The distribution of complex organic molecules in Barnard 5

Journal article, 2026

Complex organic molecules (COMs) are prevalent in cold and dense molecular cloud environments and are formed by a combination of gas phase and grain surface chemical reactions. However, open questions remain about the exact formation pathways of many COMs and the desorption mechanisms of molecular ices at low temperatures. The aim of this study was to investigate the distribution of COMs in the area of the so-called methanol hotspot in the Barnard 5 dark cloud (B5-Hotspot) to better characterize the efficient desorption of COMs in that region, and to set observational constraints on the low-temperature formation of COMs. The IRAM 30 m and OSO 20 m telescopes were used to make pointed observations toward two positions close to B5-Hotspot, i.e., B5-Edge and B5-East 189, targeting transitions of CH3OH, CH3CHO, CH3OCHO, and CH3OCH3, as well as the five-atom COMs HCOOH and CH2CO. Emission from all the targeted COMs is detected toward both B5-Edge and B5-East 189. Non-LTE (local thermodynamic equilibrium) radiative transfer models for methanol do not suggest significant differences in the physical conditions of B5-Edge, B5-Hotspot, and B5-East 189, as probed by gas kinetic temperature T-k and H-2 volume density n(H-2). However, the derived values of T-k and n(H-2) for B5-Hotspot using a new set of collisional rate coefficients differ significantly from previous estimates. Our results indicate that the efficient formation and desorption of COMs is not confined to B5-Hotspot, but is also active at the nearby positions B5-Edge and B5-East 189. When compared to COM data from the literature, our data supports a correlation between CH3OCHO and CH3OCH3, as well as the surface formation of CH3CHO by hydrogenation of CH2CO.