Chemical complexity in the Horsehead photodissociation region

The interstellar medium is known to be chemically complex. Organic molecules with up to 11 atoms have been detected in the interstellar medium, and are believed to be formed on the ices around dust grains. The ices can be released into the gas-phase either through thermal desorption, when a newly formed star heats the medium around it and completely evaporates the ices; or through non-thermal desorption mechanisms, such as photodesorption, when a single far-UV photon releases only a few molecules from the ices. The first one dominates in hot cores, hot corinos and strongly UV-illuminated PDRs, while the second one dominates in colder regions, such as low UV-field PDRs. This is the case of the Horsehead were dust temperatures are ~20-30K, and therefore offers a clean environment to investigate what is the role of photodesorption. We have carried-out an unbiased spectral line survey at 3, 2 and 1mm with the IRAM-30m telescope in the Horsehead nebula, with an unprecedented combination of bandwidth high spectral resolution and sensitivity. Two positions were observed: the warm PDR and a cold condensation shielded from the UV field (dense core), located just behind the PDR edge. We summarize our recently published results from this survey and present the first detection of the complex organic molecules HCOOH, CH2CO, CH3CHO and CH3CCH in a PDR. These species together with CH3CN present enhanced abundances in the PDR compared to the dense core. This suggests that photodesorption is an efficient mechanism to release complex molecules into the gas-phase in far-UV illuminated regions.Comment: 15 pages, 7 figures, 7 tables, Accepted in Faraday discussions 16

Dynamics of filaments of scroll waves

This has been written as a chapter for "Engineering Chemical Complexity II", and as such does not have an abstract.Comment: 18 pages, 10 figure

Chemical complexity in NGC1068

We aimed to study the chemistry of the circumnuclear molecular gas of NGC1068, and to compare it with those of the starburst galaxies M82 and NGC253. Using the IRAM-30m telescope, we observed the inner 2 kpc of NGC1068 between 86.2 GHz and 115.6 GHz. We identified 35 spectral features, corresponding to 24 different molecular species. Among them, HC3N, SO, N2H+, CH3CN, NS, 13CN, and HN13C are detected for the first time in NGC1068. Assuming local thermodynamic equilibrium (LTE), we calculated the column densities of the detected molecules, as well as the upper limits to the column densities of some undetected species. The comparison among the chemistries of NGC1068, M82, and NGC253, suggests that, apart from X-rays, shocks also determine the chemistry of NGC1068. We propose the column density ratio between CH3CCH and HC3N as a prime indicator of the imprints of starburst and AGN environments in the circumnuclear interstellar medium. This ratio is, at least, 64 times larger in M82 than in NGC1068, and, at least, 4 times larger in NGC253 than in NGC1068. Finally, we used the UCL_CHEM and UCL_PDR chemical codes to constrain the origin of the species, as well as to test the influence of UV radiation fields and cosmic rays on the observed abundances.Comment: 8 pages, 2 figures, 3 tables. Proceedings of the "The Central Kiloparsec in Galactic Nuclei", 29 August - 2 September 2011, Bad Honnef, German

Limits on chemical complexity in diffuse clouds: search for CH3OH and HC5N absorption

Context: An unexpectedly complex polyatomic chemistry exists in diffuse clouds, allowing detection of species such as C2H, C3H2, H2CO and NH3 which have relative abundances that are strikingly similar to those inferred toward the dark cloud TMC-1 Aims: We probe the limits of complexity of diffuse cloud polyatomic chemistry. Methods: We used the IRAM Plateau de Bure Interferometer to search for galactic absorption from low-lying J=2-1 rotational transitions of A- and E-CH3OH near 96.740 GHz and used the VLA to search for the J=8-7 transition of HC5N at 21.3 GHz. Results: Neither CH3OH nor HC5N were detected at column densities well below those of all polyatomics known in diffuse clouds and somewhat below the levels expected from comparison with TMC-1. The HCN/HC5N ratio is at least 3-10 times higher in diffuse gas than toward TMC-1. Conclusions: It is possible to go to the well once (or more) too ofte