Calculated optical depth for all five sources in wavelength range of interest. Gray shaded area shows the optical-depth 3σ uncertainty level. Although the spectra are extracted off-source (see Table A.1), some CO emission lines are still seen in the spectra. The CN stretching mode vibration band is shaded in light blue. The emission line between the ¹³CO₂ ice feature and the CN stretching band is the H₂ 0 – 0 S(10) line (marked with the dashed magenta line).