Complex organic molecules in strongly UV-irradiated gas

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 2 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 H 2 CO J K a ,K c = 5 1,5 → 4 1,4 and C 18 O J = 3 → 2 emission at 0.9 mm. Despite being a harsh environment, we detect more than 250 lines from COMs and related precursors: H 2 CO, CH 3 OH, HCO, H 2 CCO, CH 3 CHO, H 2 CS, HCOOH, CH 3 CN, CH 2 NH, HNCO, and HC 3 N (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 CH 3 CN and H 2 CO, 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 10 11 -10 13 cm −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, CH 3 O, CH 3 NC, CH 3 CCH, CH 3 OCH 3 , HCOOCH 3 , CH 3 CH 2 OH, CH 3 CH 2 CN, and CH 2 CHCN). 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, H 2 CO and CH 3 CN survive in the extended gas directly exposed to the strong FUV flux (T k = 150 -250 K and T d ≳ 60 K), whereas CH 3 OH only arises from denser and cooler gas clumps in the more shielded PDR interior (T k = 40 -50 K). The nondetection 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/H 2 CO/CH 3 OH ≃ 1/5/3 abundance ratio. These ratios are different from those inferred in hot cores and shocks. Taking into account the s.cuadrado@icmm.csic.es. ⋆ Based on observations obtained with the IRAM 30m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). Europe PMC Funders GroupAuthor Manuscript Astron Astrophys. Author manuscript; available in PMC 2017 November 13. Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts 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.

The Detection of Hot Cores and Complex Organic Molecules in the Large Magellanic Cloud

“…Thus, the chemistry of COMs detected in N 113 is similar to that of the Galaxy, indicating that regions where they exist are likely shielded from UV radiation (see Cuadrado et al 2017). Both grain reactions on warm dust and post-desorption ion-molecule chemistry could form CH 3 OCH 3 and CH 3 OCHO (Garrod & Herbst 2006;Taquet et al 2016).…”

The water line emission and ortho-to-para ratio in the Orion Bar photon-dominated region

“…This OPR agrees with OPRs at high thermal values from other tracers reported for the Orion Bar. Indeed an OPR of 2.8 ± 0.6 was derived for c − C 3 H 2 (Cuadrado et al 2015) and OPRs of the order of three were inferred for H 2 CO, H 2 CS and H 2 CCO Cuadrado et al 2017).…”

Sulfur Chemistry in L1157-B1

“…However, the o-H 2 CS probability distributions of the temperature and density are highly degenerate and so higher density, low temperature solutions exist for that spin isomer. The p-H 2 CS fits are better constrained because at low densities, H 2 CS transitions form separate ladders in a rotation diagram in which transitions of the same K a quantum number follow separate linear trends (Cuadrado et al 2017). This can be seen in Figure 6 where the p-H 2 CS transitions form two ladders.…”