Extraction and fitting of the 4 µm region. Left: adopted local continua, along with CH₃OH spectra determined from the 3.53 µm band fitting used to aid the continuum placement. The spectral region of the HDO O–D stretching mode is indicated in magenta. Right: fits to the extracted optical depth spectra (am. = amorphous; cr. = crystalline). The shaded gray regions indicate 3σ uncertainty levels. The same combination of pure and H₂O-rich CH₃OH spectra as used to fit the 3.53 µm feature is used in this fit. The best-fitting temperatures of the amorphous and crystalline HDO ice components are 132 K and 47 K for HOPS 370 and 138 K and 90 K for IRAS 20126, respectively (see Sect. 3.3 and Appendix B for more details).