  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 um (2092.3
cm-1) which we attribute to the vibrational stretching mode of the 13CO isotope
in pure CO icy grain mantles. This is the first detection of solid 13CO in the
interstellar medium. The 13CO band is a factor of 2.3 narrower than the apolar
component of the 12CO 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. 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 high quality multi-component
solid 12CO spectrum of NGC 7538 : IRS9 with that of the previously studied low
mass source L1489 IRS. The narrow absorption by apolar CO ices is present in
both spectra, but much stronger in NGC 7538. 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 relative 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 12CO band is used to
derive the 12CO/13CO abundance ratio in apolar ices. A ratio of 71+/-15 is
deduced, in good agreement with gas phase CO studies (~77) and the solid
12CO2/13CO2 ratio of 80+/-11 found in the same line of sight. The implications
for the chemical path along which CO2 is formed are discussed.
