2000A&A...362.1109B

We present large scale (approx. 15x15arcmin) on-the-fly maps of the Cepheus B molecular cloud in the J=3-2 and 2-1 transitions of ¹²CO, ¹³CO and C¹⁸O obtained with the KOSMA 3m-telescope. We use these maps to study the spatial variation of the excitation conditions of the molecular gas. Additional Ci observations allow to refine our analysis. The gas temperature is dominated by UV radiation from identified adjacent and 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 ¹²CO line ratios at the hot core position are inconsistent with this scenario and hence give evidence for additional heating mechanisms. Local volume densities are ∼2x10⁴cm^–3 while the average volume density of Cepheus B is less than 10³cm^–3; thus, Cepheus B is highly clumped and the volume of these clumps fills only 2% to 4% of the whole cloud. Deep penetration of FUV radiation into the clumpy medium is consistent with the detection of [CI] emission from within the cloud. Abundance ratios of atomic carbon to CO are ≥0.2. The ¹³CO/C¹⁸O integrated line intensity ratio rises significantly above the isotopic element ratio at the cloud edges where C¹⁸O intensities are low. Possible explanations for this common observation in terms of fractionation and selective photodissociation on clump surfaces in a clumpy cloud are discussed.