1994A&A...291...89J

We present the results of a mm-wave molecular line search in the Large Magellanic Cloud obtained with the SEST in the area of the N159 complex of HII regions. Isotopomers of CO, CS, SO, CN, HCN, HNC, HCO⁺ and H₂CO have been detected. Some of these molecules have been observed in two transitions allowing excitation temperature estimates. For ¹³CO and ¹²CO we find temperatures close to, or in excess of 10K, while C¹⁸O, CS and HCO⁺ show temperatures between 5 and 10K. All observed isotopomer intensity ratios suggest small optical depths provided that Galactic isotopic abundance ratios apply. From the ¹²C- and ¹³C- isotopomer data of CS and HCO⁺, combined with the C³⁴S line emission we estimate the gas phase ¹²C/¹³C abundance ratio to be 50⁺²⁵_–20 in N159. The ¹⁸O/¹⁷O abundance ratio is observed to be a factor of about 2 lower than in the Galaxy, and another factor of 2 lower than in starburst galaxies. Using the OSO 20m telescope, we have mapped the Galactic molecular cloud associated with the HII region S138 in the 3 mm transitions of most of the molecules detected in the LMC. These data have been used to investigate the effects of linear resolution on line ratios and fractional abundances. Based upon cloud luminosities, normalized to that of the CO(1-0) line, the detected lines in the LMC are weaker by a factor of 1.5 to 3 relative to those of the S138 cloud. The exception is HCO⁺, which shows stronger normalized emission in N159. The normalized luminosities of the S138 cloud are similar to those observed in a sample of strong CO galaxies. Fractional abundances of the detected molecules have been estimated using the conversion factor from CO emission to H₂ column density. While the derived fractional abundances of S138 generally agree to within a factor of 3 with those reported for Orion KL and TMC-1, all molecular concentrations observed in the LMC are typically a factor of 10 lower.