A primordial origin for molecular oxygen in comets: A chemical kinetics
  study of the formation and survival of O$_2$ ice from clouds to disks
  Molecular oxygen has been confirmed as the fourth most abundant molecule in
cometary material O$_2$/H$_2$O $\sim 4$ %) 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 disks, and iii) during luminosity
outbursts in disks. 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 ($\geq 10^5$ cm$^{-3}$), and a
moderate cosmic ray ionisation rate ($\leq 10^{-16}$ s$^{-1}$) while a
temperature of 20 K, slightly higher than the typical temperatures found in
dark clouds, also enhances the production of O$_2$. Disk models show that O$_2$
can only be formed in the gas phase in intermediate disk layers, and cannot
explain the strong correlation between O$_2$ and H$_2$O in comet 67P/C-G
together with the weak correlation between other volatiles and H$_2$O. However,
primordial O$_2$ ice can survive transport into the comet-forming regions of
disks. 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.
