CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy

Abstract

Context. Single-dish observations suggest that the abundances of organic species in star-forming regions of the outer Galaxy, which are characterised by sub-solar metallicities, are comparable to those found in the local Galaxy. Aims. To understand this counter-intuitive result and avoid a misleading interpretation due to beam dilution effects at these large distances, spatially resolved molecular emission maps are needed to correctly link the measured abundances and local physical properties. Methods. We observed several organic molecules with the Atacama Large Millimeter Array towards WB89-671, the source with the largest galactocentric distance (23.4 kpc) of the project CHEMical complexity in star-forming regions of the OUTer Galaxy (CHEMOUT) at a resolution of ~15 000 au. We compared the observed molecular abundances with chemical model predictions. Results. We detected emission of c-C3H2, C4H, CH3OH, H2CO, HCO, H13CO+, HCS+, CS, HN13C, and SO. The emission morphology is complex, extended, and different in each tracer. In particular, the most intense emission in H13CO+, H2CO and c-C3H2 arises from two millimeter-continuum infrared-bright cores. The most intense CH3OH and SO emission predominantly arises from the part of the filament that lacks continuum sources. The narrow line widths across the filament indicate quiescent gas in spite of the two embedded protostars. The derived molecular column densities are comparable with those in local star-forming regions, and they suggest an anti-correlation between hydrocarbons, ions, HCO, and H2CO on the one hand, and CH3OH and SO on the other. Conclusions. The static chemical models that match the observed column densities best favour low-energy conditions that are expected at large galactocentric radii, but they also favour carbon elemental abundances that exceed those derived by extrapolating the [C/H] galactocentric gradient at 23 kpc by three times. This would indicate a flatter [C/H] trend at large galactocentric radii, which is in line with a flat abundance of organics. However, to properly reproduce the chemical composition of each region, models should include dynamical evolution.