High‐Resolution 4.7 Micron Keck/NIRSPEC Spectra of Protostars. II. Detection of the¹³CO Isotope in Icy Grain Mantles

The high-resolution ( R = 25,000) infrared M -band spectrum of the massive protostar NGC 7538 IRS 9 shows a narrow absorption feature at 4.779 μm (2092.3 cm -1 ) that we attribute to the vibrational stretching mode of the 13 CO isotope in pure CO icy grain mantles. This is the first detection of 13 CO in icy grain mantles in the interstellar medium. The 13 CO band is a factor of 2.3 narrower than the apolar component of the 12 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 N 2 -rich ices and thermally processed O 2 -rich ices. In addition, we report new insights into the nature and evolution of interstellar CO ices by comparing the very high resolution multicomponent solid 12 CO spectrum of NGC 7538 IRS 9 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 IRS 9. It is superposed on a smooth broad absorption feature well fitted by a combination of CO 2 and H 2 O-rich laboratory CO ices. The abundances of the latter two ices, scaled to the total H 2 O 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 CO 2 or polar ices. Finally, the decomposition of the 12 CO band is used to derive the 12 CO/ 13 CO abundance ratio in apolar ices. A ratio of 12 CO/ 13 CO = 71 ± 15 (3 σ) is deduced, in good agreement with gas-phase CO studies (~77) and the solid 12 CO 2 / 13 CO 2 ratio of 80 ± 11 found in the same line of sight. The implications for the chemical path along which CO 2 is formed are discussed.