2003A&A...399.1063B

We present infrared spectra of gas-phase CO₂ around 15µm toward 14 deeply embedded massive protostars obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory. Gas-phase CO₂ has been detected toward 8 of the sources. The excitation temperature and the gas/solid ratio increase with the temperature of the warm gas. Detailed radiative transfer models show that a jump in the abundance of two orders of magnitude is present in the envelope of AFGL 2591 at T>300K. No such jump is seen toward the colder source NGC 7538 IRS9. Together, these data indicate that gas-phase CO₂ shows the same evolutionary trends as CO₂ ice and other species, such as HCN, C₂H₂, H₂O, and CH₃OH. The gas-phase CO₂ abundance toward cold sources can be explained by gas-phase chemistry and possible freeze-out in the outer envelope. Different chemical scenarios are proposed to explain the gas-phase CO₂ abundance of 1-2x10^–6 for T>300K and of ∼10^–8 for T<300K toward AFGL 2591. The best explanation for the low abundance in the warm exterior is provided by destruction of CO₂ caused by the passage of a shock in the past, combined with freeze-out in the coldest part at T<100K. The high abundance in the interior at temperatures where all oxygen should be driven into H₂O is unexpected, but may be explained either by production of OH through X-ray ionization leading to the formation of abundant gas-phase CO₂, or by incomplete destruction of evaporated CO₂ for T>300K.