Astron. Astrophys. 332, 493-502 (1998)

The C¹⁸ O/C¹⁷ O ratio in the Large Magellanic Cloud

A. Heikkilä ¹, L.E.B. Johansson ¹ and H. Olofsson <sup>2

1</sup> Onsala Space Observatory, S-439 92 Onsala, Sweden
² Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden

Received 28 May 1997 / Accepted 5 January 1998

Abstract

We report detections of J =2-1 line emission from the carbon monoxide isotopomers ¹³ CO, C¹⁸ O and C¹⁷ O in the molecular clouds N159W, N113, N44BC, and N214DE in the Large Magellanic Cloud (LMC). ¹³ CO and C¹⁸ O lines were observed in two additional clouds: N159S in the LMC and N27 in the Small Magellanic Cloud (SMC). While ¹³ CO was detected in both of them, only upper limits to the C¹⁸ O 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 C¹⁸ O/C¹⁷ O abundance ratio of 1.6 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 C¹⁸ O/C¹⁷ O abundance ratio as a measure of the elemental ¹⁸ O/¹⁷ O abundance ratio. Provided that current theories of the nucleosynthesis involving O apply, then the low ¹⁸ O/¹⁷ O 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 ¹⁸ O/¹⁷ O 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 ¹³ CO/C¹⁸ O ratio (the LMC average is 30), possibly indicating a low ¹⁸ O abundance here as well. Our data suggests a correlation between the ¹⁸ O/¹⁷ O abundance ratio and the metallicity. The high ¹⁸ O/¹⁷ O 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.

Key words: ISM: abundances galaxies: abundances galaxies: evolution galaxies: ISM Magellanic Clouds

Send offprint requests to: A. Heikkilä

SIMBAD Objects

Contents

• 1. Introduction
• 2. The observed sources
• 3. Observations
• 4. Results
• 5. Discussion
  • 5.1. Interstellar processes affecting abundance estimates
  • 5.2. What does the ¹⁸ O /¹⁷ O ratio tell us?
    • 5.2.1. Metallicity
    • 5.2.2. Star-formation history
• 6. Summary
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
helpdesk.link@springer.de