SIMBAD references

Observations of photon dominated regions (PDRs) show that variations of the ¹³CO/C¹⁸O integrated line intensities are closely correlated to the total C¹⁸O brightness along the line of sight (l.o.s.). From these intensity variations a change of the N(¹³CO)/N(C¹⁸O) column density ratio from values of about 5 to 40 or larger was deduced in the past. We argue that this is naturally explained by a model of a clumpy cloud: low ratios are observed at positions of high l.o.s. column density, where the line emission is dominated by a few large clumps, while high ratios originate at positions covered by many small clumps, having low l.o.s. column density. Quantitative agreement of our models with observations of the S140 and the Orion Bar PDR is achieved without any additional assumptions about the source geometry, thus avoiding the problems of previous models with plane parallel geometry, which required an ad hoc correlation between the l.o.s. column density and the attenuation toward the exciting sources. We conclude, that at least for high radiation field environments the C¹⁸O integrated line intensity is not a valid column density tracer.