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ScienceWeek
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)
Nature http://www.nature.com/nature
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ASTROPHYSICS: ON INTERSTELLAR MOLECULAR HYDROGEN
The following points are made by L. Hornekµr et al (Science 2003 302:1943):
1) The formation of molecular hydrogen in interstellar molecular clouds is the key first step in both the formation of stars and in the evolution of molecular complexity in the interstellar medium (ISM). It is widely accepted that (i) there is no efficient gas phase route for H2 formation from H atoms at the low temperatures and densities of the ISM and (ii) H2 is formed by H atom recombination on dust grains that are integrally associated with the interstellar molecular clouds (1).
2) To account for the astronomically observed H2 formation rate in clouds (2,3), the surface recombination of H atoms on grains to produce H2 must be a very efficient process at T ~ 10 K (1). However, previous laboratory experiments on H2 formation on surfaces of astrophysical relevance were interpreted as implying that the recombination is thermally activated (4), requiring T ~ 20 K (5) for surfaces thought to be important in dark interstellar clouds.
3) The formation of each H2 molecule also releases 4.5 eV of energy. How this energy is partitioned amongst the various degrees of freedom is crucial in understanding the temporal and chemical evolution of molecular clouds. For example, partitioning of the reaction energy into H2 kinetic energy heats the cloud, while vibrational excitation of the H2 or excitation of the grain surface are rapidly (on astronomical time scales) dissipated via radiation that escapes the cloud. Thus, the time scale for collapse of a cloud to yield cores of high density where stars ultimately form depends critically on the energy disposal in H2 formation. In addition, if a large fraction of the 4.5 eV goes into local heating of the grain, this can thermally catalyze other chemical reactions on the grain and/or desorb other atoms or molecules adsorbed in close vicinity on the grains. Unfortunately, at present no definitive aspects of this energy partitioning can be directly inferred from observational astronomy.
4) The chemical nature of the dust grains is not well characterized, but astronomical data and studies of meteorites show that they are principally composed of silicate and carbonaceous material. In diffuse (low density) clouds, the dust grains are bare, but in dark (dense) clouds, the grains are covered with "ice" mantles, principally amorphous solid water with added CO, CO2, methanol, and other molecules. Though the size distribution of the interstellar grains is accepted to follow a power law, their morphology is not well determined. Hence, it is unknown whether the typical 0.1 micron diameter particles thought to be important in the formation of H2 in the ISM are solid compact particles, porous structures, open fractal-like structures, or other possible modifications.
5) In summary: Detailed laboratory experiments on the formation of HD from atom recombination on amorphous solid water films show that this process is extremely efficient in a temperature range of 8 to 20 kelvin, temperatures relevant for H2 formation on dust grain surfaces in the interstellar medium (ISM). The fate of the 4.5 electron volt recombination energy is highly dependent on film morphology. The authors suggest these results indicate that grain morphology, rather than the detailed chemical nature of the grain surface, is most important in determining the energy content of the H2 as it is released from the grain into the ISM.
References (abridged):
1. D. Hollenbach, E. E. Salpeter, Astrophys. J. 163, 155 (1971)
2. M. Jura, Astrophys. J. 197, 575 (1975)
3. C. Gry et al., Astron. Astrophys. 139, 675
4. N. Katz, I. Furman, O. Biham, V. Pironello, G. Vidali, Astrophys. J. 522, 305 (1999)
5. G. Manico, G. Raguni, V. Pironello, J. Roser, G. Vidali, Astrophys. J. 548, L253 (2001)
Science http://www.sciencemag.org
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WATER-ICE CHEMISTRY OF THE INTERSTELLAR MEDIUM
The following points are made by F. Borget et al (J. Am. Chem. Soc. 2001 123:10668):
1) Up to 120 different molecules have been identified in the interstellar medium, and to understand their formation and evolution, numerous theoretical models, which include detailed physical and chemical pathways, have been developed. However, for these models, experimental data relating to grain surface processes and chemistry in ices have been conspicuously absent. The gas-grain interaction, involving heterogeneous reactions on grain surfaces or in the icy mantles, is nevertheless assumed to play an important role in the formation of molecules in the interstellar medium.
2) Effectively, there are vast quantities of water ice in the low-temperature region of the Solar System which have been detected by infrared spectroscopy. Present on the satellites of the outer planets and on comets, water ice is also believed to be an important constituent of interstellar dust. Concerning the pressure and temperature conditions existing in the interstellar medium, it is reasonably expected that the ice is amorphous. For these reasons, particular interest has been focused on small molecules interacting with amorphous ice surfaces.
3) The physical and structural properties of amorphous solid water have been the subject of numerous reports and reviews, and from theoretical and experimental studies of ice clusters and microporous amorphous ice, several aspects of the surface of ice have been revealed. Detailed investigations coupled with computer modeling have identified three characteristic water-ice surface molecules: a) those with a dangling OH bond; b) those with a dangling bond not occupied by H, called "dangling O"; and c) 4-coordinated molecules, called "s-4", that are distorted from the usual tetrahedral symmetry. In particular, it has been established through spectroscopic studies that dangling OH groups are abundant at the surface of both crystalline and amorphous ice.
J. Am. Chem. Soc. http://pubs.acs.org/JACS
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