NGC 1333-IRAS 4A C^18^O, NO and O_2_ spectra

The deep HIFI spectrum fails to show O_2_ at the velocity of the dense protostellar envelope, implying one of the lowest abundance upper limits of O_2_/H_2_ at <6x10^-3^ (3 sigma). The O_2_/CO abundance ratio is less than 0.005. However, a tentative (4.5 sigma) detection of O_2_ is seen at the velocity of the surrounding NGC1333 molecular cloud, shifted by 1km/s relative to the protostar.

For the protostellar envelope, pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (~0.7-1x10^6^-years), during which atomic and molecular oxygen are frozen out onto dust grains and fully converted to H_2_O, to avoid overproduction of O_2_ in the dense envelope. The same model also reproduces the limits on the chemically related NO molecule if hydrogenation of NO on the grains to more complex molecules such as NH_2_OH, found in recent laboratory experiments, is included.

The tentative detection of O_2_ in the surrounding cloud is consistent with a low-density PDR model with small changes in reaction rates. The low O_2_ abundance in the collapsing envelope around a low-mass protostar suggests that the gas and ice entering protoplanetary disks is very poor in O_2_.