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

Abstract

We discuss the abundances of interstellar CH, CH+, and CN in the Magellanic Clouds, derived from spectra of seven SMC and 13 LMC stars obtained (mostly) with the VLT UVES. CH and/or CH+ have now been detected toward three SMC and nine LMC stars; CN is detected toward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly all the optical detections of these molecular species in interstellar media beyond the Milky Way. In the LMC, the CH/H2 ratio is comparable to that found for diffuse Galactic molecular clouds in four sight lines but is lower by factors of 2.5-4.0 in two others. In the SMC, the CH/H2 ratio is comparable to the local Galactic value in one sight line but is lower by factors of 10-15 in two others. The abundance of CH in the Magellanic Clouds thus appears to depend on local physical conditions and not just on metallicity. In both the SMC and the LMC, the observed relationships between the column density of CH and those of CN, CH+, Na I, and K I are generally consistent with the trends observed in our Galaxy. Using existing data for the rotational populations of H2 in these sight lines, we estimate temperatures, radiation field strengths, and local hydrogen densities for the diffuse molecular gas. The inferred temperatures range from about 45 to 90 K, the radiation fields range from about 1 to 900 times the typical local Galactic field, and the densities (in most cases) lie between 100 and 600 cm-3. Densities estimated from the observed N(CH), under the assumption that