Astronomy and Astrophysics, volume 332, 493-502 (1998/4-2)
The C18O/C17O ratio in the Large Magellanic Cloud.
HEIKKILAE A., JOHANSSON L.E.B. and OLOFSSON H.
Abstract (from CDS):
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.