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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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