Giant molecular clouds

To date, researchers have identified more than 100 different molecules, composed of up to 13 atoms, in the interstellar medium [16]. Most were initially detected at microwave and (sub)millimetre frequencies, and the discoveries have reached far beyond the mere existence of molecules. Newly discovered entities such as difhise mterstellar clouds, dense (or dark) molecular clouds and giant molecular cloud complexes were characterized for the first time. Indeed, radioastronomy (which includes observations ranging from radio to submillunetre frequencies) has dramatically changed our perception of the composition of the universe. Radioastronomy has shown that most of the mass in the interstellar medium is contained in so-called dense... 

GHz spectral line surveys of tliree regions of the W3 giant molecular cloud complex [21]. From such studies, which reveal dramatic differences in the THz spectmm of various objects, molecular astrophysicists hope to classify the evolutionary state of the cloud, just as optical spectra are used to classify stars. 

Figure 3.12 Line of sight through three Giant Molecular Clouds to the Hubble Space Telescope. (Reproduced by courtesy of StscI and NASA)...

There are some variations in the composition of chemical clouds as they evolve in time from dark clouds such as TMC-1 to giant molecular clouds such as Orion where the presence of light from young stars initiates photochemistry. The Orion molecular cloud chemical inventory contains several saturated species such as ethanol (CH3CH2OH) and its CN analogue (CH3CH2CN), the simplest carboxylic acid (CH3COOH, acetic acid) and methylamine (CH3 NH2). 

Giant molecular clouds the GMCs have a lifetime of order 106—10s years and are the regions of new star formation. The Orion nebula (Orion molecular cloud, OMC) is some 50 ly in diameter and 1500 ly from Earth. The temperature within the cloud is of order 10 K and the atomic density is 106 cm-3. The chemical composition is diverse and contains small diatomic molecules, large polyatomic molecules and dust particles covered with a thick ice mantle. 

This is one of the fastest known reactions and occurs with nearly every collision to produce H3+, which has been seen in diffuse and giant molecular clouds. However, because the rate of reaction is so fast the detection of H2+ is going to be unlikely as it is quickly removed by the chemistry. Once H3+ is formed it quickly protonates a number of species, particularly CO to form HCO+ and also O to form OH+, leading to the other ions containing protons seen in Table 5.1. The construction of networks of possible reactions requires a knowledge of the fundamental chemical physics of molecules and the possible chemistry in the local environment. 

Repeat the calculation above for a giant molecular cloud with a density of 106... 

Concepts Solar nebula The collapsing giant molecular cloud that leads to the formation of a star, specifically our Sun with its associated debris in the form of meteorites, meteors and comets... 

Chemical networks The collection of connected chemical reactions that form the description of a complex chemical system. Applications include giant molecular clouds, atmospheres and cellular metabolism. 

Cycle of star formation The collapse of a giant molecular cloud forms a star nuclear synthesis within the star produces more elements the star ages and ultimately dies in a supernova event elements are thrown into the interstellar medium to form a giant molecular cloud. 

Giant molecular cloud (GMC) A region of space with a larger molecular density of 10s cm-3 and a rich chemical composition. The GMC may also contain young stellar objects. 

Interstellar medium (ISM) The tenuous medium between the stars with molecular densities as low as 1 molecule cm-3 rising to 106 molecules cm-3 in giant molecular clouds. Temperatures may be as low as 10-40 K. 

Jeans mass The mass of giant molecular cloud that is required for it to collapse spontaneously under the force of gravity. 

Solar nebula The giant molecular cloud from which the Sun formed. 

Spectral mapping Using a known transition in a molecule, such as the 155 GHz transition in CO, to map the column density or concentration of the molecule within a giant molecular cloud. 

Giant molecular clouds collapse to form stars and solar systems, with planets and debris left over such as comets and meteorites. Are comets and meteorites the delivery vehicles that enable life to start on many planets and move between the planets as the solar system forms, providing water and molecules to seed life The planets have to be hospitable, however, and that seems to mean wet and... 

Throughout the book I have tried to constrain the wonders of imagination inspired by the subject by using simple calculations. Can all of the water on the Earth have been delivered by comets if so, how many comets How do I use molecular spectroscopy to work out what is happening in a giant molecular cloud Calculations form part of the big hard-sell for astrochemistry and they provide a powerful control against myth. I have aimed the book at second-year undergraduates who have had some exposure to quantum mechanics, kinetics, thermodynamics and mathematics but the book could easily be adapted as an introduction to all of these areas for a minor course in chemistry to stand alone. 

Another idea under investigation can be labeled self enrichment of the molecular cloud. The idea uses the observation that in a giant molecular cloud, serial star formation may occur, where the massive stars in the first batch of stars to form explode as supernova and trigger a second round of star formation, the massive stars of which explode and trigger yet another round of star formation. As each supernova explodes, it dumps short-lived nuclides... 

Stars form in dense cores within giant molecular clouds (see Fig. 1.4, Alves et al. 2001). About 1 % of their mass is in dust grains, produced in the final phases of stellar evolution. Molecular clouds are complex entities with extreme density variations, whose nature and scales are defined by turbulence. These transient environments provide dynamic reservoirs that thoroughly mix dust grains of diverse origins and composition before the violent star-formation process passes them on to young stars and planets. Remnants of this primitive dust from the Solar System formation exist as presolar grains in primitive chondritic meteorites and IDPs. 

Classification Cold dense cloud Outflowing C-rich circum-stellar envelope Warm molecular cloud/star-forming region Giant molecular cloud... 

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