A primordial origin for molecular oxygen in comets: a chemical kinetics study of the formation and survival of O2 ice from clouds to discs

Molecular oxygen has been confirmed as the fourth most abundant molecule in cometary material (O-2/H2O similar to 4 per cent) and is thought to have a primordial nature, i.e. coming from the interstellar cloud from which our Solar system was formed. However, interstellar O-2 gas is notoriously difficult to detect and has only been observed in one potential precursor of a solar-like system. Here, the chemical and physical origin of O-2 in comets is investigated using sophisticated astrochemical models. Three origins are considered: (i) in dark clouds; (ii) during forming protostellar discs; and (iii) during luminosity outbursts in discs. The dark cloud models show that reproduction of the observed abundance of O-2 and related species in comet 67P/C-G requires a low H/O ratio facilitated by a high total density (>= 10(5) cm(-3)), and a moderate cosmic ray ionization rate (<= 10(-16) s(-1)) while a temperature of 20 K, slightly higher than the typical temperatures found in dark clouds, also enhances the production of O-2. Disc models show that O-2 can only be formed in the gas phase in intermediate disc layers, and cannot explain the strong correlation between O-2 and H2O in comet 67P/C-G together with the weak correlation between other volatiles and H2O. However, primordial O-2 ice can survive transport into the comet-forming regions of discs. Taken together, these models favour a dark cloud (or 'primordial') origin for O-2 in comets, albeit for dark clouds which are warmer and denser than those usually considered as Solar system progenitors.