Carbon isotopes in PDRs

We propose to perform deep integrations for a significant detection of the 13CII and 13CI hyperfine transitions towards five PDRs where previous observations showed marginal, inconsistent or particularlypuzzling results. These included anomalous hyperfine ratios andstrong variations of the fractionation within one source. The observations will measure the absolute C+ fractionation, they willallow to distinguish the effects of isotopeselective photodissociationand chemical fractionation, and constrain the total isotopic ratio of 13C to 12C. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]