Gas-phase and Surface Chemistryin the Massive Star-Forming Region RCW 120

Molecules in the interstellar medium form both in the gas phase and on dust particles. The chemical pathways for molecular formation are not yet well understood in detail, so the question of which pathway predominates for a particular molecule remains open. We analysed broadband emission spectra of a dense molecular clump in the RCW120 region, obtained with the APEX telescope in the 200–260 GHz range, to investigate molecular formation pathways in regions of massive star formation at an early evolutionary stage. The correlations between the derived molecular column densities, obtained under the LTE assumption, were investigated. An excess of methanol was found in the southern part of the dense clump compared to its northern part, while the abundance of other molecules, such as CH $${}_{3}$$ CN and CH $${}_{3}$$ CCH, remain comparable. The methanol abundance is also elevated relative to other oxygen-bearing molecules, such as OCS and SO. To identify possible causes of the enhanced methanol abundance in the southern part of the clump, we conducted simulations using the astrochemical model Presta in a two-phase approximation, accounting for chemical processes in both the gas phase and the mantles of dust grains. Modeling showed that the enhanced methanol abundance in the gas phase may be due to its photodesorption from icy mantles. At an $$A_{V}$$ value between $$4^{\textrm{m}}$$ and $$6^{\textrm{m}}$$ , methanol efficiently desorbs from ice mantles of dust grains upon interaction with photons, but it is not yet destroyed by UV radiation in the gas phase. A strong linear correlation between molecular column densities indicates that these molecules form in the same phase—either in the gas phase or on dust. However, integrated intensity maps of these molecules may differ, as seen for CCH and CH $${}_{3}$$ CN. If molecules form in different phases—one in the gas phase and the other in dust mantles—no correlation is observed, as in the case of CCH and CH $${}_{3}$$ OH. The weak correlation between methanol and oxygen-bearing molecules that form on dust suggests that only the upper part of the dust mantles, rich in CO ice, is being destroyed in the southern part of the clump.