2014MNRAS.438.1127S

High spectral resolution ( ∼ 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 (3874Å), CH+ (3957, 4232Å) and CH (3886, 4300Å) 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+ (4232Å) and CH (3886, 4300Å) transitions. Detection rates are ∼ 45 percent for CN and ∼ 67 percent for the other lines in the POP sample, and ∼ 10^–15 percent for CH+ and CH lines in the additional sample. CH and CH+ are well correlated between log[N(CH)/cm2] ∼ 12-14, implying that these clouds are CH±like CH and not CN-like CH. CH is also very well correlated with Nai D in the range log[N(NaI/cm2]) ∼ 12.2-14.2. A few sightlines show tentative velocity shifts of ∼ 2 km/s 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 13CH+ in a sightline towards HD 76341. No 13CH+ is detected, placing a limit on the 13CH+ to 12CH+ ratio of ∼ 0.01. If a formal fit is attempted, the equivalent width ratio in the two isotopes is a factor ∼ 90 but with large errors.