Fine-tuning the complex organic molecule formation: Sulfur and CO ice as regulators of surface chemistry

Grain-surface chemistry plays a crucial role in the formation of molecules of astrobiological interest, including H_2S and complex organic molecules (COMs). These are commonly observed in the gas phase toward star-forming regions, but their detection in ices remains limited. Combining gas-phase observations with chemical modeling is therefore essential for advancing our understanding of their chemistry. The goal is to investigate the factors that promote or hinder molecular complexity combining gas-phase observations of CH_3OH, H_2S, OCS, N_2H^+, and C^18O with chemical modeling in two prototypical dense cores: Barnard-1b and IC348. We observed millimeter emission lines of CH_3OH, H_2S, OCS, N_2H^+, and C^18O along strips using the IRAM 30m and Yebes 40m telescopes. We used the gas-grain chemical model Nautilus to reproduce the observed abundance profiles, adjusting parameters such as initial sulfur abundances and binding energies. H_2S, N_2H^+, and C^18O gas-phase abundances vary up to one order of magnitude toward the extinction peak. The CH_3OH abundance remains quite uniform. Our chemical modeling revealed that these abundances can only be reproduced assuming a decreasing sulfur budget, which lowers H_2S and enhances CH_3OH abundances. Decreasing binding energies, which are expected in CO-rich apolar ices, are also required. The sulfur depletion required to explain H_2S is generally higher than that needed by CH_3OH, suggesting an unknown sulfur sink. These findings highlight the intricate relationship between sulfur chemistry and COM formation, driven by the competition between sulfur and CO for hydrogen atoms. The formation of COMs begins in the low-density envelopes of molecular clouds. The growth of CO ice and the progressive sequestration of hydrogen atoms are critical in determining whether chemical complexity can develop. Our study highlights that molecular complexity is closely tied to sulfur chemistry within dense cores, offering crucial insights into the early stages of star and planet formation.