On the origin of O$_2$ and other volatile species in comets
  Molecular oxygen, O$_2$, was recently detected in comet 67P by the ROSINA
instrument on board the Rosetta spacecraft with a surprisingly high abundance
of 4 % relative to H$_2$O, making O$_2$ the fourth most abundant in comet 67P.
Other volatile species with similar volatility, such as molecular nitrogen
N$_2$, were also detected by Rosetta, but with much lower abundances and much
weaker correlations with water. Here, we investigate the chemical and physical
origin of O$_2$ and other volatile species using the new constraints provided
by Rosetta. We follow the chemical evolution during star formation with
state-of-the-art astrochemical models applied to dynamical physical models by
considering three origins: i) in dark clouds, ii) during forming protostellar
disks, and iii) during luminosity outbursts in disks. The models presented here
favour a dark cloud (or "primordial") grain surface chemistry origin for
volatile species in comets, albeit for dark clouds which are slightly warmer
and denser than those usually considered as solar system progenitors.
