Studies of Interstellar Molecular Gas in the Magellanic Clouds

Results of observational studies of interstellar molecular gas clouds in the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), two nearby metal-poor and dust-poor dwarf galaxies, are presented. Spectral line radiation emitted by interstellar molecules has been used to probe the physical state and the chemical composition of individual molecular clouds. In addition, observations of the C18O/C17O ratio in a few Galactic and extragalactic sources are presented. Two molecular species, deuterated formyl ion (DCO+) and hydrogen sulphide (H2S), were detected for the first time in an extragalactic source (N159W in the LMC). Moreover, a definite detection of C17O outside the Local Group has been obtained. The density and the temperature of the molecular gas in the Magellanic Clouds are estimated to be typically 104-106cm-3 and 15-30,K, respectively. The abundances of trace molecules (relative to H2) in clouds in the LMC are typically five to twenty times lower than in Galactic disc clouds. These underabundances are most likely caused by a combination of the lower metallicity and a more rapid photodissociation in the LMC. The effects of the metallicity and far-ultraviolet radiation are further emphasized in two of the clouds, where trace molecules are even less abundant: the cloud (N27) studied in the SMC and a cloud located near the centre of the 30 Doradus nebula in the LMC. Several features, most notably the abundance of C2H as well as the correlations of the HCO+/CO and the C2H/CO ratios with the star-formation activity, suggest that the chemistry in molecular clouds in the LMC and the SMC largely resembles that in Galactic photon-dominated regions. The gas-phase 12C/13C and 32S/34S abundance ratios are found to be similar to those measured in Galactic clouds. In particular, no trend of a higher 12C/13C ratio due to the low metallicity is discernible. On the other hand, the 18O/17O abundance ratio, the average in the LMC is estimated to be 1.6\ub10.3, is significantly lower than in Galactic clouds (by a factor of two) and centres of starburst galaxies (by a factor of five). Provided that the current understanding of stellar nucleosynthesis applies, a low 18O/17O abundance ratio indicates that massive stars have contributed only little to the enrichment of the interstellar medium. This suggests that the star-formation history in the LMC has been characterised by a steep initial mass function together with a low average star-formation rate