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ASTROPHYSICS: ON STELLAR SYNTHESIS OF CHEMICAL COMPOUNDS

The following points are made by Sun Kwok (Nature 2004 430:985):

1) Work on stellar nucleosynthesis in the 1950s has led to our current realization that most of the chemical elements are synthesized in stars. Helium is made by hydrogen burning in the core during the main sequence and in a shell above the core in the red giant phase. The element carbon is created by helium-burning (the triple-alpha process), first through core burning and later through shell burning above an electron-degenerate carbon-oxygen core. For massive (more than ten solar masses, > 10 M-Sun) stars, direct nuclear burning continues with the production of oxygen, neon, magnesium, silicon and so on, cumulating in the synthesis of iron, the heaviest element possible through direct nuclear burning. The other heavy elements, from yttrium and zirconium to uranium and beyond, are produced by neutron capture followed by decay(1).

2) For the majority of stars (~95%, corresponding to stars with initial masses of less than 8 M-Sun), direct nuclear burning does not proceed beyond helium, and carbon is never ignited. Most of the nucleosynthesis occurs through slow neutron capture (the s process) during the asymptotic giant branch (AGB), a brief phase (~10^(6) yr) of stellar evolution where hydrogen and helium burn alternately in a shell. These newly synthesized elements are raised to the surface through periodic "dredge-up" episodes, and the observation of short-lived isotopes in stellar atmospheres provides direct evidence that nucleosynthesis is occurring in AGB stars(2).

3) However, the atomic form is not the only form in which chemical elements can occur. Molecules are major constituents of planetary atmospheres and the Solar System contains many solid bodies, such as asteroids and terrestrial planets. Since the late 1960s, advances in millimeter-wave observation have led to the detection of over 120 molecules in the interstellar medium. Although a variety of chemical models have been developed to explain how molecular synthesis could occur in the interstellar medium, we have no direct observational knowledge of the timescales on which these reactions occur. The discovery of a large number of gas-phase molecules as well as submicrometer-sized solid-state particles of various chemical compositions in the circumstellar envelopes of AGB stars has led to the realization that extensive molecular synthesis occurs during this brief phase of stellar evolution. These envelopes are expanding and have dynamical timescales of several thousand years only, providing the first definite indication that production of molecules and solids of high complexity can occur rapidly in the circumstellar environment.

4) Although it was commonly assumed that stellar grains are destroyed during their journey through the interstellar medium by radiation and shocks(3), the recent discoveries of the chemical richness of circumstellar grains have raised the possibility that circumstellar molecular synthesis may have significant implications for the chemical enrichment of the Galaxy, or even of the early Solar System.

5) In summary: Recent isotopic analysis of meteorites and interplanetary dust has identified solid-state materials of pre-solar origin. We can now trace the origin of these inorganic grains to the circumstellar envelopes of evolved stars. Moreover, organic (aromatic and aliphatic) compounds have been detected in proto-planetary nebulae and planetary nebulae, which are the descendants of carbon stars. This implies that molecular synthesis is actively happening in the circumstellar environment on timescales as short as several hundred years. The detection of stellar grains in the Solar System suggests that they can survive their journey through the interstellar medium and that they are a major contributor of interstellar grains.(4,5)

References (abridged):

1. Wallerstein, G. et al. Synthesis of the elements in stars: forty years of progress. Rev. Mod. Phys. 69, 995-1084 (1997)

2. Lattanzio, J. in Planetary Nebulae: Their Evolution and Role in the Universe (eds Kwok, S., Dopita, M. & Sutherland, R.) 73-81 (ASP, San Francisco, 2003)

3. Jones, A. P., Tielens, A. G. G. M., Hollenbach, D. J. & McKee, C. F. Grain destruction in shocks in the interstellar medium. Astrophys. J. 433, 797-810 (1994)

4. Kwok, S. Effects of mass loss on the late stages of stellar evolution. Phys. Rep. 156, 113-146 (1987)

5. Olofsson, H. The neutral envelopes around AGB and post-AGB objects. IAU Symp. 178: Molecules in Astrophysics: Probes and Processes (ed. van Dishoeck, E.) 457-468 (Kluwer, Dordrecht, 1997)

Nature http://www.nature.com/nature

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ON THE INTERSTELLAR MEDIUM

The following points are made by Ralf I. Kaiser (Chem. Rev. 2002 102:1309):

1) The physical and chemical processes leading to the formation of molecules in the interstellar medium (ISM) -- the vast voids between the stars -- fascinated scientists since the first detection of CH, CH+, and CN radicals in extraterrestrial environments 60 years ago. Although more than 50 years have passed and 121 species from molecular hydrogen to polyatomics such as the sugar glycolaldehyde, benzene, cyanopentaacetylene, and possibly the amino acid glycine have been identified so far, the enigma of how these molecules are actually formed under the harsh conditions in the interstellar medium is still under debate.(1)

2) The ISM contains approximately 10% of the mass of our Galaxy and consists of gas (99%) and submicrometer-sized grain particles (1%) with averaged number densities of 1 H atom cm^(-3) and 10^(-11) grains cm^(-3), respectively.(2-5) These data translate to pressures of approximately 10^(-18) mbar at 10 K, which is beyond any ultrahigh vacuum achieved in terrestrial laboratories so far. The chemical composition of the interstellar medium is dominated by neutral hydrogen (93.38%) and helium (6.49%), whereas biogenic elements oxygen, carbon, and nitrogen contribute 0.11% (O:C:N 7:3:1). The third-row elements neon, silicon, magnesium, and sulfur are less copious (0.002%) and have relative abundances of 8:3:3:2; all remaining elements furnish only 0.02%.

3) This elementary classification is well-reflected in the molecular composition of the interstellar medium. Molecules, radicals, and ions are ubiquitous in extraterrestrial environments and have been detected in extraordinary diversity ranging from small molecules such as hydrogen to astrobiologically important species such as the simplest sugar glycolaldehyde and possibly the amino acid glycine. Many of the species are thermally unstable and extremely reactive in terrestrial laboratories. The majority of these molecules were detected by radio telescopes observing their rotational transitions in emission; to a minor extent, infrared, visible, and ultraviolet astronomy fostered their identification. Diatomic molecules with second- and third-row elements are particularly prevalent; in particular, carbon and silicon bearing species. CP and PN are the only phosphorus-containing molecules identified so far; NO, NS, and SO represent the sole extraterrestrial radicals carrying atoms of the fifth and sixth period.

References (abridged):

1. Chemistry and Physics of Molecules and Grains in Space. Faraday Discuss.1998, 109.

2. Hollenbach; D. J.; Thronson, H. A. Interstellar Processes; Reidel: Dordrecht, 1987.

3. Tielens, A. G. G. M.; Allamandolla, L. J. In Interstellar Processes; Hollenbach, D. J., Thronson, H. A., Eds.; Reidel: Dordrecht, 1987; pp 397-469.

4. Carbon in the Galaxy: Studies from Earth and Space; NASA-SP-3061, NASA: Washington, D.C.,1990.

5. Interstellar Dust; NASA-SP-3036, NASA: Washington, D.C., 1989.

Chemical Reviews http://pubs.acs.org/journals/chreay/index.html

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ASTROCHEMISTRY: ON NITROGEN IN SPACE

The following points are made by Theodore P. Snow (Nature 2004 429:615):

1) Knauth et al(1) have recently claimed the first detection of molecular nitrogen (N2) in interstellar space. This simple diatomic molecule, made of one of the most abundant elements in the Universe, is the most common constituent of Earth's modern atmosphere. It is also a major component of the atmosphere of Saturn's moon Titan, and has been detected in trace amounts in the atmospheres of Venus and Mars. But it has proved surprisingly difficult to find N2 in any environment beyond the Solar System.

2) Chemical models of dark interstellar clouds (whose densities are usually in the range of 10^(3) to 10^(5) particles per cm^(3)) suggest that N2 should be the most abundant form of nitrogen in these regions. This leads to the prediction(2-4) that the ratio of N2 to hydrogen should be about 10^(-5). In contrast, models for diffuse interstellar clouds, which are transparent and have densities of about 10^(2) particles per cm^(3), predict a much lower N2 abundance, in the range between 10^(-9) and 10^(-8) that of hydrogen(2,5).

3) Both predictions suggest that N2 might be observable, but searches for this molecule in interstellar space have been fruitless until now. One of the difficulties in detecting interstellar N2 arises from the fact that the symmetric diatomic molecule has no allowed rotational or vibrational (dipole) transitions. Thus, N2 -- unlike most of the 120 or more species now detected in dark interstellar clouds -- cannot be detected either through millimeter-wavelength observations of rotational emission lines or through infrared spectroscopic detection of vibrational bands (absorption or emission).

4) The only viable approach to finding interstellar N2 is to search for the spectral lines created by electronic transitions in the molecule. These lines are found exclusively at far-ultraviolet wavelengths (shorter than 100 nm), for which space-based telescopes are required because the Earth's atmosphere blocks such radiation. For technical reasons, however, most ultraviolet telescopes have not covered the far-ultraviolet spectral region where the N2 bands lie. For example, the Hubble Space Telescope cuts off at about 115 nm, well above the wavelength needed for an N2 search. The Copernicus satellite -- a small mission that was developed and led by the late Lyman Spitzer and operated from 1972 until 1980 -- was the first orbiting spectroscopic observatory capable of far-ultraviolet searches for N2 in interstellar space, but no detection was achieved.

References (abridged):

1. Knauth, D. C., Andersson, B. -G., McCandliss, S. R. & Moos, H. W. Nature 429, 636-638 (2004)

2. Viala, Y. P. Astron. Astrophys. Suppl. 64, 391-437 (1986)

3. Womack, M., Ziurys, L. M. & Wyckoff, S. Astrophys. J. 393, 188-192 (1992)

4. Bergin, E. A., Langer, W. D. & Goldsmith, P. F. Astrophys. J. 441, 222-243 (1995)

5. Black, J. H. & Dalgarno, A. Astrophys. J. Suppl. 34, 405-423 (1977)

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