On the Star-formation History in the LMC: Observations of the Interstellar C¹⁸O/C¹⁷O Ratio

Extract

The re-cycling of gas between stars and the interstellar medium (ISM) leads to a gradual metal-enrichment of a galaxy. Accordingly, information on the chemical evolution of a galaxy, e.g., its star-formation history (SFH), is contained in the chemical composition of the ISM. In this context, the abundance ratio of the rare oxygen isotopes, ¹⁸O/¹⁷O (usually taken as the C¹⁸O/C¹⁷O column density ratio), appears to be a particularly promising probe of the SFH. According to present understanding of stellar nucleosynthesis, ¹⁷O is mainly produced in intermediate-mass stars (say a few to ten M_⊙) while ¹⁸O is synthesised in massive stars (say >10M_⊙) (e.g., Prantzos et al. 1996). Thus, the ¹⁸O/¹⁷O abundance ratio possibly reflects the relative number of massive stars compared to intermediate-mass stars, and thereby (qualitatively) constrains the SFH in terms of the average star-formation rate (SFR) and the initial mass-function (IMF). However, it should be remembered that the stellar nucleosynthesis of ^17,18O is not yet fully understood, leaving room for other interpretations of the ¹⁸O/¹⁷O ratio.