The interstellar C18O/C17O ratio in the solar neighbourhood:: The ρ Ophiuchus cloud

Observations of up to ten carbon monoxide (CO and isotopomers) transitions are presented to study the interstellar (CO)-O-18/(CO)-O-17 ratio towards 21 positions in the nearby (d similar to 140 pc) low-mass star forming cloud rho Oph. A map of the (CO)-O-18 J = 1-0 distribution of parts of the cloud is also shown. An average (CO)-C-12-O-18/(CO)-C-12-O-17 isotopomeric ratio of 4.11 +/- 0.14, reflecting the O-18/O-17 isotope ratio, is derived from Large Velocity Gradient (LVG) calculations. From LTE column densities we derive a ratio of 4.17 +/- 0.26. These calculations also show that the kinetic temperature decreases from about 30 K in the cloud envelope to about 10 K in the cloud cores. This decrease is accompanied by an increase of the average molecular hydrogen density from 10(4) cm(-3) to greater than or similar to 10(5) cm(-3). Towards some lines of sight (CO)-O-18 optical depths reach values of order unity.