CH₃OCH₃ in Orion-KL: a striking similarity with HCOOCH₃^⋆

¹ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac France
e-mail: brouillet@obs.u-bordeaux1.fr; despois@obs.u-bordeaux1.fr; baudry@obs.u-bordeaux1.fr
² CNRS, LAB, UMR 5804, 33270 Floirac, France
³ Department of Physics and Astronomy, University of Århus, Ny Munkegade 120, 8000 Århus C, Denmark
e-mail: favre@phys.au.dk
⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
e-mail: awootten@nrao.edu; aremijan@nrao.edu
⁵ Naval Research Laboratory, Code 7210, Washington, DC 20375, USA
e-mail: tom.wilson@nrl.navy.mil
⁶ Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
e-mail: francoise.combes@obspm.fr
⁷ Laboratoire de Physique des Lasers, Atomes et Molécules, Université de Lille1, UMR 8523, 59655 Villeneuve d’ Ascq Cedex, France
e-mail: georges.wlodarczak@univ-lille1.fr

Received: 11 July 2012
Accepted: 3 December 2012

Abstract

Context. Orion-KL is a remarkable, nearby star-forming region where a recent explosive event has generated shocks that could have released complex molecules from the grain mantles.

Aims. A comparison of the distribution of the different complex molecules will help in understanding their formation and constraining the chemical models.

Methods. We used several data sets from the Plateau de Bure Interferometer to map the dimethyl ether emission with different arcsec spatial resolutions and different energy levels (from E_up = 18 to 330 K) to compare with our previous methyl formate maps.

Results. Our data show remarkable similarity between the dimethyl ether (CH₃OCH₃) and the methyl formate (HCOOCH₃) distributions even on a small scale (1.8″    ×    0.8″ or  ~500 AU). This long suspected similarity, seen from both observational and theoretical arguments, is demonstrated with unprecedented confidence, with a correlation coefficient of maps  ~0.8.

Conclusions. A common precursor is the simplest explanation of our correlation. Comparisons with previous laboratory work and chemical models suggest the major role of grain surface chemistry and a recent release, probably with little processing, of mantle molecules by shocks. In this case the CH₃O radical produced from methanol ice would be the common precursor (whereas ethanol, C₂H₅OH, is produced from the radical CH₂OH). The alternative gas phase scheme, where protonated methanol CH₃OH$\begin{array}{}\mathrm{+}\\ \mathrm{2}\end{array}$ is the common precursor to produce methyl formate and dimethyl ether through reactions with HCOOH and CH₃OH, is also compatible with our data. Our observations cannot yet definitely allow a choice between the different chemical processes, but the tight correlation between the distributions of HCOOCH₃ and CH₃OCH₃ strongly contrasts with the different behavior we observe for the distributions of ethanol and formic acid. This provides a very significant constraint on models.