Backlighting young stellar objects in the Central Molecular Zone An ensemble-averaged abundance structure of methanol ices

Context. The Central Molecular Zone (CMZ) of the Milky Way contains a substantial reservoir of dense molecular gas, where numerous young stellar objects (YSOs) and dense cloud cores have been identified, offering valuable opportunities to investigate star formation in the nuclear regions of spiral galaxies. However, the large distance and severe foreground extinction complicate detailed interpretation, particularly of infrared absorption features from various ice species that trace the chemical composition and evolutionary state of these embedded objects. Aims. To better characterise YSOs and dense cores in this region, we combined spectra from multiple YSOs, each likely backlit by a giant star, allowing us to probe their outer layers and derive an ensemble-averaged ice abundance profile. Methods. We obtained L-band spectra of 15 point-like sources with extremely red colours using GNIRS on Gemini North, enabling measurements of the CH₃OH absorption feature at 3.535 μm. A subset of these sources was also observed in the K band. To better constrain the foreground extinction and H₂O ice column densities, we combined these data with K-band and mid-infrared spectra from our previous observations using NASA/IRTF and Spitzer/IRS. Results. We found that the CH₃OH abundance in the CH₃OH-CO₂ ice mixture (traced by the 15.4 μm shoulder) lies between 2-5%, reaffirming previous findings and confirming that it is systematically lower than the 5-15% typically observed in the Galactic disc. Furthermore, by using the local excess of foreground extinction as a proxy for the projected distance between a backlit source and the centre of a YSO, we found that the CH₃OH abundance relative to solid CO₂ remains at the ∼10% level in the inner regions of the envelope, but increases sharply to ∼30% in the outer regions. Conclusions. The relatively low methanol ice abundance may reflect the unique chemical environment of the CMZ, including variations in elemental abundances and surface reaction pathways on dust grains. However, our results offer an alternative interpretation: since our sample is biased towards massive and luminous YSOs, intense heating from the central protostar may have caused substantial sublimation of methanol ice in the inner regions of their envelopes, thereby systematically lowering the observed CH₃OH/H₂O ice ratios.