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Astrochemical modelling and observational data

What kinds of observations are involved?

Molecules may be detected in astronomical observations in almost all regions of the spectrum, from radio to the UV. Most molecular species are detected in cold interstellar gas through their rotational transitions and these tend to give rise to emissions in the radio region of the spectrum, where wavelengths are around 1 mm. For example, the CO molecule emits in the 1 - 0 rotational transition at a wavelength of 2.6 mm. The CO molecule may also be detected in warmer regions through vibrational transitions; for example, the 1 - 0 vibrational transition in CO occurs in the near IR at about 4.7 micrometres. In electronic transitions, CO shows a complex absorption spectrum that occurs at wavelengths shorter than about 110 nanometres. Around 130 different molecular species have been detected in interstellar and circumstellar clouds by observations over the electromagnetic spectrum. Making the detection and measuring the intensity of several spectral lines can provide basic information, such as the local gas density and temperature, but much more information can be obtained by the use of models that describe in some detail the chemical and physical processes operating in the regions being observed. It is the modelling process that identifies the potential significance of reactions in the chemical network, and highlights where further study in the laboratory may be needed.

What is astrochemical modelling?

The simplest astrochemical models assume that the physical conditions, i.e. the density, temperature, and radiation fields remain unchanged while the chemistry proceeds towards steady state. The modeller then must identify a network of reactions that form and destroy all the relevant species, i.e. the species of interest and also precursor species. For example, the molecule CH may arise from a radiative association of C+ with H2 to form the ion CH2+ which then dissociatively recombines with electrons to form CH. The network must contain all the reactions that form and destroy C+, H2,and electrons, and reactions destroying CH, including the effects of radiation fields. So even this simple system will require a network of perhaps 20 reactions. In practice, networks of astrochemical models may typically contain hundreds of species and thousands of reactions. This creates a huge demand for reaction rate data at both low and high temperatures which is being met by heroic efforts of laboratory and theoretical chemists. Reactions include both gas phase and surface processes. When the models are sufficiently reliable, they can be used to provide detailed information on the gas in which the detected molecules are found. We may be able to define the total elemental abundances, the local electromagnetic radiation field, the flux of cosmic rays, the amount of dust present, and so on.

More complex models describe the situations in which the physical conditions may also be changing. For example, the gas may be in motion, either infalling in the process of star formation, or being blown away from newly formed stars. These astrochemical models are capable of describing the evolution of gas from an initial state to another, and so the chemistry as observed may be a "Chemical Clock" that allows us to infer how far along an evolutionary path one particular system might be. This is useful in describing star and galaxy formation, or the disruption of gas around newly formed stars. Thus, the intellectual return from chemical modelling is very significant for our understanding of astronomy.

What is the link between the modelling and the fundamental chemistry?

The two major areas of laboratory and theoretical study in the UCL Centre for Cosmic Chemistry and Physics are reactions of atomic hydrogen to form molecular hydrogen at cold surfaces, and the formation of more complex molecules at cold surfaces. Why have we chosen to concentrate on these types of surface reactions?

Molecular hydrogen is an interesting case. This species is the most abundant molecule in the Universe, and a significant fraction of non-stellar baryonic matter is in this form. Although atomic hydrogen is abundant, the direct reaction of two hydrogen atoms to form H2 is strongly forbidden. Exchange reactions such as CH + H -> C + H2 can be shown to be far to slow to account for the high H2 abundance. Reaction networks via H- and H2+ can be important in some circumstances, though each has a slow rate-limiting step. Perforce, one must look for another mechanism, and the one that was proposed half a century ago, but without much experimental basis, was the reaction of hydrogen atoms at the surfaces of interstellar dust to form molecules. Early models showed that the required efficiency of this reaction would need to be high, between 10% and 100%. Recently, it has become possible to investigate this type of process both in the laboratory and by fundamental quantum mechanical calculations. One can now ask the following questions: Does this process work? How does it depend on the physical and chemical nature of the surface? Is it dependent on the gas and surface temperatures? How does the H2 molecule emerge from the surface? Is it kinetically and internally excited? How much energy from the excess available in the reaction is transmitted to the surface? The UCL experiment seeks to provide answers to these questions. The answers have direct and important impacts on the astrochemical models and our understanding of the interstellar gas.

Surface reactions to form other molecules are clearly necessary. Water ice was originally detected by its pure vibrational transition near 3 micrometres. It was clear from these observations that water ice was abundant in denser interstellar clouds. However, little water was likely to be present in the gas phase, and therefore the ice could not be formed simply by deposition of H2O from the gas on to the surfaces of dust grains. Therefore, it seems that the ice must form by adsorption of O atoms and conversion in situ to ices. The overall efficiency of the process of ice formation is required to be fairly high. Similarly, methanol is seen to be abundant in some interstellar clouds, at levels that cannot be provided by the most comprehensive networks of gas phase reactions. One is therefore forced to consider the possibility of conversion of CO molecules to methanol at surfaces, by hydrogenation reactions. These are the types of processes that are under investigation in our laboratories at UCL, and also by theoretical investigation. It is clear that surface processes are important in astrochemistry, and our work is providing the firm foundation for the inclusion of such surface networks in the astrochemical models.

What are the present activities of the astrochemical observers and modellers at UCL?

The current applications that are being made of astrochemical models in the UCL Centre are wide-ranging. We have a long-standing interest in star-formation, and the use of astrochemistry as a tool to describe the evolution of gas from low to high density and ultimately incorporation into a star or disruption back into diffuse gas. Where the latter happens, there may be locations where gas not incorporated into the star is subjected to powerful stellar-driven winds that erode the gas and create a characteristic chemistry. A more recent development has been the application of ideas that we have developed from our studies in the Milky way Galaxy to more distant galaxies. Our studies show that we can use our models to describe the physical and chemical conditions in even quite distant galaxies which we can now observe when the Universe was only about one tenth of its present age.

This page last modified 26 October, 2007 by John Edridge

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