Warm interstellar medium

Two types of models have been proposed that use this general picture as the basis for understanding volatile depletions in chondrites. Yin (2005) proposed that the volatile element depletions in the chondrites reflect the extent to which these elements were sited in refractory dust in the interstellar medium. Observations show that in the warm interstellar medium, the most refractory elements are almost entirely in the dust, while volatile elements are almost entirely in the gas phase. Moderately volatile elements are partitioned between the two phases. The pattern for the dust is similar to that observed in bulk chondrites. In the Sun s parent molecular cloud, the volatile and moderately volatile elements condensed onto the dust grains in ices. Within the solar system, the ices evaporated putting the volatile elements back into the gas phase, which was separated from the dust. Thus, in Yin s model, the chondrites inherited their compositions from the interstellar medium. A slightly different model proposes that the fractionated compositions were produced in the solar nebula by... 

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. 

The interstellar medium constitutes 10 % of the mass of the galaxy. It can be subdivided into environments with very low-density hot gas, environments with warm intercloud gas, and regions with denser and colder material (23). H and He gas are the major components of interstellar clouds molecules and submicron dust particles are only present in small concentration (22). Through gas phase reactions and solid-state chemistry, gas-grain interactions can build up complex organic molecules. Silicate and carbon-based micron-sized dust particles provide a catalytic surface for a variety of reactions when they are dispersed in dense molecular clouds 24). In cold clouds such dust particles... 

The interstellar medium (ISM) contains different environments showing large ranges in temperature between 10 K in dense clouds and up to 200 K in warm circumstellar envelopes and densities between 100 to 10 hydrogen atoms cm. Dense clouds are characterized by very low temperatures (10-30 K) and... 

Several important observations of the abundances of the C varieties of molecules In diffuse clouds have recently become available. Hawkins and Jura (1987) determined accurate CH+Z CH" " column density ratios for 4 lines of sight. The inferred ratio (43 4) was found to be remarkably uniform in the different directions, and is a factor of two lower than the terrestrial isotope ratio of 89. If CH" " is indeed formed in a warm region without fractionation, the measured CH / CH ratio should be directly representative of the [ C)/[ C] abundance ratio of carbon in all forms in the interstellar medium. 

The presence of reactive ions like CH in the diffuse interstellar medium has been a challenge for interstellar chemistry since CH is easily destroyed by reactions with H2 but slow to form under the known physical conditions of the diffuse interstellar medium. It now appears that a warm chemistry can develop in the tiny dissipative structures of the interstellar turbulence, enabling the formation of transient species like CH and SH [43], The opening up of the sub-millimetre sky by the Herschel telescope has led to the discovery of several new reactive ions, enabling a better characterization of their chemistry. In the future, these tracers should bring interesting constraints on the properties of the interstellar turbulence. 

---