CO mapping and multi-line-analysis of Cepheus B

We present large scale (approx. 15 x 15 arcmin) on-the-fly maps of the Ceph eus B molecular cloud in the J = 3-2 and 2-1 transitions of (CO)-C-12, (CO) -C-13 and (CO)-O-18 obtained with the KOSMA 3m-telescope. We use these maps to study the spatial variation of the excitation conditions of the molecul ar gas. Additional Cr observations allow to refine our analysis. The gas temperature is dominated by UV radiation from identified adjacent a nd embedded sources plus additional heating by a hot core in the north-west of the cloud. A spherical clump PDR model allows to reproduce the observed line ratios at most positions. Only (CO)-C-12 line ratios at the hot core position are inconsistent with this scenario and hence give evidence for ad ditional heating mechanisms. Local volume densities are similar to 2(.)10(4 ) cm(-3) while the average volume density of Cepheus B is less than 10(3) c m(-3); thus, Cepheus B is highly clumped and the volume of these clumps fil ls only 2% to 4% of the whole cloud. Deep penetration of FUV radiation into the clumpy medium is consistent with the detection of [C-I] emission from within the cloud. Abundance ratios of atomic carbon to CO are greater than or equal to 0.2. The (CO)-C-13/(CO)-O-18 integrated line intensity ratio rises significantly above the isotopic element ratio at the cloud edges where (CO)-O-18 intens ities are low. Possible explanations for this common observation in terms o f fractionation and selective photodissociation on clump surfaces in a clum py cloud are discussed.