Hubble Space Telescope Measurements of Vacuum Ultraviolet Lines of Interstellar CH

Abstract

Three interstellar absorption lines near 1370 Å seen toward ζ 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)×10¹¹, (3.8+/-0.7)×10¹¹, and (1.1+/-0.6)×10¹⁰ s^-1 for λλ1368, 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 Å 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 Å, 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 Å. Comparisons with results of available theoretical calculations for the four CH bands are presented.