Hydrogen molecular clouds

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) ... 

The solar system abundances of the elements are the result of the Big Bang, which produced hydrogen and helium, 7.5 billion years of stellar nucleosynthesis, which produced most of the rest of the elements, and the physical processes that mixed the materials together to form the Sun s parent molecular cloud. The unique features of the solar system composition may also reflect the stochastic events that occurred in the region where the Sun formed just prior to solar system formation. 

Carbonaceous material (Fig. 12.8b) is intimately mixed with silicates and is very abundant (carbon abundance averages 13% and varies up to 50%) in CP IDPs. Some carbon is elemental (graphite), but C-H stretching resonances in infrared spectra show that aliphatic hydrocarbons are also present. Polycyclic aromatic hydrocarbons (PAHs) also occur. Nanodiamonds have been identified in cluster IDPs, but not in smaller CP IDPs. Enormous D/H and 15N/14N anomalies have been measured in bulk IDPs, and the hydrogen isotopic anomalies are correlated with organic-rich domains. Ratios of D/H as high as 25 times the solar ratio suggest the presence of molecular cloud materials. 

Obviously, by decreasing the temperature down to 15 K (the typical temperature of a dense molecular cloud) and reducing dramatically the pressure (in the gas phase of a molecular cloud there are from 102 to 106 particles cm3 in comparison to our atmosphere that under normal conditions contains 1019 molecules/cm3), with a further constrain of the lack of surfaces, the effectiveness of the recombination reaction between atomic hydrogen becomes negligible in comparison to that discussed above on laboratory conditions. In fact, at the temperature of a dense molecular cloud of the interstellar medium (10-20 K), the recombination rate is negligible (Herbst 1995, 2001 Henning 1998). 

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). 

Hydrogen chloride is the first chlorine-bearing interstellar molecule to have been detected. Its lowest rotational transition (J = 1 -> 0) at 625.9 GHz has been observed in the Orion Molecular Cloud (OMC-1) in emission with the Kuiper Airborne Observatory, (Blake, Keene, and Phillips, 1985) since atmospheric opacity at this... 

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... 

Wilson et al. (1972) have recently reported the first positive 1SN isotopic detection in an astronomical source. They observed in the Orion molecular cloud the J = 2 -> 1 transitions of the rare hydrogen cyanide isotopic species H13C14N and H12C15N whose rest frequencies are 172677.7 MHz and 172108.1 MHz, respectively (Winnewisser et al., 1971). For these two molecules and in almost all the other isotopic abundance determinations, which can presently be given only with large error margins ( 20 %—50%), the isotopic ratios are found to have essentially terrestrial values. 

The chemical dynamics, reactivity, and stability of carbon-centered radicals play an important role in understanding the formation of polycyclic aromatic hydrocarbons (PAHs), their hydrogen-dehcient precursor molecules, and carbonaceous nanostructures from the bottom up in extreme environments. These range from high-temperature combustion flames (up to a few 1000 K) and chemical vapor deposition of diamonds to more exotic, extraterrestrial settings such as low-temperature (30-200 K), hydrocarbon-rich atmospheres of planets and their moons such as Jupiter, Saturn, Uranus, Neptune, Pluto, and Titan, as well as cold molecular clouds holding temperatures as low as 10... 

In the following two sections the discussion will be confined to the chemistry of dense molecular clouds, where one can assume that effects of photodissoziation and photoionization are unimportant, quite in contrast to diffuse clouds. Furthermore, in dense clouds hydrogen is predominantly in the molecular form, H2, and reactions... 

Fig. 1. Interstellar formation scheme illustrating the CH, CH, C H and higher hydrocarbon cycle. The left side of the reaction cycle pertains to tenous clouds (Uj, 100 cm ), whereas the right hand side is more appropriate to areas where is present, i.e. dense molecular clouds (n 10 -10 cm" ). The thick arrows indicate assumed preferential reaction paths leading to the higher order hydrocarbons. The following processes are involved (v, e) photoionization (v, H) photodissociation (e, v) radiative recombination (H) (Hj, v) radiative association (e, H), (e, Hj) dissociative electron recombination. (Hj, H) hydrogen abstraction reaction (C, H) charge exchange (M, M ) metal charge exchange metal = Mg, Fe, Ca, Na,...

Inheritance of 0-poor gas from the presolar molecular cloud is based on the well-known phenomenon of photochemical self-shielding, in which photolysis of CO in the cloud interior affects preferentially the less-abundant isotopic species C O and C O. Yurimoto and Kuramoto (2002) proposed that the and atoms thus formed can react with hydrogen to form water ice. When a portion of the cloud collapses to form the solar nebula, the ice evaporates to form a... 

In the laboratory, the study of the properties of pure polyynes and poly-cumulenes is inhibited because of their extreme reactivity with oxygen and the formation of cross linked chains. In space, refractory dust made of silicates or carbonaceous material are formed in the atmosphere of evolved stars and released into the interstellar medium (ISM). In dense molecular clouds ( Hydrogen > 10 cm, 10-20 K) atoms and molecules that... 

Kaiser, R.I. Balucani, N. Asvany, O. Lee, Y.T. Crossed molecular beam experiments of radical-neutral reactions relevant to the formation of hydrogen deficient molecules in extraterrestrial environments. In Astrochemistry from Molecular Clouds to Planetary Systems. Mihn, Y.C., van Dishoek, E.F., Eds., Astronomical Society of the Pacific - lAU Series, Volume 197, 2000, 251-264. 

---