Spectral decomposition of the mid-IR spectrum of Elias 29 protostar. a: optical depth scale of the observed spectrum between 2.5 and 20 μm, and the seven ice components used to fit the observed spectrum. The best fit is shown by the dashed green line. The residual spectrum is shown below this panel. b–f: zoom onto short spectral ranges highlighting the contribution of small features. In these plots, the spectral features of amorphous carbon, a−C(:H), that were not used in the spectral decomposition are shown by the dashed grey lines above the Elias 29 spectrum and the ice data components. This illustrates a potential contribution of features other than vibrational modes of ice species. Panel b: L-band red wing absorption excess that is not entirely reproduced by the ice components (see Sect. 4.2). Panels c and d: CO₂ and CO features. Panel e: important contributions of UV and cosmic-ray processed samples. Finally, panel f highlights the contribution of the broad NH₃ ice feature at 9 μm, CH₃CH₂OH at 9.6 μm, and CH₃OH at 9.8 μm.