2017A&A...603A.124C

We investigate the presence of complex organic molecules (COMs) in strongly UV-irradiated interstellar molecular gas. We have carried out a complete millimetre (mm) line survey using the IRAM 30 m telescope towards the edge of the Orion Bar photodissociation region (PDR), close to the H₂ dissociation front, a position irradiated by a very intense far-UV (FUV) radiation field. These observations have been complemented with 8.5'' resolution maps of the H₂CO J_Ka,Kc=5_1,5-4_1,4 and C¹⁸O J=3-2 emission at 0.9mm. Despite being a harsh environment, we detect more than 250 lines from COMs and related precursors: H₂CO, CH₃OH, HCO, H₂CCO, CH₃CHO, H₂CS, HCOOH, CH₃CN, CH₂NH, HNCO, H₂¹³CO, and HC₃N (in decreasing order of abundance). For each species, the large number of detected lines allowed us to accurately constrain their rotational temperatures (T_rot) and column densities (N). Owing to subthermal excitation and intricate spectroscopy of some COMs (symmetric- and asymmetric-top molecules such as CH₃CN and H₂CO, respectively), a correct determination of N and T_rot requires building rotational population diagrams of their rotational ladders separately. The inferred column densities are in the 10¹¹-10¹³cm^–2 range. We also provide accurate upper limit abundances for chemically related molecules that might have been expected, but are not conclusively detected at the edge of the PDR (HDCO, CH₃O, CH₃NC, CH₃CCH, CH₃OCH₃, HCOOCH₃, CH₃CH₂OH, CH₃CH₂CN, and CH₂CHCN). A non-thermodynamic equilibrium excitation analysis for molecules with known collisional rate coefficients suggests that some COMs arise from different PDR layers but we cannot resolve them spatially. In particular, H₂CO and CH₃CN survive in the extended gas directly exposed to the strong FUV flux (T_k=150-250K and T_d≤60K), whereas CH₃OH only arises from denser and cooler gas clumps in the more shielded PDR interior (T_k=40-50K). The non-detection of HDCO towards the PDR edge is consistent with the minor role of pure gas-phase deuteration at very high temperatures. We find a HCO/H₂CO/CH₃OH~=1/5/3 abundance ratio. These ratios are different from those inferred in hot cores and shocks. Taking into account the elevated gas and dust temperatures at the edge of the Bar (mostly mantle-free grains), we suggest the following scenarios for the formation of COMs: (i) hot gas-phase reactions not included in current models; (ii) warm grain-surface chemistry; or (iii) the PDR dynamics is such that COMs or precursors formed in cold icy grains deeper inside the molecular cloud desorb and advect into the PDR.