Ultraviolet scattering properties and the size distribution of interstellar grains

Recent work related to the determination of the size distribution of interstellar grains is reviewed briefly. It is shown that the scattering properties of interstellar grains determined from ultraviolet observations of reflection nebulae point toward the existence of a bimodal size distribution of grains, consisting of classical wavelength sized grains and a class of much more numerous smaller particles, which scatter far ultraviolet light isotropically with near unit albedo. The implications of this result are discussed briefly.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

Desorption of molecules from interstellar grains and the heating of dust clouds

Temperature fluctuations induced in very small (r10^–3 ) interstellar grains by the absorption of photons from the ultra-violet radiation field or by energy released on molecule formation are shown to lead to significant gas heating due to thermal desorption of condensed atoms or molecules. For clouds with N(H)=1–10×10²⁰ cm^–2, heating rates due to this process are comparable to direct heating by cosmic rays or grain photoelectrons.     

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.     

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.

---

. , . , . . .

     

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.     

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.     

Electric charge of grains and interstellar radioactivity

We investigate the electric potential of high speed interstellar grains trapping radioactive nuclei which are suggested to exist in interstellar medium by the study of meteorite samples and the gamma-ray line observation. These radioactive nuclei, e.g.²²Na,²⁶Al,⁴⁴Ti and⁶⁰Fe bring about a drastic change in grain potential. Grains with larger radius than the critical radiusa _c attain a negative potential when grains are traversing a typical intercloud region.     

Large interstellar molecules

We propose that photocycloaddition reactions in molecular complexes in normal interstellar clouds will create unusually large molecules. These may be sufficiently radiation stable to be circulated with the interstellar gas, and so provide convenient nucleation centres for growth of loosely bound grains in dark regions.     

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.     

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.     

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.     

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.     

The temperature of interstellar iron grains

In order for catalytic reactions to occur in interstellar dust clouds, it is necessary for the temperature of the grains to be about an order of magnitude hotter than usually calculated for grains of dielectric materials. However, transition metal (e.g., iron) grains should be fairly abundant, and because they absorb strongly in the visible and ultraviolet regions of the interstellar radiation field, they have equilibrium temperatures 133 K. This was determined by a new method which utilizes Semi-Empirical free electron theory parameters in the infrared, coupled with a reiteration scheme which takes into consideration the change of conductivity of the iron with temperature.     

Equilibrium temperatures of interstellar iron grains

Because of the relatively low number densities found in typical interstellar clouds, molecules observed there must be produced by a combination of both two-body gas-phase reactions and surface reactions. The latter type includes various catalytic reactions, such as the formation of H₂ on transition metal grains. These reactions are very temperature dependent, the grain temperature appearing in the exponential of the rate equations. Because of the small heat capacities of the grains due to their small sizes, they may be subject to considerable fluctuations in temperature. This problem is examined for iron grains and found to be minimal for sizes greater than 100 Å. Steady-state equilibrium temperatures are then calculated for a size distribution of iron particles ranging from 10³ to 10⁹ atoms per grain by a refined method of an earlier work by one of us (RGT). The results are that iron grain temperatures are significantly greater than those of dielectric grains of comparable size in the same radiation field.     

The temperature of interstellar oxide grains

Grain temperatures have been derived for small oxide grains in a typical interstellar radiation field. Detailed Mie calculations have been used to determineQ _abs from 1000 Å to 30 m. Longward of 30 m, there is no optical data available but the fact that such grains are likely highly defected implies that a ^–1 dependence in the far-infrared may be appropriate.     

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.     

Growth of interstellar grains by coagulation

A discussion is given of the spontaneous coagulation of interstellar grains due to their natural Brownian motion. It is shown that the predicted present day size of such grains, assuming such a mechanism being operative, is in reasonable agreement with observations for three different environments that have been considered: namely, normal interstellar gas clouds, molecular clouds and the atmospheres of cool stars.