HIGH RESOLUTION 4.7 μm KECK/NIRSPEC SPECTRA OF PROTOSTARS. II: DETECTION OF THE CO ISOTOPE IN ICY GRAIN MANTLES

The high resolution (R=25,000) infrared M band spectrum of the massive protostar NGC 7538 : IRS9 shows a narrow absorption feature at 4.779 μm (2092.3 cm) which we attribute to the vibrational stretching mode of the CO isotope in pure CO icy grain mantles. This is the first detection of CO in icy grain mantles in the interstellar medium. The CO band is a factor of 2.3 narrower than the apolar component of the CO band. With this in mind, we discuss the mechanisms that broaden solid state absorption bands. It is shown that ellipsoidally shaped pure CO grains fit the bands of both isotopes at the same time. Slightly worse, but still reasonable fits are also obtained by CO embedded in N2–rich ices and thermally processed O2–rich ices. In addition, we report new insights into the the nature and evolution of interstellar CO ices by comparing the very high resolution multi-component solid CO spectrum of NGC 7538 : IRS9 with that of the previously studied low mass source L1489 IRS. The narrow absorption of apolar CO ices is present in both spectra, but much stronger in NGC 7538 : IRS9. It is superposed on a smooth broad absorption feature well fitted by a combination of CO2 and H2O–rich laboratory CO ices. The abundances of the latter two ices, scaled to the total H2O ice column, are the same in both sources. We thus suggest that thermal processing manifests itself as evaporation of apolar ices only, and not the formation of CO2 or polar ices. Finally, the decomposition of the CO band is used to derive the CO/CO abundance ratio in apolar ices. A ratio of CO/CO=71±15 (3σ) is deduced, in good agreement with gas phase CO studies (∼77) and the solid CO2/ CO2 ratio of 80±11 found in the same line of sight. The implications for the chemical path along which CO2 is formed are discussed. Subject headings: Infrared: ISM—ISM: molecules—ISM: abundances—stars: formation—stars: individual (NGC 7538 : IRS9)—astrochemistry