2022A&A...664A.154F

Context. The outer Galaxy is an environment with a lower metallicity than the regions surrounding the Sun and for this reason the formation and survival of molecules in star-forming regions located in the inner and outer Galaxy are expected to be different.
Aims. To gain understanding of how chemistry changes throughout the Milky Way, it is crucial to observe the outer star-forming regions of the Galaxy in order to constrain models adapted for lower metallicity environments.
Methods. The project 'chemical complexity in star-forming regions of the outer Galaxy' (CHEMOUT) is designed to address this problem by observing a sample of 35 star-forming cores at Galactocentric distances of up to ∼23 kpc with the Institut de RadioAstronomie Millimetrique (IRAM) 30 m telescope in various 3 mm and 2 mm bands. In this work, we analyse observations of methanol (CH₃OH), one of the simplest complex organic molecules and crucial for organic chemistry in star-forming regions, and of two chemically related species, HCO and formaldehyde (H₂CO), towards 15 out of the 35 targets of the CHEMOUT sample. More specifically, we consider only the targets for which both HCO and H₂CO were previously detected, which are precursors of CH₃OH.
Results. We detected CH₃OH in all 15 targets. The emission is associated with an extended envelope, as the average angular size is ∼47'' (i.e. ∼2.3 pc at a representative heliocentric distance of 10 kpc). Using a local thermodynamic equilibrium approach, we derive CH₃OH excitation temperatures in the range ∼7-16 K and line widths ≤4 km s^–1, which are consistent with emission from a cold and quiescent envelope. The CH₃OH fractional abundances with respect to H₂ range between ∼0.6 x 10^–9 and ∼7.4 x 10^–9. These values are comparable to those found in star-forming regions in the inner and local Galaxy. H₂CO and CH₃OH show well-correlated line velocities, line widths, and fractional abundances with respect to H₂, indicating that their emission originates from similar gas. These correlations are not seen with HCO, suggesting that CH₃OH is likely more chemically related to H₂CO than to HCO.
Conclusions. Our results have important implications for the organic and possibly pre-biotic chemistry occurring in the outermost star-forming regions of the Galaxy, and can help to set the boundaries of the Galactic habitable zone.