THE (CO)-O-18 (CO)-O-17 RATIO IN THE LARGE-MAGELLANIC-CLOUD/

We report detections of J=2-1 line emission from the carbon monoxide i sotopomers (CO)-C-13, (CO)-O-18 and (CO)-O-17 in the molecular clouds N159W, N113, N44BC, and N214DE in the Large Magellanic Cloud (LMC).(CO )-C-13 and (CO)-O-18 lines were observed in two additional clouds: N15 9S in the LMC and N27 in the Small Magellanic Cloud (SMC). While (CO)- C-13 was detected in both of them, only upper limits to the (CO)-O-18 line emission were obtained. Statistical-equilibrium excitation and ra diative transfer calculations were made to infer molecular column dens ities from the observed line intensities. We estimate an average gas-p hase (CO)-O-18/(CO)-O-17 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 (CO)-O-18/(CO)-O-17 abundance ratio as a measure of the elemental O-18/O-17 abundance ratio. Provided that current theo ries of the nucleosynthesis involving (17),O-18 apply, then the low O- 18/O-17 ratio suggests that massive stars have contributed little to t he 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 Galax y) together with a low average star-formation rate. This explanation c ontrasts with the present situation in prominent star-formation region s in the LMC, such as 30 Doradus, which form stars at a considerable r ate and appear to have initial mass functions similar to star clusters in the Galaxy. The apparent spatial constancy of the O-18/O-17 abunda nce 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. I n the SMC we obtained a lower limit of 17 for the (CO)-C-13/(CO)-O-18 ratio (the LMC average is 30), possibly indicating a low O-18 abundanc e here as well. Our data suggests a correlation between the O-18/O-17 abundance ratio and the metallicity. The high O-18/O-17, abundance rat io in centres of starburst galaxies could reflect a high metallicity, mainly caused by a high star-formation rate, possibly but not necessar ily together with an initial mass function biased towards massive star s.