Abstract
We have detected emission from the two lowest rotational transitions of C17O from NGC 253, M 82, and, tentatively, IC 342. Observations of the corresponding transitions of C18O were used to determine O-18/O-17 isotope ratios, which are 8.3(+/- 1.3) toward M 82, 10(+/- 3) toward NGC 253 and greater-than-or-similar-to 5.5 toward IC 342. These values are much higher than the approximately 3.6 found in Galactic sources (e.g. Wannier 1980; Penzias 1981) and two are definitely larger than the solar system ratio, 5.5. The results can be understood if, during a previous epoch of star formation, high mass stars were preferentially formed near the centers of NGC 253 and M 82, if we assume that O-18 is mainly produced in high mass stars. This leads to an underabundance of O-17 relative to O-18. This interpretation is also consistent with the high ratios of (CO)-C-12/(CO)-C-13 observed in NGC253 and M 82, which suggest an underabundance of C-13.