OBSERVATIONS OF INTER-STELLAR [(H2CO)-C-13]-[(H2CO)-O-18] ABUNDANCE RATIOS

The 4388-MHz transition of H2C18O and the 4593-MHz transi- tion of H213CO have been measured in 10 molecular clouds. The median value of the [H213CO]/[H2C18O] abundance ratio (7.7) is higher than values obtained in other studies. The mean value for the three cases near the Galactic Centre (10.5 ± 0.6) is 50 per cent higher than the value outside the centre (7.0±0.5), which is closer to the terrestrial value of 5.5 for the [13C/12C]/ [180/160] abundance ratio. © Royal Astronomical Society.