Gas-phase CO2 toward massive protostars

Abstract

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 T > 300 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 1-2 × 10-6 for T > 300 K and of ∼10-8 for T < 300 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 T < 100 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 T > 300 K.