1998A&A...332..493H

We report detections of J=2-1 line emission from the carbon monoxide isotopomers ¹³CO, C¹⁸O and C¹⁷O in the molecular clouds N159W, N113, N44BC, and N214DE in the Large Magellanic Cloud (LMC). ¹³CO and C¹⁸O lines were observed in two additional clouds: N159S in the LMC and N27 in the Small Magellanic Cloud (SMC). While ¹³CO was detected in both of them, only upper limits to the C¹⁸O line emission were obtained. Statistical-equilibrium excitation and radiative transfer calculations were made to infer molecular column densities from the observed line intensities. We estimate an average gas-phase C¹⁸O/C¹⁷O abundance ratio of 1.6±0.3 in the LMC. This is significantly lower than typical values found in Galactic clouds (by a factor of two) and in centres of starburst galaxies (by a factor of five). We use the C¹⁸O/C¹⁷O abundance ratio as a measure of the elemental ¹⁸O/¹⁷O abundance ratio. Provided that current theories of the nucleosynthesis involving ^17,18O apply, then the low ¹⁸O/¹⁷O ratio suggests that massive stars have contributed little to the metal enrichment of the interstellar medium in the LMC in the past. This may be caused by a steep initial mass function (which appears to be the case for field stars in the Magellanic Clouds and in the Galaxy) together with a low average star-formation rate. This explanation contrasts with the present situation in prominent star-formation regions in the LMC, such as 30 Doradus, which form stars at a considerable rate and appear to have initial mass functions similar to star clusters in the Galaxy. The apparent spatial constancy of the ¹⁸O/¹⁷O abundance ratio, the nominal values for the individual clouds vary between 1.6 and 1.8, indicates a well mixed interstellar medium and/or that the star-formation activity took place globally in the LMC in the past. In the SMC we obtained a lower limit of 17 for the ¹³CO/C¹⁸O ratio (the LMC average is 30), possibly indicating a low ¹⁸O abundance here as well. Our data suggests a correlation between the ¹⁸O/¹⁷O abundance ratio and the metallicity. The high ¹⁸O/¹⁷O abundance ratio in centres of starburst galaxies could reflect a high metallicity, mainly caused by a high star-formation rate, possibly but not necessarily together with an initial mass function biased towards massive stars.