Molecular clouds densities

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 lifetime of the molecular cloud is considered to be a time line running from cloud formation, star evolution and finally dispersion in a period that is several tci. The chemistry of the TMC and, to a good approximation, all molecular clouds must then be propagated over a timescale of at most 20 million years. The model must then investigate the chemistry as a function of the age of the cloud, opening the possibility of early-time chemistry and hence species present in the cloud being diagnostic of the age of the cloud. The model should then expect to produce an estimated lifetime and the appropriate column densities for the known species in the cloud. For TMC-1 the species list and concentrations are shown in Table 5.4. 

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

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. 

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. 

In truth, star formation from molecular clouds is no easy subject to study. This is because the processes involved change the density from 10 g cm to about 1 g cm within a space of only a few tens of millions of years. Only the force of gravity, whose long range plays a key role, is able to produce such staggering compression rates. 

Mezey, P. G. (1998) The proof of the metric properties of a fuzzy chirality measure of molecular electron density clouds. J. Molec. Struct. (Theochem.) 455, 183-190. 

The life cycle of stars begins in cold dark molecular clouds. A particularly dense region in the cloud may become gravitationally unstable and collapse. As the density increases, the temperature rises due to the conversion of gravitational potential energy to thermal energy. 

Stars form when dense regions of cold molecular clouds undergo gravitational collapse. In dense molecular cloud cores, the temperature is in the order of 10-20 and the gas density... 

However, it cannot convert atomic to molecular hydrogen under interstellar conditions. Nor is the three-body associative process possible because the density of a dense molecular cloud involves, say, 104 particles/mL the chance that a third body strikes the H2 collision complex before it dissociates so as to stabilize it, is zero under considered conditions. There is, however, a finite but exceedingly small possibility that a molecule of hydrogen can be formed in the gas phase in interstellar conditions the rate constant is in fact extremely low K = 10-31 s not sufficient to explain the amount of molecular hydrogen present in the Universe (Pirronello and Avema 1988). 

Interstellar medium The dust, molecular clouds, and neutral hydrogen that lie between the stars of this galaxy, generally in the plane of the Milky Way, but whose density is highly variable... 

Molecular astronomy of carbon molecules is very rich. Of about 120 known interstellar molecules more than three-quarters contain carbon atoms diatomic molecules include CO, CN, C2, CH, CH+, CN+, and CO+ polyatomic include CH2, CH4, C2H2j CH OH, CH3CH2OH, H2CO and HNC large complex unsaturated radicals and polycyclic aromatic hydrocarbons are also detected. These all play a role in the thermochemistry of interstellar clouds. The 2.6-millimeter line of CO diagnoses density and temperature in molecular clouds, as do other molecules. 

The above fuzzy electron density membership functions reflect the relative contributions of the fuzzy, three-dimensional charge clouds of the various molecular electron density distributions to the total electronic density of molecular family L. 

The density domain approach was first proposed [4] as a tool for the description of chemical bonding where the complete shape information of the molecular electron density was taken into account. Density domains are formal bodies of electron density clouds enclosed by MIDCOs defined by eq. (1) [or by eq. (2) if there is no need to specify the nuclear configuration K],... 

A pictorial analogy between macroscopic clouds of various densities and molecular charge densities can be used here. A density domain DD(a) is analogous to a cloud we could see if our eyes were adjusted to notice only densities equal to or higher than the threshold a. By readjusting our eyes, different "density domains" of clouds of higher or lower density threshold values could be observed. 

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. 

The density and temperature distribution of interstellar matter, contrary to its elemental composition, is strongly inhomogeneous. At least three different phases exist (e.g. Tielens 2005) (i) extended low-density bubbles of hot ionized gas (hot interstellar medium or HIM, mass fraction 0.003, volume fraction 0.5), resulting from series of SN explosions in mass-rich stellar clusters (ii) cold and dense clouds of neutral gas (cold and neutral interstellar medium or CNM, mass fraction 0.3, volume fraction 0.01), resulting from sweeping up of warm gas and (iii) a warm, either ionized or neutral, medium in between (warm interstellar medium or WIM, mass fraction 0.5, volume fraction 0.5). The essential properties of the three phases are indicated in Fig. 2.4. The coolest and most massive of the clouds are the molecular clouds (MC, mass fraction 0.2, volume fraction 0.0005), a separate component, that are the places of star formation, where new stars are formed as stellar clusters with total masses between about 200 and several 106 M0. 

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