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

TL;DR

This study demonstrates that methanol can form in early interstellar ices rich in water and poor in CO through surface reactions involving methane and hydroxyl radicals at very low temperatures, expanding understanding of methanol formation in space.

Contribution

It introduces a new formation pathway for interstellar methanol in water-rich ices prior to CO freeze-out, supported by laboratory experiments at cryogenic temperatures.

Findings

Methanol forms via CH4 + OH reaction at 10-20 K.

The formation yield is about 20 times lower than the CO + H route.

Surface tunneling enhances the methanol formation process.

Abstract

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. 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. 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. 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). 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.