Interstellar Polarization

Polar carbon-rich species such as hexapentaeneyhdene (124) have been identified in interstellar clouds,which has attracted some attention and interest in CeH2 compounds. The existence of 124 in interstellar space is remarkable, since this structure is 50 kcal/mol [CCSD(T)/cc-pVTZ] less stable than hexatriyne (125), which, however, is nonpolar and therefore difficult to observe radioastro-nomically. At the same level of theory the three isomeric benzdiynes 126-128 are higher in energy than 125 by 39.4, 46.3, and 56.2 kcal/mol, respectively. ... 

An interstellar dust cloud containing aligned particles may be looked upon as a linearly birefringent (and possibly linearly dichroic) medium (van de Hulst, 1957, p. 58) the cloud acts like a retarder. We showed at the end of Section 2.11 that linearly polarized light becomes circularly polarized upon transmission by a retarder. The first clear evidence for this kind of polarization mechanism was reported by Martin (1972), where light from the Crab Nebula was the source of linearly polarized incident light. 

Coyne, G. V., T. Gehrels, and K. Serkowski, 1974. Wavelength dependence of polarization XXVI. The wavelength of maximum polarization as a characteristic parameter of interstellar grains, Astron. J., 79, 581-589. 

Martin, P. G., 1974. Interstellar polarization from a medium with changing grain alignment, Astrophys. J., 187, 461-472. 

M. Jura (University of California, U.S.A.). There has been a report from a shuttle experiment that the uv polarization of the interstellar extinction continued through the 220 nm feature. Is this consistent with the notion that since you would expect the polarization produced by grains, for example picket-fence molecules, to be lined up by a magnetic field, how can this occur if the molecules are spherical ... 

The wide variety of interstellar molecules detected so far in our Galaxy (see Table 6) are composed of the most abundant chemically reactive elements, i.e. H, C, N, 0, Si and S. The selection of detected molecules is influenced by molecular and observational considerations i) the molecules must be polar ii) they must have sufficient vapor pressure for their laboratory spectra to be known, iii) of the known spectra, only the most intense transitions can be expected to be observable in interstellar space, and iv) the frequencies of these transitions have to be located within the Earth s atmospheric windows Only molecules which satisfy these conditions are amenable to radio techniques. 

On the other hand reactions of hydride radicals with each other lead to the formation of astrophysically important, but often non-polar molecules such as HCCH, H2CCH2, H3CCH3. Correspondingly HNNH, H2NNH2 and in particular HOOH and HSSH, which are all polar, may very well be detectable in interstellar space. Surface reactions of the hydride radicals with other radicals and molecular fragments produce more complex molecules. For example cyano-acetylene, HCC-CN may well have been formed this way. All detected interstellar polyatomic molecules can be explained this way (see Table-8) and some hitherto undetected, but important ones, can be predicted to exist in interstellar space. Their observation or absence in interstellar space may then in conjunction with laboratory results shed more light on possible chemical pathways. 

Various theories exist, but one with considerable support takes note of the fact that some meteorites have brought to earth amino acids that are partially enantiomeric, with 3-15% enantiomeric excess of the L-amino acids (36). They are not normal amino acids, but they have a methyl group on the alpha carbon in place of the usual hydrogen so they cannot racemize. It is believed that they are formed as racemates in interstellar space, but then are deracemized partially by circularly polarized light emitted by synchrotron processes at neutron stars. 

Matrajt G, Pizzarello S, Taylor S, Brownlee D. Concentration and variability of the AIB amino acid in polar micrometeorites Implications for the exogenous delivery of amino acids to the primitive Earth. Meteorit. Planet. Sci. 2006 128 7412-7413. Busemann H, Young, AE, Alexander, CMO D, Hoppe, P, Mukhopadhyay, S, Nitter, LR. Interstellar chemistry recorded in organic matter from primitive meteorites. Science 2006 312 727-730. 

In the cold (10-30 K) interstellar clouds the detection of carbon clusters is still incomplete but the centro-symmetric carbon chain could possibly be detected by their far-IR bending rotation transitions. The C3 molecule was recently detected in one cold dust cloud [104]. The observations of hydrogen-containing polar chain molecules towards star f-Ophiuchi and laboratory studies of the electronic transitions of a number of homologous series lead to an important conclusion on the role of carbon chains as potential molecules in the so-called diffuse interstellar bands (DIBs). These carbon-chain molecules would have to be very long, e.g. HC H, C2 , C 11 neutral chains, cations or (C ) anions (except for odd-numbered... 

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