Interstellar medium dense molecular cloud

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). 

There is convincing observational evidence that the placental interstellar medium (ISM) from which the solar system originated was a dense molecular cloud (Wasserburg et al., 1982 1979). In fact, the recent evidence of the presence of short-lived nuclei in meteorites requires that the free-fall time scale for gravitational collapse (tft) be less than or comparable to the mean lifetime of Al ( 10 yrs), i.e. ttt 4.10 / /n < 10 yrs, which requires nn lOVcc, a value typical of molecular clouds. Since molecular clouds are observed to be a major feature in our galaxy, they constitute a most reliable starting point for the processes that will eventually lead to the formation of stars and planetary systems (Falk and... 

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... 

Allamandola L. J., Sandford S. A., Tielens A. G. G. M., and Herbst T. M. (1993) Diamonds in dense molecular clouds a challenge to the standard interstellar medium paradigm. Science 260, 64-66. 

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... 

HNC is found primarily in dense molecular clouds, though it is ubiquitous in the interstellar medium. HNC is formed primarily through the dissociative recombination of HNCH and H2NC, and it is destroyed primarily through ion-neutral reactions with Hs and C. Rate constants are taken from udfa.net, and data on fractional abundances is taken from here. Rate calculations were done at 3.16x10 years, which is considered early time, and at 20K, which is a typical temperature for dense molecular clouds. 

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... 

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 solar system probably formed from the collapse of a fragment of a molecular cloud—a cold, dense portion of the interstellar medium... 

A contracting molecular cloud is not equally dense throughout. As in the interstellar medium, some regions of a cloud are more dense than others. Thus, when the cloud contracts, it does not... 

If one requires kj. at a different, usually lower, temperature from that of the SIFT measurements, it is best to proceed with the aid of theory which is reliable as far as the temperature variation is concerned. Theorj provides the only means of allowing for the excitation conditions in the interstellar medium being different from those in the laboratory. The difference may be great for example the molecidar hydrogen in dense interstellar clouds is thought to have its rotational levels in true thermal equilibrium whereas the molecular hydrogen used in most laboratory experiments consists of the normal 3 to 1 ortho to para mixture. 

Abstract This chapter presents a description of the interstellar medium. It starts with a summary of the interstellar medium stmcture and how the various phases are related to each other. The emphasis is put on molecular clouds, and on their densest regions, the dense cores, which are the birth place of stars. The evolution of matter during the star formation process and its observable consequences, especially in term of chemical composition is presented. The next section is dedicated to the constituents of the interstellar medium, with separate presentations of the gas species and the dust grains. Methods used by astronomers to derive useful information on the structure, temperature, ionization rate of interstellar environments as well as magnetic fields are briefly described. The last part of the chapter presents the telescopes and their instruments used for studying the interstellar medium across the electromagnetic spectrum. 

In Fig. 4.1, we show the results of a model for dense cloud conditions (a temperature of 10 K, a hydrogen atom density of 2 x 10" cm andavisualextinctionof 10), which includes only gas-phase processes, except for the production of molecular hydrogen, which occurs on granular surfaces. The chemical processes that are involved in the chemistry of the interstellar medium are described in Sect 4.3 of this chapter. In addition to the parameters described in Sect. 4.2.1, the geometry of the object, the presence of mixing, and physical dymamics can influence the chemical composition as well (Sects. 4.2.2 and 4.2.3). 

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