Formation of interstellar methanol ice prior to the heavy CO freeze-out stage

¹ Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: dqasim@strw.leidenuniv.nl
² INAF–Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy
³ School of Electronic Engineering and Computer Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK
⁴ School of Physical Sciences, STEM, Open University, Milton Keynes MK7 6AA, UK
⁵ Institute for Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Drive, Honolulu 96822-1839, USA

Received: 24 November 2017
Accepted: 17 January 2018

Abstract

Context. The formation of methanol (CH₃OH) on icy grain mantles during the star formation cycle is mainly associated with the CO freeze-out stage. Yet there are reasons to believe that CH₃OH also can form at an earlier period of interstellar ice evolution in CO-poor and H₂O-rich ices.

Aims. This work focuses on CH₃OH formation in a H₂O-rich interstellar ice environment following the OH-mediated H-abstraction in the reaction, CH₄ + OH. Experimental conditions are systematically varied to constrain the CH₃OH formation yield at astronomically relevant temperatures.

Methods. CH₄, O₂, and hydrogen atoms are co–deposited in an ultrahigh vacuum chamber at 10–20 K. OH radicals are generated by the H + O₂ surface reaction. Temperature programmed desorption – quadrupole mass spectrometry (TPD–QMS) is used to characterize CH₃OH formation, and is complemented with reflection absorption infrared spectroscopy (RAIRS) for CH₃OH characterization and quantitation.

Results. CH₃OH formation is shown to be possible by the sequential surface reaction chain, CH₄ + OH → CH₃ + H₂O and CH₃ + OH → CH₃OH at 10–20 K. This reaction is enhanced by tunneling, as noted in a recent theoretical investigation Lamberts et al. (2017, A&A, 599, A132). The CH₃OH formation yield via the CH₄ + OH route versus the CO + H route is approximately 20 times smaller for the laboratory settings studied. The astronomical relevance of the new formation channel investigated here is discussed.