Showing papers by "N. C. Wickramasinghe published in 1977"

Polysaccharides and infrared spectra of galactic sources

Abstract: OBSERVATIONS over the infrared waveband 2–30 µm available for a number of astronomical objects are shown here to be reconcilable with the transmittance properties of polysaccha-rides. Using an experimentally determined transmittance spectrum for cellulose we can readily relate astronomical data in the 2–4-µm, 8–13-µm and 15–30-µm wavebands and we obtain close fits to astronomical spectra in these several bands. From this detailed spectral agreement we consider it reasonable to infer the detection of interstellar polysaccharides. The identification of this highly complex macromolecule, presumably formed by an abiogenic processing of interstellar formaldehyde, could have a profound bearing on interstellar chemistry including the evolution of prebiotic molecules.

On the computation of optical properties of heterogeneous grains

Abstract: The Maxwell-Garnet (MG) expression for the effective dielectric function of a composite medium consisting of small spherical inclusions embedded in a homogeneous matrix is derived. It is shown that the MG theory accounts for dipole-dipole interactions among the inclusions and that the validity of the theory does not depend on the matrix being non-absorbing. Moreover, by comparing the Mie theory with the MG theory it is shown that the MG dielectric function is correct at least to terms linear in the volume fraction of inclusions. For a sphere ‘carved out’ of such a medium we then have a prescription for calculating cross sections and scattering phase functions. The scattering properties of a small heterogeneous sphere described by the MG dielectric function are identical to those of a small layered sphere with the same outer radius and volumes of the two constituents. In the small particle (Rayleigh) limit we discuss two illustrative examples: (a) a porous sphere, (b) spherical inclusions in a ‘waxy’ dielectric matrix. In both cases we obtain the conditions for resonance absorption. Finally, we apply these results to evaluate the role of porosity in individual graphite grains, and of a waxy matrix containing small spherical graphite inclusion, on the graphite particle resonance absorption at 2200 A. Increasing porosity and increasing the volume fraction of matrix material have similar effects in shifting the resonance to longer wavelengths.

Identification of the λ2, 200Å interstellar absorption feature

Abstract: A BROAD absorption feature centred on λ2, 200A with a half-width of ∼ 300A appears in the spectra of reddened stars1–3. This conspicuous feature in the interstellar extinction curve, might hold an important clue to the identity of a major component of interstellar matter, but it has defied identification for over a decade. Here we identify this band as representing the integrated effect of a set of bicyclic compounds, each with the empirical formula C8H6N2. Such nitrogenated structures could form in stellar mass flows of the type which we have also discussed4. A significant mass fraction of all interstellar material might exist in this form.

Spectroscopic Evidence for Interstellar Grain Clumps in Meteoritic Inclusions

Abstract: Aromatic material extracted from the Murchison meteorite shows an absorption at λ2 200 A similar to the well-known interstellar absorption feature at this wavelength. This suggests a link between interstellar grain clumps containing organics and the presolar component of carbonaceous chondrites.

Prebiotic molecules and interstellar grain clumps.

Abstract: INTERSTELLAR molecules detected by radioastronomical techniques in clouds such as OMC 1 and 2 in the Orion Nebula and Sgr B2 in the galactic centre span a wide range of types and complexity. Among the heaviest of the molecules recently discovered is cyanodiacetylene1 (H—C≡C—C≡C—C≡N). There have been earlier detections of precursors to the simplest known amino acid glycine (formic acid and methanimine), and probable detections of polyoxymethylene polymers and co-polymers2–5 in interstellar clouds. We discuss here a possible identification of organic molecules of even greater complexity, and its implications for the start of biological activity.

Prebiotic polymers and infrared spectra of galactic sources

Abstract: INFRARED absorption features characteristic of molecular dust clouds in the Galaxy may be assigned to complex organic polymers or prebiotic polymers. It could be argued that such highly stable, complex polymers evolve due to radiation processing of molecular mantles on interstellar grains—essentially by a type of natural selection which operates in the interstellar medium. A large fraction of all C, N, O elements in the interstellar medium could be condensed in the form of these stable polymers. Such interstellar material may also account for a significant fraction of the ‘insoluble organic matter’ which is found in carbonaceous chondrites.

Origin and nature of carbonaceous material in the galaxy

Abstract: ASTRONOMERS generally believe that the carbonaceous material emerging from stars must be in the form of graphite, the most stable condensed form of carbon, and that such emergence must be confined to situations where the C/O ratio exceeds unity, such as in the atmospheres of carbon stars. We argue here that this state of affairs remains valid for mass flows from stars of sufficiently low surface temperatures, but it is not correct for low density flows from stars with colour temperatures ≳ 4,000 K (or for oscillatory stars with colour temperatures that go above 4,000 K for a portion of their cycle). In the latter case we show that carbonaceous material comprised mainly of polysaccharides will be able to condense.

Polyoxymethylene co-polymers on grains

Abstract: Conditions prevalent in dense molecular clouds are shown to favour the polymerization of H2CO molecules and the deposition of formaldehyde co-polymer mantles, with typical radii ∼10−5 cm, on smaller refractory grains. If a significant fraction of such co-polymer coated grains are expelled with systematic gas flows into the general interstellar medium, these moderately refractory grains may be responsible for the bulk of interstellar extinction and polarization at optical wavelengths. Mie calculations for a mixture consisting of iron, graphite and POM particles are presented as an example where POM grains of radii 0.15 μ dominate the extinction at optical wavelengths, providing a satisfactory overall fit to a range of extinction data. A size distribution of POM needles with a ‘mean’ radius 0.15 μ also provides good agreement with data on interstellar linear as well as circular polarization. Suitably end-capped and stabilized co-polymer-coated grains, with either silicate or graphite cores, may survive at temperatures ∼450 K under interstellar ambient conditions and be responsible for the 10 μ emission feature in many sources. Theoretically computed band profiles of the 10 μ-feature in POM coated grains, in general, provide better agreement with observations than most types of silicate grains considered so far. We also note that an unexplained dip at λ≃10 μ in the 8–12 μ feature of the infrared source OH 231.8+4.2 may be a signature of POM grains; likewise, a persistent 3.3 μ emission feature in many different types of infrared source could be attributed to the CH stretching mode in formaldehyde co-polymer grains.

Pre-biotic molecules in Martian dust clouds

Abstract: THE search for life on Mars has so far proved disappointing1. Experiments on the Viking package were designed on the presumption that any microbial life on Mars would be endowed with metabolic processes similar to present-day microorganisms on Earth. Analyses of gases released in soil heating experiments2 have provided no evidence for significant abundances of highly complex organic molecules. These data are however, limited to only two landing sites and moreover the compositional studies carried out so far reflect properties of only a relatively thin dust layer on the Martian surface. Dust sucked up into the atmosphere from large areas of the planet's surface would have been witnessed in the 1971 storm, representing a much broader sampling of Martian surface material. Here we propose that a broad absorption feature centred on a wavelength close to 2,200 A, derived from scattering studies of these dust clouds, may be a signature of highly complex organic or prebiotic molecules.

Organic Molecules in Interstellar Dust: A Possible Spectral Signature at λ2200 Å?

Abstract: The λ2 200 A interstellar absorption band, generally attributed to graphite grains, could equally well arise from π→* electronic transitions in conjugated double bonds of organic molecules. These molecules, which should comprise ∼ 10% of the total interstellar dust mass, may be lodged within clumps of 100 A-sized refractory grains.

On the Production of Positive Molecular Ions in Cometary Comas

Abstract: Positively charged molecular ions, such as H2O+, which have been observed in cometary. comas, may be efficiently produced by the evaporation of positively charged clathrate grains of radii in the range ∼10−6–10−5 cm. Such grains may be expelled from nuclei of comets, along with gaseous molecules. Grain charging occurs via interaction with solar ultraviolet photons and/or solar wind protons. Observational data on the total quantities as well as the distributions of H2O and H2O+ in cometary comas are shown to be in accord with detailed model calculations.