The interaction between interstellar dust, ice and gas plays a major role for the chemistry in regions where star and planets form. Different reactions occur in the gas and on the ice mantles of dust grains in these regions, and consequently the mutual exchange of matter between the two phases is what regulates the chemical evolution of newborn stars and planets. Methanol (CH₃OH) is a key molecule in this process, as it predominantly forms through the sequential addition of hydrogen atoms to condensed CO molecules. Once present on the ice mantles, it is considered a fundamental precursor of more complex interstellar species.
To determine the importance of the various chemical processes governing this interplay, I conduct multi-wavelength observational studies and obtain relative abundances of solid- and gas-phase molecules, particularly of methanol. This methodology allows to calculate gas-to-ice ratios and directly access the efficiencies of condensation and desorption processes. The selected targets are the cold protostellar envelopes of low-mass stars belonging to three nearby star-forming regions, with distinct physical conditions and histories. By comparing these it is possible to test the dependencies of the chemical evolution of protostars on the large-scale environment from which they form.
The calculated CH₃OH gas-to-ice ratios of the order of ∼ 10^–3 - 10^–4 validate previous experimental and theoretical predictions, consistent with a considerably efficient non-thermal desorption mechanism in cold envelopes. Similarities in the gas-to-ice ratios in different nearby star-forming regions suggest that the CH₃OH-mediated chemistry in the outer protostellar envelopes is relatively independent on variations of the physical conditions. This might explain the ubiquitous presence of methanol in a variety of interstellar and circumstellar environments. The combination of millimetric and infrared observations presented in this thesis has proven to be an essential tool to cast light onto the small-scale variations in the ice chemistry and its relation to the physics on large scales in star-forming regions. Thereby, these studies serve as a critical pathfinder for future work with the James Webb Space Telescope and the Atacama Large Millimeter/submillimeter Array to constrain further the routes to the formation of complex molecules during the embedded stages of star formation.