively. These rates are significantly higher than those associated with radiative decays, and thus are readily attributable to predissociation of the Rydberg state. A fourth interstellar line near 1271 A has been conjectured by Watson to be the strongest transition in the 4d-X Rydberg band of CH. We detected this line along four sight lines and our spectrum syntheses show that with f(1271.02) = 0.007 +/- 0.002, it is also consistent with the known column densities of CH. In addition, we conducted a search for the F-X band of CH near 1549 A, and successfully discovered two of its absorption features along four sight lines. The astronomical oscillator strengths derived for these features are f(1549.05) = 0.021 +/- 0.006 and f(1549.62) = 0.013 +/- 0.003. Finally, the X Per sight line provided us with an astronomical detection of another CH band, via two D-X features near 1694 A. Comparisons with results of available theoretical calculations for the four CH bands are presented.