SIMBAD references

Observations of up to ten carbon monoxide (CO and isotopomers) transitions are presented to study the interstellar C¹⁸O/C¹⁷O ratio towards 21 positions in the nearby (d∼140pc) low-mass star forming cloud ρ Oph. A map of the C¹⁸O J=1-0 distribution of parts of the cloud is also shown. An average ¹²C¹⁸O/¹²C¹⁷O isotopomeric ratio of 4.11±0.14, reflecting the ¹⁸O/¹⁷O 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 30K in the cloud envelope to about 10K in the cloud cores. This decrease is accompanied by an increase of the average molecular hydrogen density from 10⁴cm^–3 to >10⁵cm^–3. Towards some lines of sight C¹⁸O optical depths reach values of order unity.