C-12/C-13 ISOTOPE RATIO ACROSS THE GALAXY FROM OBSERVATIONS OF C-13 C-18 IN MOLECULAR CLOUDS

We have studied the carbon isotopic ratio across the Galaxy by observing nine interstellar clouds in the doubly rare isotope of carbon monoxide, 13C18O, and in the rare isotopic species 12C18O. The goal in this study was to improve the data from optically thin carbon monoxide emissions. We find a systematic gradient in the 12C/13C isotopic ratio across the Galaxy, ranging from about 30 in the inner part at 5 kpc to about 70 at 12 kpc, with a Galactic center value of 24. Near the solar radius the average ratio is 57, but it is somewhat larger, about 67, in Orion. Carbon monoxide isotopic ratios are smaller than those of formaldehyde, but they have roughly the same slope across the Galaxy. Our result of 57 for the local CO ratio lies between the values from various CH+ absorption measurements.