The Theory of Cosmic Grains

Light scattering and absorption by small homogeneous particles can be worked-out exactly for spheres and infinite cylinders. Homogeneous particles of irregular shapes, when averaged with respect to rotation, have effects that can in general be well-approximated by reference to results for these two idealised cases. Likewise, small inhomogeneous particles have effects similar to homogeneous particles of the same average refractive index. Thus most problems can be solved to a satisfactory approximation by reference to the exact solutions for spheres and cylinders, which are fully stated here in the early part of the book. The sum of scattering and absorption, the extinction, is too large to be explained by inorganic materials, provided element abundances in the interstellar medium are not appreciably greater than solar, H 0 and NH3 being essentially excluded in the 2 general medium, otherwise very strong absorptions near 3p,m would be observed which they are not. A well-marked extinction maximum in the ultraviolet near 2200A has also not been explained satisfactorily by inorganic materials. Accurately formed graphite spheres with radii close to O.02p,m could conceivably provide an explanation of this ultraviolet feature but no convincing laboratory preparation of such spheres has ever been achieved.

Klappentext

The problem of the composition of cosmic dust grains has stubbornly defied solution for over half a century. A succession of models have been proposed and their properties worked out for comparison with an ever-expanding body of relevant observational data. The authors began their pioneering work in this field in the 1960s by challenging the then popular ice grain theory. Most controversially they later hypothesised that condensed organic matter in the galaxy is of biological origin, thus linking an old astronomical problem with the question of the origin of terrestrial life. In this book the authors develop the theory of Cosmic Grains on a broad front starting logically from basic mathematical and astronomical premises. The reader is guided through a historical progression of ideas on the nature of grains, leading ultimately to the authors' own point of view, which shows through a clear predictive sequence the important role of complex organic material in the interstellar grains.

Inhalt

1. Introduction.- 1.1. Early Ideas.- 1.2. Trumpler's Method of Estimating Interstellar Extinction.- 1.3. The First Colour Measurements.- 1.4. The Oort Limit.- 1.5. Data Relating to Interstellar Clouds.- 1.6. Correlation Between Gas and Dust Clouds.- 1.7. Composition of Grains.- 2. Electromagnetic Properties of Small Particles.- 2.1. Homogeneous Spherical Particles.- 2.2. Composite Spheres.- 2.3. Infinite Cylinders.- 2.4. Rayleigh Scattering by Ellipsiods.- 2.5. Heterogeneous or Porous Grains.- 2.6. Absorption Crosssections, Bulk Absorption Coefficient and Emissivity.- 2.7. Two Special Cases.- 3. Interstellar Extinction and Polarisation.- 3.1. Equation of Transfer.- 3.2. Observations of Interstellar Extinction, Definition of Colour Indices and Colour Excesses.- 3.3. Observations of Interstellar Polarisation.- 3.4. Diffuse Interstellar Bands.- 4. Reflection Nebulae and the Diffuse Galactic Light.- 4.1. Introductory Remarks.- 4.2. Apparent Size of Reflection Nebulae.- 4.3. Observations of NGC 7023.- 4.4. Observations of Reflection Nebula Around Merope.- 4.5. Multiple Scattering Models of Reflection Nebulae.- 4.6. Diffuse Galactic Light.- 5 Interactions between Dust, Gas and Radiation.- 5.1.Introductory Remarks.- 5.2. Grain Temperatures for Sandard Grains.- 5.3.Temperature Spikes in Very Small Grains.- 5.4.Electrostatic Gharge on Grains.- 5.5.Rotation of Grains.- 5.6.Radio Waves from Grains.- 5.7.Molecule Formation.- 5.8. Growth and Destruction of Grains.- 5.9. Effects of Radiation Pressure.- 5.10. Gyration About the Magnetic Field.- 5.11. Alignment of Grains.- 5.12. Depletion of Elements from the Gas Phase.- 6. Inorganic Theories of Grain Formation.- 6.1. Interstellar Condensation.- 6.2. Condensation of Graphite Grains.- 6.3. Condensation of Grains in CoolOxygenrich Giant Stars.- 6.4. Coremantle Grains.- 7. The Organic Grain Model.- 7.1. Introductory Remarks.- 7.2. Polymerisation of Formaldehyde.- 7.3. From Formaldehyde to Polysacharides.- 7.4. Polysacharide Formation in Stellar Mass Flows.- 7.5. HAC, PAH and QCC Models.- 7.6. Fischer Tropsch Reactions in the Gas Phase.- 7.7. The Biological Grain Model.- 8. Models of the Extinction and Polarisation of Starlight.- 8.1. Introduction.- 8.2. The Visual Extinction Curve.- 8.3. The Ultraviolet Extinction Curve: Extinction Curves for Graphite Grains.- 8.4. Polarisation Constraints.- 8.5. An Organic/Biologic Grain Model.- 8.6. Analysis of a Biological Grain Model.- 8.7. Refinements to Biological Extinction Model.- 9. Spectroscopic Identifications.- 9.1. Introduction.- 9.2. The 8 13µm Features in Astronomy.- 9.3. The 8 40µm Flux from the Trapezium Nebula.- 9.4. The 3.4µm Band: Proof that Grains are Mainly Organic.- 9.5. Modelling the 2.9 4µm IR Data for GC-IRS7.- 9.6. How Much Water Ice?.- 9.7. Sources with Spectra in the 2 14µm Waveband.- 9.8. Evidence for PAH.- 9.9. Aromatic Molecules and the Diffuse Optical Bands.- 10. Dust in External Galaxies.- 10.1. Introduction.- 10.2. The Magellanic Clouds: LMC and SMC.- 10.3. M82 and Other Galaxies.- 10.4. Particles of High Infrared Emissivity.- 10.5. The Ejection of Iron Whiskers from Galaxies.- 10.6. The Microwave Background.