HCO and CHOH maps of the Orion Bar photodissociation region^*

¹ ESO, Karl-Scharzschild-Strasse 2, 85748 Garching-bei-München, Germany e-mail: sleurini@mpifr.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁴ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d'Hères, France

Received: 13 May 2009
Accepted: 4 December 2009

Abstract

Context. A previous analysis of methanol and formaldehyde towards the Orion Bar concludes that the two molecular species may trace different physical components, methanol the clumpy material, and formaldehyde the interclump medium.

Aims. To verify this hypothesis, we performed multi-line mapping observations of the two molecules to study their spatial distributions.

Methods. The observations were performed with the IRAM-30 m telescope at 218 and 241 GHz, with an angular resolution of . Additional data for H₂CO from the Plateau de Bure array are also discussed. The data were analysed using an LVG approach.

Results. Both molecules are detected in our single-dish data. Our data show that CH₃OH peaks towards the clumps of the Bar, but its intensity decreases below the detection threshold in the interclump material. When averaging over a large region of the interclump medium, the strongest CH₃OH line is detected with a peak intensity of ~0.06 K. Formaldehyde also peaks on the clumps, but it is also detected in the interclump gas.

Conclusions. We verified that the weak intensity of CH₃OH in the interclump medium is not caused by the different excitation conditions of the interclump material, but reflects a decrease in the column density of methanol. The abundance of CH₃OH relative to H₂CO decreases by at least one order of magnitude from the dense clumps to the interclump medium.