Gas-phase CO toward massive protostars^*

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
² SRON National Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
³ Department of Physics and Astronomy, Denison University, Granville, Ohio 43023, USA

Corresponding author: A. M. S. Boonman, boonman@strw.leidenuniv.nl

Received: 4 November 2002
Accepted: 16 December 2002

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  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.