Letter to the Editor Constraints on the abundance of solid O2 in dense clouds from ISO-SWS and ground-based observations

The Short Wavelength Spectrometer (SWS) onboard the Infrared Space Observatory (ISO) has been used to search for solid O2 in cold dense clouds at 6.45 μm. Additional constraints on the O2 abundance are obtained from analysis of the 4.67 μm 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 30×10−6 and 15×10−6 relative to nH . 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.