H$_{mathsf 2}$CO and CH$_{mathsf 3}$OH maps of the Orion Bar photodissociation region

摘要

Context. A previous analysis of methanol andformaldehyde towards the Orion Bar concludes that the two molecularspecies may trace different physical components, methanol the clumpymaterial, and formaldehyde the interclump medium. Aims. To verify this hypothesis, we performedmulti-line mapping observations of the two molecules to study theirspatial distributions. Methods. The observations were performed with theIRAM-30mtelescope at 218 and 241GHz, withan angular resolution of .Additional data for H2CO from the Plateau deBure array are also discussed. The data were analysed using an LVGapproach. Results. Both molecules are detected in oursingle-dish data. Our data show that CH3OH peakstowards the clumps of the Bar, but its intensity decreases below thedetection threshold in the interclump material. When averaging over alarge region of the interclump medium, the strongest CH3OHline is detected with a peak intensity of 0.06K. Formaldehyde also peaks on theclumps, but it is also detected in the interclump gas. Conclusions. We verified that the weak intensity ofCH3OH in the interclump medium is not caused bythe different excitation conditions of the interclump material, butreflects a decrease in the column density of methanol. The abundance ofCH3OH relative to H2COdecreases by at least one order of magnitude from the dense clumps tothe interclump medium. Key words: ISM: individual objects: OrionBar - ISM: abundances - ISM: molecules - ISM: structure