The isotopic abundances of interstellar oxygen

The uniformity of the relative abundance of O-17 and O-18 in the Galaxy's giant molecular clouds, where it varies by less than about 5% between the galactic center and sources as much as 12 kPc away, is thought to reflect the similarity in the galactic processes which introduce the two isotopes into the interstellar medium. The processes are (1) novae explosions, in the case of O-17 and (2) mass loss from low-to-intermediate mass stars, in the case of O-18. Of the two known small-scale deviations from the general uniformity, the O-18/O-17 ratio in the IRC+10216 evolving star envelope, which is found to be an order of magnitude below the galactic value, is nevertheless consistent with present understanding of nucleosynthesis, while the 40% O-18/O-17 enhancement in the solar system remains an enigma.