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

Abstract

Context. The formation of methanol (CH 3 OH) 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 CH 3 OH also can form at an earlier period of interstellar ice evolution in CO-poor and H 2 O-rich ices. Aims. This work focuses on CH 3 OH formation in a H 2 O-rich interstellar ice environment following the OH-mediated H-abstraction in the reaction, CH 4 + OH. Experimental conditions are systematically varied to constrain the CH 3 OH formation yield at astronomically relevant temperatures. Methods. CH 4 , O 2 , and hydrogen atoms are co–deposited in an ultrahigh vacuum chamber at 10–20 K. OH radicals are generated by the H + O 2 surface reaction. Temperature programmed desorption – quadrupole mass spectrometry (TPD–QMS) is used to characterize CH 3 OH formation, and is complemented with reflection absorption infrared spectroscopy (RAIRS) for CH 3 OH characterization and quantitation. Results. CH 3 OH formation is shown to be possible by the sequential surface reaction chain, CH 4 + OH → CH 3 + H 2 O and CH 3 + OH → CH 3 OH 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 CH 3 OH formation yield via the CH 4 + 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.