Molecular clouds

Herbst E 1987 Gas phase chemical processes in molecular clouds Interstellar Prooesses ed D J Hollenbach and H A Tronson (Dordrecht Reidel) pp 611-29... 

For remote sensing, spectroscopy at THz frequencies holds the key to our ability to remotely sense enviromnents as diverse as primaeval galaxies, star and planet-fonuing molecular cloud cores, comets and planetary atmospheres. 

In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

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... 

Figure Bl.4.3. (a) A schematic illustration of the THz emission spectrum of a dense molecular cloud core at 30 K and the atmospheric transmission from ground and airborne altitudes (adapted, with pennission, from [17]). (b) The results of 345 GHz molecular line surveys of tlu-ee cores in the W3 molecular cloud the graphics at left depict tire evolutionary state of the dense cores inferred from the molecular line data [21],...

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. 

Perhaps the source with the largest fractional abundances of unsaturated complex molecules is the small condensation in the Taurus complex known as TMC-1 (Taurus Molecular Cloud -1) which, although it shows evidence for substructure, can be considered to have average physical conditions (n = 104 cm-3 and T = 10 K). Table 2 contains a comparison of observed4 abundances in TMC-1 for almost 50... 

One can view a quiescent molecular cloud as a one-dimensional PDR with % = 1. Here, instead of spherical shells representing outer and inner layers, one has one-dimensional slabs. The advantage of such shell models65 over homogeneous models of the inner portions of clouds is that the roles of the outer layers can be accounted for such roles are especially important for atoms (e.g. C) and radicals (e.g. OH, CH). Small dense clouds, known as translucent clouds, have particularly salient outer portions which should be included in models. 

Wilson, T. L. Johnston, K. J. (Eds.). The Structure and Content of Molecular Clouds, Lecture Notes in Physics 439 Springer-Verlag New York, 1994. 

Studies of the isomers of this species were undertaken because of a particularly vexing problem relating to the chemistry in interstellar molecular clouds. This is that propynal, HC=C-CHO, has been observed but its isomer propadienone, H2C=C=C=0, has not, although searches have been made in the most molecule-rich clouds, the Taurus Molecular Cloud and Sagittarius B2.85 A possible route to these isomers is the reaction,86... 

Fig. 3.13 151 GHz Spectrum (excerpt) from the nucleus of the molecular cloud G 327.3-0.6 in the southern sky. The molecular species identified are shown. Peaks which are not labelled are not yet clearly identified. (The numbers used to label the molecular cloud, G 327.3-0.6, are galactic coordinates). With personal permission of Prof. Hjalmarsson, Chalmers University of Technology, Goteborg, Sweden, from the Proceedings of the First European Workshop on Exo/Astrobiology, Frascati, 21-23 May 2001, ESA SP-496...

As an example, we shall discuss the interstellar synthesis of a compound which is produced on Earth in millions of tons per year methanol. This simplest alcohol was obtained by Robert Boyle in 1661 from the dry distillation of wood. In the molecular clouds of the universe, it is likely that hydrogenation of CO on the surface of dust particles occurs according to the following scheme (Tielens and Charnley, 1997) ... 

Four years of study led to the discovery of glycine in the millimetre wavelength range in the hot molecular clouds of Sagittarius (around 81,500 light years away), Orion KL and W51. We can only conjecture as to the mechanism of its formation. Ion-molecule reactions in the gas phase, as well as UV photolytic processes in molecular ice, have been discussed. 

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are exuaterresuial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

The extinction towards a star at 450 nm is 0.24 and is attributed to an interstellar cloud containing dust particles with an extinction coefficient of 0.0032 pc-1. Calculate the diameter of the intervening molecular cloud, expressing your answer in light-years. 

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

The observed transition intensity ratio for R(1)/R(0) of CO in a molecular cloud is 1.10. Calculate the temperature of the molecular cloud. 

The R(0) transition is observed in 12CO and 13CO in two molecular clouds along the line of sight. Calculate the relative velocities of the two clouds if the R(0) transitions in each molecule are coincident. Comment on the possible reason for the relative velocities. 

The general extent of the molecular cloud can be mapped within the sky but the physical conditions and stellar activity lead to different chemical regimes, all of which must be considered if the chemistry of the ISM is to be understood. A... 

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. 

The estimate of the distance must now consider the estimate of the interstellar extinction Av, best estimated by the reddening Av can take several values and in calculations of molecular cloud chemistry typical values are of order 1 but may be as much as 5. The distance calculation in Equation 5.1 can be significantly perturbed so that an A v of 2.4 can reduce the apparent distance by a factor of 3. 

In principle, it is now possible to construct a complete network of interconnecting chemical reactions for a planetary atmosphere, a hot molecular core or the tail of a comet. Once the important reactions have been identified the rate constants can be looked up on the database and a kinetic model of the atmosphere or ISM molecular cloud can be constructed. Or can it Most of the time the important reactions are hard to identify and if you are sure you have the right mechanisms then the rate constants will certainly not be known and sensible approximations will have to be made. However, estimates of ISM chemistry have been made with some success, as we shall see below. 

One useful trick in solving complex kinetic models is called the steady-state approximation. The differential equations for the chemical reaction networks have to be solved in time to understand the variation of the concentrations of the species with time, which is particularly important if the molecular cloud that you are investigating is beginning to collapse. Multiple, coupled differentials can be solved numerically in a fairly straightforward way limited really only by computer power. However, it is useful to consider a time after the reactions have started at which the concentrations of all of the species have settled down and are no longer changing rapidly. This happy equilibrium state of affairs may never happen during the collapse of the cloud but it is a simple approximation to implement and a place to start the analysis. 

The improvement in the rate of chemical reactions is reversed when temperature is cooler and at temperatures as low as 30 K (a warm comer of TMC-1) the exponential term is of order 10-279 and nearly all reactions between neutral species are frozen out at 50 K. Two important classes of reactions survive radical-radical chemistry and ion-molecule chemistry. The importance of these different reaction types will become apparent later with the construction of the models of molecular clouds. For the moment, however, laboratory measurements of reactions in radicals such as C2H have shown that even with temperatures as low as 15 K the rate constant for reactions of the type ... 

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