The Distribution of Complex Organic Molecules in Barnard 5

We present an observational study aiming to set constraints on the formation mechanisms of complex organic molecules (COMs) in cold and dense molecular cloud sources. The so-called methanol hotspot in B5 (B5-Hotspot) is an ideal testbed for this purpose. It is a cold (~9.5 K) and dense (~6.8 x 10-4 cm-3) source, characterized by high abundances of gas phase CH3OH (~5.4 x 10-9) and H2O (~2.8 x 10-9) w.r.t. H2. In addition, the larger COMs CH3CHO, CH3OCHO, and CH3OCH3 are identified with abundances around 10-10. It is known from mappings that the abundance of CH3OH is peaking at B5-Hotspot, while larger COMs have been studied only with pointings towards this position. However, it is largely undetermined what mechanism is causing the efficient release of methanol and water. In our study, we made pointed observations with the IRAM 30m and Onsala 20m telescopes towards two positions close to B5-Hotspot: one position with a lower AV at the edge of the B5 ridge and one position with a higher AV, i.e., the dense core B5-East 189. We targeted the same COMs identified at B5-Hotspot and find that abundances of larger COMs are peaking together with methanol at B5-Hotspot. We argue that this can be explained most easily if these COMs are formed alongside methanol at the surface of dust grains and released by the same efficient desorption mechanism. It requires chemical modelling to test if the observed COM abundance patterns could also be produced by the gas phase formation of COMs.