Interstellar molecules, listed

Table 1. Known Interstellar Molecules Listed by Number of Atoms... 

A hundred or so different molecular species have been spotted in molecular or circumstellar clouds. Most interstellar molecules are organic molecules, that is, they are carbon-based. This indicates that chemical evolution does occur on a cosmic scale. What is more, many molecules on the interstellar list are fundamental building-blocks for the construction of biological structures. 

From an analysis such as described here, the relative abundances of interstellar molecules N(molecule)/N(H2) in various sources can be determined. Table 2 shows a representative list of abundances in two sources, TMC1 and Orion (OMC1). The relative abundance of a species with respect to H2 is more uncertain that the absolute column density because it is normally obtained by dividing the absolute column density by the 13CO value and then estimating the ratio of the 13CO to H2 column densities (Irvine et al. 1985). Thus, one should not assume better than one order of magnitude accuracy in the numbers shown in Table 2. Still, several trends can be discovered in this table ... 

Within the last four years, nearly 30 different molecules have been identified in the interstellar gas clouds of our Galaxy. This advance has been made possible in part by improved techniques of radio astronomy, and has added a large variety of new interstellar molecules to the list of the three radicals CN, CH, and CH+, known from their ultraviolet spectra since before 1940 (Adams,... 

Since Hartmann s 1904 discovery, astronomers have learned that a variety of elements and compounds exist in the interstellar medium, alheit at very low concentrations. In fact, Hartmann s original discovery was based on his observation of the spectrum of calcium in the ISM. Today, astrochemists know of at least 123 interstellar molecules. The chart on page 24 lists some of the more common of these molecules. Notice that some familiar elements and compounds have heen identified in the ISM, including ozone (03), hydrogen chloride... 

An update of lists of Chemical Carcinogens and Interstellar Molecules. 

Table 5.2 lists some of the molecules which have been detected. It is interesting to note that some of them, such as the linear triatomics C2H, HCO and N2H, were found in the interstellar medium before they were searched for and found in the laboratory. In all molecules, except OH and NH3, the transitions observed are rotational in nature. 

It is reasonable to consider the assumption that life began, somehow, among one of the mixtures of small organic molecules that are produced by abiotic processes. The only natural examples in hand today are the components of meteorites that have fallen to Earth (see Section 5.2.1) and particles returned by the Stardust mission. Spectroscopy has also yielded partial lists of the organic molecules in interstellar space and interplanetary dust clouds. 

Molecules interact with the surfaces of solids in almost every environment in the universe. In addition to purely intellectual interest, we customarily justify studying these interactions on technological grounds, heterogeneous catalysis and the fabrication of microchips being the most frequently listed applications. However the field is much more broadly relevant the adsorption and desorption of atoms and molecules on the surfaces of dust grains is very important to molecule formation in the interstellar medium, reactions on the surfaces of ice crystals is important in atmospheric chemistry and reactions at surfaces determine the behaviour of medical implants in our bodies. 

The number of molecules, diatomic and polyatomic, detected in interstellar space, continues to increase, and now totals well over one hundred. At the beginning of the third millenium 26 diatomic species have been observed these are listed in table 10.1. Details of some of the spectroscopic transitions observed and their analyses are described later in this chapter. 

Table 17.1. Observed interstellar and circumstellar molecules The species listed are observed by their rotational emission spectra unless...

A number of molecules have been detected in the interstellar medium, in circumstellar envelopes around evolved stars, and comae and tails of comets through observation of their microwave, infrared, or optical spectra. The following list gives the molecules and the particular isotopic species that have been reported so far. Molecules are listed by molecular formula in the Hill order. All species not footnoted otherwise are observed in interstellar clouds, while some are also found in comets and circumstellar clouds. The list was last updated in October 2008 and lists 162 molecules (298 isotopic forms). 

A recent count puts the number of distinct molecules detected in interstellar space at around 150. The list includes small diatomic molecules that are common on Earth (e.g., CO, N2, O2), high-energy diatomic radicals that have exceedingly short lifetimes on Earth (e.g., HO-, HC-), and all the way up to organic compounds like acetone, ethylene glycol, and benzene. 

Photoionization of atoms and molecules by the interstellar radiation field can occur only if their lowest ionization potential is less than 13.6 eV. The ionization potentials of a number of molecules of astrophysical interest are collected in Table 2. The table lists the lowest ionization threshold, wherever data are available. Note that this threshold may be substantially lower than the vertical ionization potential of the molecule. For example, for CH4, the ionization threshold is about 12.5 eV, whereas the vertical ionization potential is 14.2 eV. Thus, the photoionization cross section of CH4 is extremely small at threshold, and becomes substantial only at energies larger than 14.2 eV, resulting in a negligible... 

Tables 3 and 4 list the resulting photoionization rates in the unattenuated interstellar radiation field of Draine (1978 cf. Eq. (3)) for a number of important atomic and molecular species. Although many of the atomic cross sections and rates are accurately known, the rates for most molecules are uncertain by at least a factor of two. An order of magnitude estimate of the photoionization rates of species not listed in Table 4 can be obtained by assuming a constant cross section of 10 cm above the (vertical) ionization threshold. Some prototypical calculations for different thresholds are included at the bottom of Table 4. Comparison with rates for species for which the cross sections are known shows that the assumption of rrion = 10 cm most likely results in an underestimate of the rate by a factor of a few. The variations of the photoionization rates with depth for the cases discussed in 5.3 are included as well. It is expected that virtually all of the molecular ionizations lead to the formation of a stable molecular ion.

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