Dissociative Recombination and the build-up of complex molecules in the Interstellar
Medium
111W. D. Geppert, M. Hamberg , E. Vigren , R. D. Thomas1, V. Zhaunerchyk1, E. S. Wirström2, C. M.
Persson2b, T. J. Millar3, J. Semaniak4 M. Kamińska4, F. Österdahl1, J. H. Black2, Å. Hjalmarson2, P.
Bergman2, M. Holmgren2, F. Hellberg5, and M. Larsson1
1

Stockholm University, Department of Physics, Roslagstullbacken 21, SE-10691
Stockholm, Sweden
2Onsala Space Observatory, SE-439 92 Onsala, Sweden
3
Queen’s University Belfast, University Road, Belfast, BT7 1NN, UK
4
Institute of Physics, Świętokrzyska Academy, PL-25406, Kielce, Poland
5Manne Siegbahn Laboratory, Frescativägen 24, SE-104 05 Stockholm, Sweden
methanol, ethanol, dimethyl ether and formic acid are produced in the gas-phase or on grain surfaces.
As gas-gas production pathways very often ion-neutral reactions leading to the protonated form of thespecies followed by dissociative recombination to yield the final product have been invoked.
In the case of methanol, a feasible gas-phase production process is unlikely. The rate of radiativeassociation of CH3+ and H2O leading to CH3OH2+ has been found to be far to low to explain theobserved methanol abundances [1] and, on top of that, only a minor fraction of methanol (3 %) is
produced in the dissociative recombination of the latter ion [2]. Introduction of these new findings into
state-of-the art model calculations of dark clouds yielded that the proposed gas-phase mechanism is by
far insufficient to explain the observed methanol abundances. On the other hand, successivehydrogenation of CO on icy grain surface by H atoms has been found to produce methanol [3].

Figure 1: Spectra of the 2-1 rotational line group of 13CH3OH (black) and 12CH3OH (gray) around 96 GHz

It now remains to be proven by direct observation that interstellar methanol really originates fromgrain surfaces. This can be done by the so-called, isotope labelling a posteriori, which was firstsuggested by Charnley et al. [4]. The method is based on the fact that 13C is preferably accumulated in
CO at low temperature, which in turn results in a 13C deficiency in other molecules forming from ionneutral reactions in the gas-phase [5]). Assuming that this selective fractionation remains unaltered by
the processes of adsorption and desorption, the 12C/13C ratio in various molecules could be used todistinguish between formation from CO on cold grains and gas-phase formation.
We therefore performed observations of the 12C/13C ratio for C18O and methanol in several massive
young stellar objects, one pair of compact HII regions and one source hosting several young stellar
objects. The 2-1 rotational line groups of the 2 isotopomers around 96 GHz were observed using the20m telescope at Onsala Space Observatory. (see Fig. 1). With one exception (where the 12C moleculelines are very probably optically thick) the agreement of the 12C/13C ratio on the two compounds isexcellent. This points to a surface origin of interstellar methanol, which is also corroborated by the factthat a strong correlation of abundances between methanol and formaldehyde has been found in several
star-forming regions [6].
Furthermore, the role of dissociative recombination in the formation of other, more complex
molecules detected in star-forming regions like ethanol, dimethyl ether and ethanol will be discussed.
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