SIMBAD references

The ¹⁸O/¹⁷O abundance ratio is, in principle, a powerful tool to estimate the relative contributions of massive stars and low- to intermediate-mass stars to the chemical enrichment of galaxies. We present ¹⁸O/¹⁷O ratios derived from simultaneous observations of C¹⁸O and C¹⁷O 1-0 towards 51 massive star-forming regions with the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescope. Simultaneous observations of HC¹⁸O⁺ 1-0 and HC¹⁷O⁺ 1-0 with the Yebes 40m telescope towards five sources from this sample were also done to test the consistency of ¹⁸O/¹⁷O ratios derived from different isotopic pairs. From our improved measurements, resulting in smaller errors than previous work in the literature, we obtain a clear trend of increasing ¹⁸O/¹⁷O ratio with increasing Galactocentric distance (D_GC), which provides a significant constraint on Galactic chemical evolution (GCE) models. Current GCE models have to be improved in order to explain the observed C¹⁸O/C¹⁷O 1-0 gradient.