VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds

We discuss the abundances of interstellar CH, CH+, and CN in the MagellanicClouds (MC), derived from spectra of 7 SMC and 13 LMC stars obtained (mostly)with the VLT/UVES. CH and/or CH+ are detected toward 3 SMC and 9 LMC stars; CNis detected toward 2 stars. In the MC, the CH/H2 ratio is comparable to thatfound for diffuse Galactic molecular clouds in some sight lines, but is lowerby factors up to 10-15 in others. The abundance of CH in the MC thus appears todepend on local physical conditions -- and not just on metallicity. Theobserved relationships between the column density of CH and those of CN, CH+,Na I, and K I in the MC are generally consistent with the trends observed inour Galaxy. Using existing data for the rotational populations of H2, weestimate temperatures, radiation field strengths, and local hydrogen densitiesfor the diffuse molecular gas. Densities estimated from N(CH), assuming that CHis produced via steady-state gas-phase reactions, are considerably higher; muchbetter agreement is found by assuming that the CH is made via the (stillundetermined) process(es) responsible for the observed CH+. The UVES spectraalso reveal absorption from the diffuse interstellar bands at 5780, 5797, and6284 A in the MC. On average, the three DIBs are weaker by factors of 7-9 (LMC)and about 20 (SMC), compared to those observed in Galactic sight lines withsimilar N(H I), and by factors of order 2-6, relative to E(B-V), N(Na I), andN(K I). The detection of several of the ``C2 DIBs'', with strengths similar tothose in comparable Galactic sight lines, however, indicates that no single,uniform scaling factor (e.g., one related to metallicity) applies to all DIBs(or all sight lines) in the MC. (abstract abridged)Comment: 59 pages, 15 figures, 10 tables; aastex; accepted to ApJ