Clouds, interstellar molecular

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... 

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... 

IrevineW. M. (1999). The Composition of Interstellar Molecular Clouds. Space Science Reviews 90 203-218. 

Rae J. G. L., Bell N., HaAquist T. W., Pilling M. J. and Ruffle D. P. (2002). Reduced networks governing the fractional ionization in interstellar molecular clouds. Astronomy ... 

Particular fraction of the organic polymer C1.C2 3000 9 Interstellar molecular cloud... 

Irvine W. The composition of interstellar molecular clouds. Space Sci Rev 1999 90 203-18. 

Excitation temperatures in interstellar molecular clouds are typically 3 °K < Tex < 50 °K, and thus the approximation (14b) looses its validity in the shorter mm-wavelength region. 

In Section B we have discussed how the basic quantities of line emission and absorption, the excitation temperature Tex and optical depth r can be determined from observations. Energies required for rotational excitation are generally low enough (< 200 cm-1) so that the rotational levels are expected to be populated even at the very low kinetic temperatures of the interstellar molecular clouds. On the other hand, with a few exceptions such as H20 and NH3, one may assume that only the lowest energy levels of interstellar molecules are populated. Under these conditions the observable fractional column density Nx may not deviate appreciably from the total column density N of a molecule, which can be computed by means of Eq. (17) on the assumption of LTE. 

Fig. 20 shows the observed interstellar molecular lines of various isotopic species of formaldehyde, H2CO, as detected by Gardner et al., 1971. This particular line, the lowest asymmetry-doublet transition 110 — lu, is seen in absorption in the continuum radiation of the strong radio source Sgr B2, which is located behind the molecular gas cloud. Frequency is plotted along the abscissa and the ordinate is intensity, expressed in the ratio of line-to-continuum antenna temperatures. For all three formaldehyde isotopes the continuum temperature is Tc T >b Tex- This is the case because the formaldehyde molecules are in approximate equilibrium with the microwave background... 

The location of an observed molecular radio transition in its energy level scheme and its measured interstellar intensity contain important information concerning the physical state of the molecular cloud in which the transition is observed. It will therefore be an important task for future interstellar molecular research to observe and measure as many transitions of any one molecule in any one particular cloud. Doppler shifts, i.e. the difference in frequency between the rest frequency (known from laboratory measurements) and the observed interstellar line frequencies, provide information on the large scale motion of the molecular clouds while the linewidths indicate the turbulence within the clouds. 

While small carbon molecules have been found in locations as diverse as interstellar molecular clouds, carbon stars, hydrocarbon flames and laser-ablated carbon, bulk quantities are much less easily produced and have therefore been little studied. However, as with many organic intermediates, stabilization of these reactive molecules by coordination to metal centers can be achieved and is the subject of this review. The preparation of systems containing metal centers linked by carbon-atom chains is both a synthetic challenge (in spite of the first such being obtained over 45 years ago) and of considerable current relevance. 

We will not mention effects on molecular formation due to shocks and shock fronts in dense molecular clouds, nor will we discuss the chemistry of the cir-cumstellar environment, where an abundance of molecular species has been detected during the past several years. In the warm, dense envelopes of stars the abundances can be matched by chemical-equilibrium calculations, in contrast to the chemical reactions which can take place in the cold interstellar molecular clouds. For example theoretical calculations based on chemical equilibrium have been performed for the expanding molecular envelope of the cool carbon star IRC H-10216 by McCabe et al. (1980), in agreement with the observed molecular column-densities. 

The primary energy source behind the ion-molecule scheme in interstellar molecular clouds is the cosmic ray ionization of H, H2 and He, which can be transferred efficiently to less abundant atoms or species, notably C, N and O. Thus their effective time scale for ionization is reduced by a factor proportional to their abundance. In addition, exothermic feactions between positive ions and neutrals occur with no... 

Carbon based chemistry (organic chemistry) has thus been established to be of fundamental importance in interstellar molecular clouds. Similarly the observed composition of comets is dominated by carbon bearing molecules, and in the reducing atmospheres of Jupiter and Saturn the carbon chain molecules C2H2, CjH have been detected. 

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... 

The IR spectral features of individual subcomponents of chondritic IDPs have also been measured. A measurement of the —10 p.m silicate feature of GEMS produced a broad featureless band at —9.7 p.m that matches the spectra of interstellar molecular cloud dust. 

The chemical composition of ices in space is inferred theoretically on the basis of condensation theory, which predicts tlie composition of solids condensed from gas of Uie cosmic abundance of elements. In Table 9.2, chemical compositions of ices and corresponding equilibrium condensation temperatures are shown in protosolar nebula [1] and interstellar molecular clouds [2],... 

Yamamoto et al. [2] made a condensation calculation to estimate tlie chemical composition of the ice in molecular clouds and cometaiy nuclei. They assumed tlie interstellar molecular composition for tlie abundance of gas. Inter-... 

Observation of Ices in Space 9.1.2.1. Interstellar Molecular Clouds... 

Infrared spectroscopy enables us to obtain information on the chemical composition and structure of icy grains in interstellar molecular clouds [3], Table 9.3 summarizes the abundance of molecules identified [4]. Among these species, the predominance of H2O ice is clear, its abundance being one order of magnitude greater than that of all odier molecules. The molecules CO and CO2 are those next most abundant, following H2O. Small amounts of reduced molecules, hydrocarbons and NH3 are also observed. 

One of the more active and growing areas of research into the study of condensed matter is the investigation of the properties of atomic and molecular clusters. A detailed understanding of clusters is vital to the study of such diverse phenomena as condensation, the dispersion of supported catalysts, cloud formation, molecular generation on interstellar grains, - and the thermodynamic properties of powders. In addition, the study of clusters is of fundamental importance to the understanding of the transition from finite to bulk behavior. 

Interstellar dusts could have a layered structure, as proposed by Greenberg (1998), of an icy mantle covering an organic-silicate core. On the way from an interstellar molecular cloud to protosolar nebula, interstellar dust could be modified by any thermal events, e.g., nebula gas heated by shock waves on an accreting disk or the absorption of solar radiation. 

Solids based on sp hybridization, although subject of intense experimental efforts, seem to be the most elusive of the different carbon families [5]. The existence of linear chains of carbon atoms linked by alternating single and triple bonds (polyyne) or double bonds (polycumulene) with stabilizing molecular complexes at the end of the chains, has been recognized in interstellar molecular clouds and can be artificially produced by different chemical routes [6-8],... 

The species labeled (y/) have been observed in interstellar molecular clouds. The four products of dissociative recombination (17.21,17.22a,b) are all observed. The cyclic C3H2 is a very abundant interstellar species. 

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