CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy. II. Methanol formation at low metallicity

Abstract

Context. The outer Galaxy is an environment with metallicity lower than the Solar one and, because of this, the formation and survival of molecules in star-forming regions located in the inner and outer Galaxy is expected to be different. Aims. To gain understanding on how chemistry changes throughout the Milky Way, it is crucial to observe outer Galaxy star-forming regions to constrain models adapted for lower metallicity environments. Methods. The project "chemical complexity in star-forming regions of the outer Galaxy" (CHEMOUT) aims to address this problem observing a sample of 35 high-mass star-forming cores at Galactocentric distances up to ∼ 23 kpc with the Institut de RadioAstronomie Millimétrique (IRAM) 30m telescope in various 3mm and 2mm bands. In this work we analyse observations of methanol (CH 3 OH), one of the simplest complex organic molecules crucial for organic chemistry in star-forming regions, and of two chemically related species, HCO and formaldehyde (H 2 CO), towards 15 out of the 35 targets of the CHEMOUT sample. In fact, only targets previously detected in both HCO and H 2 CO, which are precursors of CH 3 OH, were considered. Results. We detected CH 3 OH 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 CH 3 OH excitation temperatures in the range ∼ 7 − 16 K and line widths ≤ 4 km s −1 , consistent with emission from a cold and quiescent envelope. The CH 3 OH fractional abundances w.r.t. H 2 range between ∼ 0.6 × 10 −9 and ∼ 7.4 × 10 −9 . These values are comparable to those found in star-forming regions in the inner and local Galaxy. H 2 CO and CH 3 OH show well correlated line velocities, line widths, and fractional abundances w.r.t. H 2 , indicating that their emission is originated from similar gas. These correlations are not seen with HCO, suggesting that CH 3 OH is likely more chemically related to H 2 CO than to HCO. Conclusions. Our results have important implications in the organic, and possibly pre-biotic, chemistry occurring in the outermost star-forming regions of the Galaxy, and can help setting the frontiers of the Galactic Habitable Zone.