Exploring Organic Environments in the Solar System (2007) / Chapter Skim
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II—The Formation, Modification, and Preservation of Organic Compounds in the Solar System, 2 Interstellar Chemistry
II—The Formation, Modification, and Preservation of Organic Compounds in the Solar System, 2 Interstellar Chemistry
Pages 21-36
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From page 21... ...
Carbon-13 (13C) is made late in the lives of red giant stars, where formation of helium, catalyzed by 12C, results in the formation of 13C, 14 N, and 15O via the CNO cycle outlined below: 12C + H 1 13N + 13N 13 C + e+ + 13C + H 1 14N + 14N + H 1 15O + 15O 15N + e+ + 15 N + H 1 12C + He 4 1For the purposes of this report, the interstellar medium is included in this search, both because Earth-based observations can be used to identify organics present there, and because it is thought that the interstellar medium was a source for organics present in the solar system's protoplanetary disk.
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From page 22... ...
The CNO cycle proceeds in the shells of the red giant stars that contain high levels of carbon, where the conversion of C to C, and to N and O, proceeds after the bulk of the hydrogen has been converted to He. The 12 13 14 15 pulsation of the red giant during this energetic process disperses the elements formed into the interstellar medium, where they serve as the starting materials for the formation of a new star.
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From page 23... ...
Astronomical observations, combined with studies of interstellar grains preserved in meteorites, suggest that the dust might consist variously of amorphous carbon, complex fullerenes, polycyclic aromatic hydrocarbons, diamond, silicon carbide, silicates, carbonates, and a host of other candidates, all with or without mantles of ices and/or organic compounds.1 Important components of the ISM are molecular clouds, which are dense, massive objects found throughout the Milky Way and in many external galaxies. In these molecular clouds -- also known as dense clouds -- the gas density is 103 to 106 particles/cm3, which is very high by interstellar standards, and their masses can be as large as a million times the mass of the Sun.
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From page 24... ...
," a 3 with which available Number HC 3 OC 2 4 and CHCN C NC CH H CHOH variety HC 2 3 5 2 2 2 6 a 7891 CH CH HC HCOCH NH c-C CH spectra, spectroscopy from structure; Observatory, + 4 electronic linear Molecules C 4 CN NC OH SH NH CHO CHO N a HC 2 H 3 3 3 3 3 2 2 N 4 or obtained 5 2 C 5 C C l-H C CH CH CH CH HC NH C "l," Astronomy data high-resolution of + by Radio 2 2 H H OH CN NH NC 2 3 3 4 rovibrational, lower-case H Si 3 2 4 3 2 CHN C NCN COH Circumstellar a 5 4 4 H 2 2 2 2 l-C c-C CH HC HC HCOOH H H H HNC SiH H emission composite National and a ? C rotational, + + + are structure; HC D + 3 HC 2 Wootten, 3 NC O S H 2 CO CN CS O 3 3 molecular pure 3 3 3 2 2 2 2 3 4 c-C l-C C C CH HCCN HCNH HNCO HNCS HOCO H H H H NH SiC C of cyclic species a Interstellar their are Alwyn of The H
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From page 25... ...
Given a typical interstellar gas density of 105 particles cm3, a complete conversion of CO to CH4should occur in molecular clouds if chemical equilibrium prevailed. In contrast, CO is the second most abundant interstellar gas-phase molecule, so abundant, in fact, that it is used to map the distribution of molecular clouds in our galaxy and in external galaxies.
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From page 26... ...
For example, the typical synthesis for building larger organic molecules involves the reaction of hydrocarbon radicals such that: CnH2 + C+ Cn H+ + H, +1 followed by the addition of two hydrogens, leading to Cn H3 . The neutral species is finally generated by + +1 dissociative electron recombination or by proton transfer to a suitable base.
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From page 27... ...
The dust grains are produced in the circumstellar shells of red giant stars that condense from the hot material (mainly silicates) emitted from the stellar surface.
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From page 28... ...
Extensive laboratory studies have shown that the ultraviolet irradiation of simulated grain mantles results in the generation of more complex organics.11-14 Unfortunately, the laboratory studies are by necessity carried out with a high ultraviolet flux and thus are not representative of interstellar conditions. Since the precise composition of the grain mantles is not known, it is not possible to accurately extrapolate from the laboratory simulations to the amounts of these compounds in the dark ISM.
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From page 29... ...
Systematic observations of the key organic compounds in a statistical sample of molecular sources will be helpful in this regard. PROTOPLANETARY DISKS The early evolution of a young stellar object proceeds with rapid and dramatic changes.18 Stars begin their lives in molecular clouds.
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From page 30... ...
The chemistry and chemical processes in young stellar objects may be considered in several different regimes as shown in Table 2.3. TABLE 2.3 The Chemistry and Chemical Processes in Young Stellar Objects Components of Molecules Detected in Molecules in the Principal Chemical Formation Stage the Protoplanetary Millimeter/Submillimeter Infrared Wavelength Processes Believed of the Disk Wavelength Region Region to Occur Protoplanetary Disk Dense cloud Molecular ions, carbon Simple ices Low-temperature Cloud fragmentation chains H2O, CO2, CO, CH3OH, chemistry collapse to protostar HC3N, CH3OH, SO, SO2 HCOOH, H2CO Ion-molecule reactions Cold envelope Simple species Ices Low-temperature Enters main accretion around protostar H2CO H2O, CO2, CH3OH chemistry phase (class 0)
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From page 31... ...
External cosmic-ray irradiation does not play much of a role in the dense inner disk, but beyond 10 AU, where the density becomes low enough that the rays are not completely attenuated, cosmic-ray irradiation produces H3 and He+ ions that can convert CO and H2 to CO2, CH4, + NH3, and HCN. Shock Waves in Protoplanetary Disks Shock waves occur in protoplanetary disks where the outflow from the protostar collides with the surrounding cloud material, and there are also accretion shocks from the material infalling onto the disk.
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From page 32... ...
While the latter are believed to have formed in the solar system, the former formed via condensation in circumstellar regions around evolved stars, including red giants, carbon stars, asymptotic giant branch stars, novas, and supernovas. Information on the nature of interstellar grains is available from two sources, astronomical observations and laboratory studies of meteorites.
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From page 33... ...
In addition to the graphitic carbon and silicates, other astronomical evidence has led researchers to suggest the presence of a variety of other grain candidates, including diamonds, ultraviolet-processed hydrogenated amorphous carbon, onion-like hyper-fullerenes, glassy carbon, aliphatic hydrocarbons, polycyclic aromatic hydrocarbons, carbonates, and silicon carbide. In summary, no one particular set of chemical or physical characteristics fits all of the available evidence.
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From page 34... ...
Tarafdar, "UV Radiation Field Inside Dense Clouds -- Its Possible Existence and Chemical Implications," Astrophysical Journal 267: 603, 1983.
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From page 35... ...
Marty, "Nitrogen Isotopic Composition of Macromolecular Organic Matter in Interplanetary Dust Particles," Geochimica et Cosmochimica Acta 67: 3773-3783, 2003.
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From page 36... ...
Marty, "Nitrogen Isotopic Composition of Macromolecular Organic Matter in Interplanetary Dust Particles," Geochimica et Cosmochimica Acta 67: 3773-3783, 2003.
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