Complex organic molecules in strongly UV-irradiated gas â

Abstract

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 H 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 HCO Ja,K = 5 → 4 and CO J = 3 → 2 emission at 0.9 mm. Despite being a harsh environment, we detect more than 250 lines from COMs and related precursors: HCO, CHOH, HCO, HCCO, CHCHO, HCS, HCOOH, CHCN, CHNH, HNCO, H CO, and HCN (in decreasing order of abundance). For each species, the large number of detected lines allowed us to accurately constrain their rotational temperatures (T) and column densities (N). Owing to subthermal excitation and intricate spectroscopy of some COMs (symmetric-and asymmetric-Top molecules such as CHCN and HCO, respectively), a correct determination of N and T requires building rotational population diagrams of their rotational ladders separately. The inferred column densities are in the 10-10 cm 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, CHO, CHNC, CHCCH, CHOCH, HCOOCH, CHCHOH, CHCHCN, and CHCHCN). 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, HCO and CHCN survive in the extended gas directly exposed to the strong FUV flux (T = 150-250 K and Tâ60 K), whereas CHOH only arises from denser and cooler gas clumps in the more shielded PDR interior (T = 40-50 K). 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/HCO/CHOH â1/5/3 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.