The spectroscopic identification of interstellar grains

It is shown that the condition of matching the 3.3–3.9 m spectrum of the galactic infrared source GC-IRS 7 leads to a remarkably tight convergence on the transmittance curve measured in the laboratory for the dessicated bacteriumE. coli. Other materials, including certain biochemicals and postulated prebiologic compounds, are shown to be deficient with regard to meeting this condition.     

Organic model of interstellar grains

Extinction efficiency of grains is calculated from the Mie formula on the premise that the grains are of organic composition. The optical constants adopted for the calculations are those ofE. coli, polystyrene and bovine albumin. The grain radiusa is assumed to obey a distribution of the formN(a) a ^– and the value of is chosen so as to make the calculated extinction curve match the observed interstellar extinction curve. Although the calculated curve gives a reasonably good fit to the observed extinction curve for wavelength less than 2100 Å, at longer wavelength region agreement is poor. It is concluded that another component is required for the organic model to be viable.     

A model of interstellar grains with surface roughness

A simple model of surface roughness on the interstellar spherical grains has been considered. A test case with dirty ice spherical grains reveals that the effect of surface roughness is to significantly enhance the extinction in the far ultraviolet compared to the equivalent smooth spheres.Receipt delayed by postal strike in Great Britain.     

Porous interstellar grains

Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of interstellar dust. Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of the nature of interstellar dust, we present a technique based on the multipole expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite, arbitrarily shaped particles. The results of this study speak in favour of core–mantle structures characterized by the presence of porosities.     

Model for surface reactions on interstellar grains — A numerical study

A model of the formation of molecules by surface reactions on interstellar grains is described and assessed numerically. The model predicts that for the molecules—other than H₂-likely to be important in the interstellar medium, the formation rates by surface reactions are insensitive to the nature of the surface. The formation rates have magnitudes which are significant when compared with other routes. The model also describes H₂ formation in high density clouds and shows it to be parameter dependent.     

On the acceleration of interstellar grains

Dust grains of radiir _g 3×10^–6 cm, injected into the intercloud medium at speeds in the range 10⁷–10⁸ cm s^–1, may be stochastically accelerated to speeds 0.1c due to scattering by irregularities in the galactic magnetic field.     

The availability of phosphorus in the bacterial model of the interstellar grains

It is shown that phosphorus availability is in close balance with the phosphorus requirement of the bacterial grain model. This correspondence, which would be fortuitous if the interstellar grains were of inorganic origin, points to the correctness of the biological model.     

On the nature of interstellar grains

Data on interstellar extinction are interpreted to imply an identification of interstellar grains with naturally freeze-dried bacteria and algae. The total mass of such bacterial and algal cells in the galaxy is enormous, 10⁴⁰ g. The identification is based on Mie scattering calculations for an experimentally determined size distribution of bacteria. Agreement between our model calculations and astronomical data is remarkably precise over the wavelength intervals µ^–1 < ;^–2 < 1.94µ^–1 and 2.5µ^–1 < ;^–1 < 3.0 ;^–1. Over the more restricted waveband 4000–5000 Å an excess interstellar absorption is found which is in uncannily close agreement with the absorption properties of phytoplankton pigments. The strongest of the diffuse interstellar bands are provisionally assingned to carotenoid-chlorophyll pigment complexes such as exist in algae and pigmented bacteria. The 2200 Å interstellar absorption feature could be due to degraded cellulose strands which form spherical graphitic particles, but could equally well be due to protein-lipid-nucleic acid complexes in bacteria and viruses. Interstellar extinction at wavelengths <1800 Å could be due to scattering by virus particles.     

Organic models of interstellar grains

A number of experimental studies have been carried out to verify the claim that interstellar grains largely consist of organic material, including biological cells. Our spectroscopic studies on biological cells and organic extracts from carbonaceous compounds have failed to identify the well-known 2200 Å interstellar extinction peak with the organic material.     

A note on the composition and structure of interstellar grains

Gürtleret al. (1981) argue that SiO could not be the material responsible for the interstellar 10 micron feature as had been proposed by Duleyet al. (1979). While Millar (1982) has quite successfully refuted the arguments of Gürtleret al. (1981), we present here some additional considerations which indicate that the model of Duleyet al. (1979) requires appreciable modification to be consistent with recent experimental results.     

The temperature of interstellar porous grains

Free-space equilibrium temperatures for porous interstellar grains are computed for different dust sizes and are compared to those expected for homogeneous normal grains for two material candidates: graphite and a lunar silicate. Relevant differences are found with the former, but are essentially negligible with the latter. The results are discussed in terms of the different optical properties produced by the porosity of the grains. The possible astrophysical implications of these findings are also mentioned.     

The maximum temperatures of interstellar grains

The maximum temperature a typical interstellar grain will attain upon absorption of a photon or chemical band formation of molecules on its surface is calculated by considering the exact Debye theory of dielectrics. Other contributions to the specific heats of solids are discussed. It is shown that the use of the approximate Debye theory whereC _v is proportional toT will lead to serious errors in the calculation of velocities of desorption of molecules from grain surfaces.     

Infrared spectra of small interstellar grains

It is shown that surface stresses in small particles will lead to a broadening and weakening of strong resonances in the infrared spectrum of interstellar dust.     

Orientation of interstellar and interplanetary grains

The explicit expressions for the orientation distribution function of interstellar and interplanetary dust grains in the anisotropic corpuscular or radiation fluxes, with consideration for the magnetic field influence, are obtained. An orientation is shown to be possible in a medium having an anisotropic temperature, which is usually the case for a non-equilibrium plasma in a magnetic field. It is noted that the small inhomogeneous dust grains should possess a specific rotation of polarization. The orientation of these dust grains is considered. The time required for the orientation is estimated. A possibility of explaining the interstellar polarization and polarization of the cometary radiation is discussed.

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Volume extinction factors of interstellar grains

Volume extinction factorsV _c have been evaluated for spherical grains at several refractive indices (m=1.6,m=1.5,m=1.33,m=1,16,m=1.1). From these results it is found that grains with higher-refractive index are more efficient for visual extinction than grains with low-refractive index.     

The formation of plum-pudding interstellar grains

A calculation is given of the growth of multicored (plum-pudding) interstellar grains as the result of simultaneous condensation of an icy matrix on small cores combined with coagulation of the resulting grains induced by radiation pressure. It is shown that starting with cores of radius 10^–6 cm in normal gas clouds generally gives rise to plum-pudding grains of radius 10^–5 cm after 10⁸–10⁹ yr.     

Carbon altotropic composition of interstellar grains

Extinktionskurven für Kern-Mantel-Teilchen und für Vielkernteilchen, die allotrope Kohlenstoffmodifikationen enthalten, werden mit Extinktionsbeobachtungen der Sterne R CrB und R Y Sgr im fernen UV sowie mit der interstellaren Extinktion verglichen. Wir zeigen, daß dieses Teilchenmodell mit der beobachteten Extinktion im fernen UV verträglich ist, finden jedoch, daß nur Teilchen, in denen amorpher Kohlenstoff von Graphit ist, die interstellare Extinktionskurve und die der R-CrB-Sterne gut wiedergeben.