Abstract

High spectral resolution (similar to 80 000) and signal-to-noise observations from the Ultraviolet and Visual Echelle Spectrograph Paranal Observatory Project (UVES-POP) are used to study the interstellar molecular lines CN (3874 angstrom), CH+ (3957, 4232 angstrom) and CH (3886, 4300 angstrom) towards 74 O- and B-type stellar sightlines. Additionally, archive data are presented for 140 ELODIE early-type stellar sightlines at R = 42 000, plus 25 FEROS at R = 48 000 and 3 UVES at R > 50 000, mainly in the CH+ (4232 angstrom) and CH (3886, 4300 angstrom) transitions. Detection rates are similar to 45 per cent for CN and similar to 67 per cent for the other lines in the POP sample, and similar to 10-15 per cent for CH+ and CH lines in the additional sample. CH and CH+ are well correlated between log[N(CH) cm(-2)]similar to 12-14, implying that these clouds are CH+-like CH and not CN-like CH. CH is also very well correlated with Na I D in the range log[N(Na I cm(-2)]) similar to 12.2-14.2. A few sightlines show tentative velocity shifts of similar to 2 km s(-1) between CH and CH+, which appear to be caused by differences in component strength in blends, and hence do not provide firm evidence for shocks. Finally, we describe a search for (CH+)-C-13 in a sightline towards HD 76341. No (CH+)-C-13 is detected, placing a limit on the (CH+)-C-13 to (CH+)-C-12 ratio of similar to 0.01. If a formal fit is attempted, the equivalent width ratio in the two isotopes is a factor similar to 90 but with large errors.