Gas-phase CO₂ toward massive protostars

We present infrared spectra of gas-phase CO2 around 15 μm 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 K. 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– for K and of ~10-8 for K 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 K. 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 K.