Aims. Solid O 2 has been proposed as a possible reservoir for molecular oxygen in dense clouds through freeze-out processes. The aim of this work is to characterize quantitatively the physical processes that are involved in the desorption kinetics of CO-O 2 ices by interpreting laboratory temperature programmed desorption (TPD) data. This information is used to simulate the behavior of CO-O 2 ices under astrophysical conditions. Methods. The TPD spectra have been recorded under ultra high vacuum conditions for pure, layered and mixed morphologies for different thicknesses, temperatures and mixing ratios. An empirical kinetic model is used to interpret the results and to provide input parameters for astrophysical models. Results. Binding energies are determined for different ice morphologies. Independent of the ice morphology, the desorption of O 2 is found to follow 0th-order kinetics. Binding energies and temperature-dependent sticking probabilities for CO-CO, O 2 -O 2 and CO-O 2 are determined. O 2 is slightly less volatile than CO, with a binding energy of 912 ± 15 versus 858 ± 15 K for pure ices. In mixed and layered ices, CO does not co-desorb with O 2 but its binding energy is slightly increased compared to pure ice whereas that of O 2 is slightly decreased. Lower limits to the sticking probabilities of CO and O 2 are 0.9 and 0.85, respectively, at temperatures below 20 K. The balance between accretion and desorption is studied for O 2 and CO in astrophysically relevant scenarios. Only minor differences are found between the two species, i.e., both desorb between 16 and 18 K in typical environments around young stars. Thus, clouds with significant abundances of gaseous CO are unlikely to have large amounts of solid O 2 .

We measured the binding energy of N 2 , CO, O 2 , CH 4 , and CO 2 on non-porous (compact) amorphous solid water (np-ASW), of N 2 and CO on porous amorphous solid water (p-ASW), and of NH 3 on crystalline water ice. We were able to measure binding energies down to a fraction of 1% of a layer, thus making these measurements more appropriate for astrochemistry than the existing values. We found that CO 2 forms clusters on np-ASW surface even at very low coverages. The binding energies of N 2 , CO, O 2 , and CH 4 decrease with coverage in the submonolayer regime. Their values at the low coverage limit are much higher than what is commonly used in gas-grain models. An empirical formula was used to describe the coverage dependence of the binding energies. We used the newly determined binding energy distributions in a simulation of gas-grain chemistry for cold cloud and hot core models. We found that owing to the higher value of desorption energy in the sub-monlayer regime a fraction of all these ices stays much longer and up to higher temperature on the grain surface compared to the single value energies currently used in the astrochemical models.

Context. According to traditional gas-phase chemical models, O 2 should be abundant in molecular clouds, but until recently, attempts to detect interstellar O 2 line emission with ground-and space-based observatories have failed. Aims. Following the multi-line detections of O 2 with low abundances in the Orion and ρ Oph A molecular clouds with Herschel, it is important to investigate other environments, and we here quantify the O 2 abundance near a solar-mass protostar. Methods. Observations of molecular oxygen, O 2 , at 487 GHz toward a deeply embedded low-mass Class 0 protostar, NGC 1333-IRAS 4A, are presented, using the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory. Complementary data of the chemically related NO and CO molecules are obtained as well. The high spectral resolution data are analysed using radiative transfer models to infer column densities and abundances, and are tested directly against full gas-grain chemical models. Results. 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 ≤6 × 10 −9 (3σ). The O 2 /CO abundance ratio is less than 0.005. However, a tentative (4.5σ) detection of O 2 is seen at the velocity of the surrounding NGC 1333 molecular cloud, shifted by 1 km s −1 relative to the protostar. For the protostellar envelope, pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (∼0.7-1 × 10 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. Conclusions. 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 .

Accurate modeling of physical and chemical processes in the interstellar medium (ISM) requires detailed knowledge of how atoms and molecules adsorb on dust grains. However, the sticking coefficient, a number between 0 and 1 that measures the first step in the interaction of a particle with a surface, is usually assumed in simulations of ISM environments to be either 0.5 or 1. Here we report on the determination of the sticking coefficient of H 2 , D 2 , N 2 , O 2 , CO, CH 4 , and CO 2 on nonporous amorphous solid water. The sticking coefficient was measured over a wide range of surface temperatures using a highly collimated molecular beam. We showed that the standard way of measuring the sticking coefficient-the King-Wells method-leads to the underestimation of trapping events in which there is incomplete energy accommodation of the molecule on the surface. Surface scattering experiments with the use of a pulsed molecular beam are used instead to measure the sticking coefficient. Based on the values of the measured sticking coefficient, we suggest a useful general formula of the sticking coefficient as a function of grain temperature and molecule-surface binding energy. We use this formula in a simulation of ISM gas-grain chemistry to find the effect of sticking on the abundance of key molecules both on grains and in the gas phase.