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According to traditional gas-phase chemical models, O2 should be abundant inmolecular clouds, but until recently, attempts to detect interstellar O2 lineemission with ground- and space-based observatories have failed. Following themulti-line detections of O2 with low abundances in the Orion and rho Oph Amolecular clouds with Herschel, it is important to investigate otherenvironments, and we here quantify the O2 abundance near a solar-massprotostar. Observations of O2, at 487 GHz toward a deeply embedded low-massClass 0 protostar, NGC 1333-IRAS 4A, are presented, using the HIFI instrumenton the Herschel Space Observatory. Complementary data of the chemically relatedNO and CO molecules are obtained as well. The high spectral resolution data areanalysed using radiative transfer models to infer column densities andabundances, and are tested directly against full gas-grain chemical models. Thedeep HIFI spectrum fails to show O2 at the velocity of the dense protostellarenvelope, implying one of the lowest abundance upper limits of O2/H2 at<6x10^-9 (3 sigma). However, a tentative (4.5 sigma) detection of O2 is seen atthe velocity of the surrounding NGC 1333 molecular cloud, shifted by 1 km/srelative to the protostar. For the protostellar envelope, pure gas-phase modelsand gas-grain chemical models require a long pre-collapse phase (~0.7-1x10^6years), during which atomic and molecular oxygen are frozen out onto dustgrains and fully converted to H2O, to avoid overproduction of O2 in the denseenvelope. The same model also reproduces the limits on the chemically relatedNO molecule. The tentative detection of O2 in the surrounding cloud isconsistent with a low-density PDR model with small changes in reaction rates.The low O2 abundance in the collapsing envelope around a low-mass protostarsuggests that the gas and ice entering protoplanetary disks is very poor in O2.

Deep observations of O2 toward a low-mass protostar with Herschel-HIFI