HIFI Spectrum of Water and Organics in the Orion Nebula. Copyright: ESA, HEXOS and the HIFI Consortium. E. Bergin

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the molecular universe

(iau symposium 280)


May 30 - June 3, 2011

Toledo (Spain)


Scientific Rationale

Since 2005, many exciting developments have occurred in astrochemistry. The role of molecules in astronomy has grown so that it is no longer an exaggeration to refer to a sizeable portion of the universe as - The Molecular Universe -, a title we have borrowed for our symposium. Recent successes in the field include advances in observational, laboratory, theoretical, and modeling work. Let us give a few examples of each.

Observational Advances: The use of millimeter interferometers has shown that hot cores, regions of complex molecular growth in the vicinity of high- and low-mass young stellar objects, are far more complex and heterogeneous than previously thought. The maps of different organic molecules in the gas phase can be so different as to not overlap appreciably, suggesting a more complex chemical history than any so far modeled. On the much larger scale of entire galaxies, molecular maps - and even entire line surveys - are starting to reveal interesting spatial differences related to different physical processes (UV photons, X-rays, shocks, hot core chemistry in trapped starburst regions). Molecules other than CO have now been detected even out to redshifts of more than 6. On a more local scale, negatively charged molecular anions have been found for the first time in a variety of sources, including the cold core TMC-1 and the envelopes of evolved stars, following laboratory work. The first spatially resolved maps of the chemistry in the outer regions of protoplanetary disks are becoming available with observed distributions that disagree with existing models. Spitzer and ground-based infrared spectroscopy are starting to reveal a surprisingly rich chemistry in the inner planet-forming zones of disks, and at the same time provide a complete inventory of ices in star- and planet-forming regions. In the realm of exo-planets, the study of their atmosphere has begun, and evidence exists for methane, carbon dioxide, and water. The inventory of molecules in cometary atmospheres continues to grow, with interesting variations found between comets of different origin.

Laboratory Advances: Laboratory studies have traditionally focused on gas-phase processes, but the emphasis has shifted significantly toward gas-grain interactions over the last 6 years. State-of-the-art surface science experiments are now being applied to chemical and physical processes on analogs of interstellar dust particles. For example, the efficiency of photodesorption has been measured for the first time for simple ices. Photodissociation of methanol ice along with impurities has partially confirmed new models of complex molecule formation on ices. The complicated formation of water ice on surfaces at low temperature, relevant to the interpretation of Herschel data, has been studied, as has the thermal evaporation of mixed ices. The formation of molecular hydrogen on high-temperature grains, a most complex process, has been thoroughly investigated. Cometary samples returned from the Stardust mission as well as meteoritic and IDP samples are studied in the laboratory with increasingly sophisticated instruments. Much effort has gone into the study of gas-phase rotational spectra at frequencies as high as 2 THz; such submillimeter-wave and far-infrared spectra will be necessary to assign spectra from Herschel, SOFIA and ALMA. In some regions, the density of lines from a small number of molecules known as weeds is so great that it is difficult to assign lines to other more interesting (pre-biotic) species. Laboratory studies of PAHs continue to be essential to interpret the wealth and variety of infrared features seen in Spitzer and future JWST spectra. In the realm of gas-phase kinetics, reactions involving negative ions (anions) are being studied to help modelers understand the chemistry of these species.

Theoretical Advances: The chemical processes that occur on dust grains are normally treated in models by so-called rate equations. In some instances, these equations are inaccurate, and more computationally intensive methods, known as stochastic approaches, must be used. New stochastic methods and approximations have been developed. Much progress has been made in creating models that combine gas-phase chemistry, which is treated by rate equations, and surface chemistry, which is treated stochastically by these new methods. Quantum chemical potentials, used sometimes with classical dynamics and sometimes with quantum mechanical dynamics, have been employed to determine accurate inelastic scattering cross sections, needed in the analysis of molecular spectral intensities.

Modeling Advances: Chemical simulations have been improved in a number of ways. The major networks of gas-phase reactions now include processes involving anions, which act as catalysts for the production of larger neutral species. Sensitivity analyses can now be used to determine the uncertainties in the predictions of calculated molecular abundances based on uncertainties in rate coefficients and physical conditions; such methods can also be used to determine to which unstudied reactions molecules are most sensitive, so that the proper reactions can be studied theoretically or in the laboratory. A new mechanism for the formation of complex molecules in hot cores has been developed. In this approach, complex gas-phase species are produced on granular surfaces as they are heating up in the presence of a young stellar object; the rising temperature allows molecules and radicals produced by photodissociation to diffuse more readily around grains and eventually evaporate. Models of protoplanetary disks, especially the inner portions, are being challenged to reproduce the mid-infrared detection of water, acetylene (HCCH) and HCN, together with future Herschel far-IR observations. These inner disks require that chemical networks be able to operate at temperatures as high as 1000 K, and various groups are developing such high-temperature networks. Models of photon-dominated regions (PDRs), relevant for a wide variety of galactic and extragalactic regions, have reached new levels of sophistication and are now available for general distribution. A gas-grain PDR-type model for water and oxygen seems to explain the puzzling low abundances of these species in the gas-phase of cold objects measured by SWAS and Odin, and will be further tested by Herschel. Hydrodynamics is being applied to more chemical models in an attempt to merge realistic dynamics and chemistry. Finally, molecular data bases coupled with radiative transfer programs continue to be developed for general distribution to provide astronomers with a variety of tools to make line intensity predictions. These tools can either be coupled with the output of sophisticated chemo-dynamical models or used in a stand-alone mode to analyze molecular observations in terms of physical conditions and molecular abundances. Such tools will become increasingly important in the preparation of observations for ALMA, JWST and ELTs, whether applied to the smallest scales in nearby protoplanetary disks or to the most distant galaxies.

In summary, this symposium will cover the entire field of astrochemistry in order to make investigators aware of the many new advances of the last 6 years, and to acquaint them with the explosive growth in the subject to be coming in the next few years in the ALMA, JWST and ELT eras.



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