Destruction of dimethyl ether and methyl formate by collisions with He⁺

¹ Department of Physics, University of Trento, Via Sommarive 14, 38123 Povo, Italy
e-mail: daniela.ascenzi@unitn.it
² Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy
e-mail: nadia.balucani@unipg.it
³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: cecilia.ceccarelli@univ-grenoble-alpes.fr
⁴ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy

Received: 6 November 2018
Accepted: 11 March 2019

Abstract

Context. To correctly model the abundances of interstellar complex organic molecules (iCOMs) in different environments, both formation and destruction routes should be appropriately accounted for. While several scenarios have been explored for the formation of iCOMs via grain and gas-phase processes, much less work has been devoted to understanding the relevant destruction pathways, with special reference to (dissociative) charge exchange or proton transfer reactions with abundant atomic and molecular ions such as He⁺, H₃⁺ and HCO⁺.

Aims. By using a combined experimental and theoretical methodology we provide new values for the rate coefficients and branching ratios (BRs) of the reactions of He⁺ ions with two important iCOMs, namely dimethyl ether (DME) and methyl formate (MF). We also review the destruction routes of DME and MF by other two abundant ions, namely H₃⁺ and HCO⁺.

Methods. Based on our recent laboratory measurements of cross sections and BRs for the DME/MF + He⁺ reactions over a wide collision energy, we extended our theoretical insights on the selectivity of the microscopic dynamics to calculate the rate coefficients k(T) in the temperature range from 10 to 298 K. We implemented these new and revised kinetic data in a general model of cold and warm gas, simulating environments where DME and MF have been detected.

Results. Due to stereodynamical effects present at low collision energies, the rate coefficients, BRs and temperature dependences here proposed differ substantially from those reported in KIDA and UDfA, two of the most widely used astrochemical databases. These revised rates impact the predicted abundances of DME and MF, with variations up to 40% in cold gases and physical conditions similar to those present in prestellar cores.

Conclusions. This work demonstrates that the accuracy of astrochemical models can be improved by a thorough characterisation of the destruction routes of iCOMs. The details of the chemical systems can, indeed, strongly affect their efficiency and significant deviations with respect to the commonly used Langevin model estimates are possible.