Polycyclic aromatic hydrocarbon lifetime in cometary environments

Observations of cometary comae in the infrared and in the near-ultraviolet suggest that polycyclic aromatic hydrocarbons (PAHs) are present in these environments. However, the chemical identity and abundance of these molecules are not clearly determined yet. Some species are probably more stable than others when submitted to the solar radiation field, and are therefore more likely to be observed. The photophysics of gas-phase PAHs in cometary environments is modelled. Photodissociation occurs when the heating by absorption of UV photons is more efficient than the radiative cooling. The lifetime of the molecules is found to depend on their size: small molecules being more stable than large ones. Furthermore, at 1 AU from the Sun, the lifetime of PAHs is found to be very short (20s for phenanthrene). This suggests that, if observed in the gas phase in cometary environments, these molecules should be produced by an extended source.     

Survival of organic phases in porous IDPs during atmospheric entry: A pulse‐heating study

Abstract— In this study, we have performed pulse‐heating experiments at different temperatures for three organic molecules (a polycyclic aromatic hydrocarbon [PAH], a ketone, and an amino acid) absorbed into microporous aluminum oxide (Al₂O₃) in order to imitate the heating of the organic molecules in interplanetary dust particles (IDPs) and micrometeorites (MMs) during atmospheric entry and to investigate their survival. We have shown that modest amounts (a few percent) of these organic molecules survive pulse‐heating at temperatures in the 700 to 900 °C range. This suggests that the porosity in IDPs and MMs, combined with a sublimable phase (organic material, water), produces an ablative cooling effect, which permits the survival of organic molecules that would otherwise be lost either by thermal degradation or evaporation during atmospheric entry.     

Complex organics in space from Solar System to distant galaxies

Recent observational and experimental evidence for the presence of complex organics in space is reviewed. Remote astronomical observations have detected \(\sim \)200 gas-phased molecules through their rotational and vibrational transitions. Many classes of organic molecules are represented in this list, including some precursors to biological molecules. A number of unidentified spectral phenomena observed in the interstellar medium are likely to have originated from complex organics. The observations of these features in distant galaxies suggests that organic synthesis had already taken place during the early epochs of the Universe. In the Solar System, almost all biologically relevant molecules can be found in the soluble component of carbonaceous meteorites. Complex organics of mixed aromatic and aliphatic structures are present in the insoluble component of meteorites. Hydrocarbons cover much of the surface of the planetary satellite Titan and complex organics are found in comets and interplanetary dust particles. The possibility that the early Solar System, or even the early Earth, have been enriched by interstellar organics is discussed.     

Molecules in white dwarfs

Molecular dissociation equilibrium calculations were done for the model atmospheres of DA and non-DA white dwarfs. Our calculations show that He ₂ ⁺ and HeH⁺ appear as most abundant molecules in the atmospheres of non-DA white dwarfs while H₂ and H ₂ ⁺ are most abundant molecules in DA white dwarfs. It is suggested that these molecules should be searched for in the atmospheres of white dwarfs.     

Cosmogenic nitrogen isotopic disequilibrium observed in meteorites

Abstract— The isotopic disequilibrium of N molecules in meteorites was examined. For a large data-set, consisting of mass compositions of N molecules extracted at 1200 °C by stepped combustion of chondrites and eucrites, we find that a cosmogenic-N-enriched component and a normal air-like N component are isotopically disequilibrated with each other. The isotopic composition of the cosmogenic-N-enriched component seems to be variable, although the cause for the variation is not clear. The abundance of indigenous atomic N in silicate minerals seems to be small. Indigenous N in meteoritic silicates, if present, may be present in the form of N molecules.     

Millimeter observations of organic molecules toward high-mass star formation region G34.26+0.15

To investigate the chemical origination of organic molecules CH_3OH, CH_3OCH_3, C_2H_5OH,CH_3OCH, CH_3 CN, C_2H_3 CN and C_2H_5 CN in the hot core associated with high-mass star formation region G34.26+0.15, Submillimeter Array observations were made with its 230 GHz receiver. The molecular gas distribution has shown that the oxygen- and nitrogen-containing molecules peak at different positions.Comparing the spatial distributions with rotational temperatures and fractional abundances of the observed molecules, we discuss the possible chemical origination of these organic molecules.     

Hot cores: probes of high-redshift galaxies?

The very high rates of second generation star formation detected and inferred in high-redshift objects should be accompanied by intense millimetre-wave emission from hot core molecules. We calculate the molecular abundances likely to arise in hot cores associated with massive star formation at high redshift, using several different models of metallicity in the early Universe. If the number of hot cores exceeds that in the Milky Way Galaxy by a factor of at least 1000, then a wide range of molecules in high-redshift hot cores should have detectable emission. It should be possible to distinguish between different models for the production of metals and hence hot core molecules should be useful probes of star formation at high redshift.     

Spectrofluorometric analysis of amino acid mixtures: Implications for future space missions

Efficient detection of organic molecules is fundamental for the success of future life detection missions. Spectrofluorometric analysis is one of the many techniques that may be used to detect organic molecules in extraterrestrial settings. A particularly important class of organic molecules to target is the amino acids on which all terrestrial life depends. This study aims to identify the optimum fluorescence excitation and emission wavelengths for 17 amino acid standards to examine the effects of amino acid concentrations, mixtures and fluorescence quenching. The results and interpretations can guide the design and operation of life detection protocols on future space missions.     

Role of HD Molecules in the Cooling of the Primordial Gas

This paper examines the effect of HD molecules on the thermochemical evolution of the primordial gas behind the fronts of shock waves that may develop during the epoch of galaxy formation. A critical shock velocity is found at which deuterium efficiently converts to HD molecules and the contribution of HD molecules to cooling is dominant. Above this value of the shock velocity the gas can cool to temperatures close to that of the cosmic background radiation. In this case the Jeans mass will depend only on the red shift and the initial density, with M_J ∝ δ _c ^−0.5 (1 + z)^0.5. For z≳45, HD molecules heat the gas and for large red shifts they generally cease to play a significant role in the thermal evolution of the gas. __________ Translated from Astrofizika, Vol. 48, No. 4, pp. 585–601 (November 2005).     

On the possibility of faint molecular lines of PO,PH, MgH+, and CN in the solar spectrum

Equivalent width calculations for some electronic and vibration-rotation transitions of the molecules PO, PH, MgH^+f, and CN have been carried out for a few umbral, photospheric, and facular model atmospheres. It appears that a few weak lines of these molecules might show up in the umbral spectrum. Le Blanc bands of CN are too weak for detection in the solar spectrum.     

Organic synthesis in the coma of Comet Hale–Bopp?

Several organic molecules have now been detected in the coma of Hale–Bopp. These species may either emanate from the nucleus, or, as has been suggested by Bockelée–Morvan et al., could be synthesized in the coma. We have modelled the gas phase chemistry which occurred in the coma of Hale–Bopp, concentrating on the observed organic molecules HCOOH, HCOOCH₃, HC₃N and CH₃CN. We find that gas phase chemical reactions are unable to synthesize the observed abundances of these molecules, so all these species are most probably present in the nuclear ice. We briefly discuss the implications of this result for the connection between cometary and interstellar ices.     

Formation of complex chemical species in astrochemistry (a review)

In recent years, a rapid growth in a new area of space studies??astrochemistry??has been observed. Its subject is the chemical evolution and chemical diversity of interstellar matter. Molecules yield unique information concerning physical conditions in the interstellar medium and, in particular, in the star-formation regions, through spectral observations of the matter in the gas-phase and dust fractions via rotational and vibrational transitions of interstellar molecules. Moreover, an understanding of the chemistry of molecules can tell us about the lifetime and history of the observed objects. Such an understanding, however, requires detailed chemical knowledge of the gas-phase reactions and grain-surface chemical processes that very often take place under rather exotic conditions strongly differing from those for chemical reactions in the laboratory. Note that the interests of chemists and astronomers in this new area are different: chemists are more likely to be interested in chemical diversity throughout the Universe, whereas astronomers are more likely to use molecules as probes of physical processes.     

Miller-Urey Synthesis in the Nuclei of Galaxies

The most promising venues for the synthesis of prebiotic molecules by Miller-Urey type processes may be found near the centres of galaxies. Explosions of supermassive stars would produce the basic chemical elements necessary to make molecules in high-density mass flows that are then acted upon by ionizing radiation, thus simulating the conditions needed for Miller-Urey type processing. This revised version was published online in July 2006 with corrections to the Cover Date.     

The RNA World and its origins

The theory of the "RNA World" states that the first molecular systems to display the properties of self-replication and evolution were RNA molecules. The origin of life not only depended crucially upon this event, but RNA molecules can even be viewed as the first "living" things. In recent years this theory has gained ascendancy over competing ideas and is now largely accepted by biologists as the most satisfactory explanation for the origin of life. The reasons for this development will be reviewed and the problem of the origin of the first RNA molecules will be discussed.     

Miller-Urey Synthesis in The Nuclei of Galaxies

The most promising venues for the synthesis of prebiotic molecules by Miller-Urey type processes may be found near the centres of galaxies. Explosions of supermassive stars would produce the basic chemical elements necessary to make molecules in high-density mass flows that are then acted upon by ionizing radiation, thus simulating the conditions needed for Miller-Urey type processing. This revised version was published online in July 2006 with corrections to the Cover Date.     

Desorption processes and the deuterium fractionation in molecular clouds

Several processes have been suggested as ways of returning accreted grain mantles to the gas, thus preventing the total removal of molecules from the gas phase in dark quiescent clouds. We attempt to distinguish between them by considering not only the calculated gas-phase abundances, but also the ratio of the abundances of deuterated species to non-deuterated species. We find that the D/H ratio in molecules is relatively model-independent, but that desorption due to the formation of H₂ on grains gives the best overall agreement with the observations.     

Interstellar molecules and the problem of laboratory data

The use of interstellar molecules as probes of physical conditions in interstellar clouds is hampered by the lack of basic laboratory data. The excitation of interstellar molecules is poorly understood because the nature of the interaction of molecules with radiation and with neutral particles is largely undknown. The mechanisms of formation and destruction of interstellar molecules are presently speculative, because little data exists in such areas as gas-phase ion-molecule reactions and exchange reactions, and reactions of various types on surfaces. Specific needs with regard to laboratory data are discussed in these and other areas. Operated by Associated Universities, Inc., under contract with the National Science Foundation.     

Organic compound alteration during hypervelocity collection of carbonaceous materials in aerogel

Abstract— The NASA Stardust mission brought to Earth micron‐size particles from the coma of comet 81P/Wild 2 using aerogel, a porous silica material, as the capture medium. A major challenge in understanding the organic inventory of the returned comet dust is identifying, unambiguously, which organic molecules are indigenous to the cometary particles, which are produced from carbon contamination in the Stardust aerogel, and which are cometary organics that have been modified by heating during the particle capture process. Here it is shown that 1) alteration of cometary organic molecules along impact tracks in aerogel is highly dependent on the original particle morphology, and 2) organic molecules on test‐shot terminal particles are mostly preserved. These conclusions are based on two‐step laser mass spectrometry (L²MS) examinations of test shots with organic‐laden particles (both tracks in aerogel and the terminal particles themselves).     

Interstellar polyynes and related species

The existence of the polyynes — molecules consisting essentially of long chains of carbon atoms — in the interstellar medium is a discovery that appears to be critical to our understanding of interstellar chemistry. The family relationships of these species to other interstellar species are explored as are their significance to theories of interstellar chemistry. Methods for deducing the numbers of related molecules are presented.     

Detection of rovibrationally excited H2 formed through the heterogeneous recombination of H atoms on a cold HOPG surface

This paper reports results from an experiment designed to measure the nascent rovibrational population of H₂ molecules that have formed through the heterogeneous recombination of H atoms on the surface of cosmic dust analogues under conditions approaching those of the interstellar medium (ISM). H₂ that has formed on a highly oriented pyrolytic graphite (HOPG) surface has been detected, using laser induced resonance-enhanced multi-photon ionization (REMPI), in the v = 1 (J= 0–3) rovibrational states at surface temperatures of 30 K and 50 K. These excited product molecules display rotational temperatures significantly higher than the target surface temperature. These first results suggest that a considerable proportion of the binding energy released on formation of the H₂ is deposited in the surface, in addition to internal excitation of the product molecules. This revised version was published online in July 2006 with corrections to the Cover Date.