2013A&A...552A.122B

One of the big questions in astrochemistry is whether complex organic molecules are formed in the gas phase after evaporation of the icy mantles of interstellar dust grains or at intermediate temperatures within these icy mantles. Dimethyl ether (CH₃OCH₃) is one of these species that may form through either of these mechanisms, but it is yet unclear which is dominant. The goal of this paper is to determine the respective importance of solid state vs. gas phase reactions for the formation of dimethyl ether. This is done by a detailed analysis of the excitation properties of the ground state and the torsionally excited states, v₁₁=1 and v₁₅=1, toward the high-mass star-forming region G327.3-0.6. With the Atacama Pathfinder EXperiment 12m submillimeter telescope, we performed a spectral line survey toward G327.3-0.6 around 1.3, 1.0, and 0.9mm as well as at 0.43 and 0.37mm. The observed CH₃OCH₃ spectrum is modeled assuming local thermal equilibrium. CH₃OCH₃ has been detected in the ground state, v=0, and in the torsionally excited states v₁₁=1 and v₁₅=1, for which lines have been detected here for the first time. The emission is modeled with an isothermal source structure as well as with a non-uniform spherical structure. In the isothermal case two components at 80 and 100K are needed to reproduce the dimethyl ether emission, whereas an abundance jump at 85K or a model with two abundance jumps at 70 and 100K fit the emission equally well for the non-uniform source model. The emission from the torsionally excited states, v₁₁=1 and v₁₅=1, is very well fit by the same model as the ground state. For non-uniform source models one abundance jump for dimethyl ether is sufficient to fit the emission, but two components are needed for the isothermal models. This suggests that dimethyl ether is present in an extended region of the envelope and traces a non-uniform density and temperature structure. Both types of models furthermore suggest that most dimethyl ether is present in gas that is warmer than 100K, but a smaller fraction of 5%-28% is present at temperatures between 70 and 100K. The dimethyl ether present in this cooler gas is likely formed in the solid state, while gas phase formation probably is dominant above 100K. Finally, the v₁₁=1 and v₁₅=1 torsionally excited states are easily excited under the density and temperature conditions in G327.3-0.6 and will thus very likely be detectable in other hot cores as well.