1999A&A...346L..57V

The Short Wavelength Spectrometer (SWS) on-board the Infrared Space Observatory (ISO) has been used to search for solid O₂ in cold dense clouds at 6.45µm. Additional constraints on the O₂ abundance are obtained from analysis of the 4.67µm solid CO absorption profile observed from the ground. We derive upper limits of 50% and 100% of solid O₂ relative to solid CO toward the protostellar sources R CrA IRS2 and NGC 7538 IRS9 respectively, corresponding to abundances of 30x10^–6 and 15x10^–6 relative to n_H. These results indicate that the abundance of solid O₂ in dense clouds accounts for less than 6% of the total oxygen budget in the interstellar medium. The reservoirs of oxygen in dense clouds are discussed, taking into account recent measurements of oxygen-bearing species.