The Theory of Cosmic Grains

315 Pages·1991·11.366 MB·English

by  F. Hoyle, N. C. Wickramasinghe (auth.)

THE THEORY OF COSMIC GRAINS
ASTROPHYSICS AND
SPACE SCIENCE LIBRARY
A SERIES OF BOOKS ON THE RECENT DEVELOPMENTS
OF SPACE SCIENCE AND OF GENERAL GEOPHYSICS AND ASTROPHYSICS
PUBLISHED IN CONNECTION WITH THE JOURNAL
SPACE SCIENCE REVIEWS
Editorial Board
R. L. F. BOYD, University College, London, England
W. B. BURTON, Sterrewacht, Leiden, The Netherlands
C. DE JAGER, University of Utrecht, The Netherlands
J. KLECZEK, Czechoslovak Academy of Sciences, Ondfejov, Czechoslavakia
Z. KOPAL, University of Manchester, England
R. LUST, Max-Planck-Institutfur Meteorologie, Hamburg, Germany
L. I. SEDOV, Academy of Sciences of the U.S.S.R., Moscow, U.S.S.R.
Z. SvESTKA, Laboratory for Space Research, Utrecht, The Netherlands
VOLUME 168
CURRENT RESEARCH
THE THEORY
OF COSMIC GRAINS
by
F. HOYLE
Bournemouth, United Kingdom
and
N. C. WICKRAMASINGHE
School ofM athematics, University ofWales,
Cardiff, United Kingdom
SPRINGER -SCIENCE+BUSINESS MEDIA, B. V.
Library of Congress Cataloging-in-Publication Data
Hoyle. Fred. Sir.
The theory of cosmlC grains I authored by F. Hoyle and N.C.
Wickramasinghe.
p. cm. -- (Astrophysics and space science 1 ibrary ; v. 168)
ISBN 978-94-010-5505-5 ISBN 978-94-011-3402-6 (eBook)
DOI 10.1007/978-94-011-3402-6
1. Cosmic grains. 2. AstrophysiCs. 3. Cosmochemistry.
1. Wickramasinghe. N.C. (Nalin Chandra). 1939- II. Title.
III. Ser ies.
QB791.2.H69 1991
523.1' 125--dc20 91-11105
ISBN 978-94-010-5505-5
Printed on acid-free paper
AII Rights Reserved
© 1991 Springer Science+Business Media Dordrecht
Originally published by Kluwer Academic Publishers in 1991
Softcover reprint of the hardcover 1s t edition 1991
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical,
including photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright owner.
CONTENTS
Acknowledgement vii
Preface IX
1. Introduction
1.1. Early Ideas 1
1.2. Trumpler's Method of Estimating Interstellar
Extinction 2
1.3. The First Colour Measurements 3
1.4. The Oort Limit 4
1.5. Data Relating to Interstellar Clouds 5
1.6. Correlation Between Gas and Dust Clouds 6
1. 7. Composition of Grains 9
2. Electromagnetic Properties of Small Particles
2.1. Homogeneous Spherical Particles 15
2.2. Composite Spheres 24
2.3. Infinite Cylinders 27
2.4. Rayleigh Scattering by Ellipsiods 34
2.5. Heterogeneous or Porous Grains 36
2.6. Absorption Cross-sections, Bulk Absorption
Coefficient and Emissivity 39
2.7. Two Special Cases 41
3. Interstellar Extinction and Polarisation
3.1. Equation of Transfer 47
3.2. Observations of Interstellar Extinction, Definition
of Colour Indices and Colour Excesses 50
3.3. Observations of Interstellar Polarisation 59
3.4. Diffuse Interstellar Bands 66
4. Reflection Nebulae and the Diffuse Galactic Light
4.1. Introductory Remarks 72
4.2. Apparent Size of Reflection Nebulae 72
4.3. Observations of NGC 7023 76
4.4. Observations of Reflection Nebula Around Merope 81
4.5. Multiple Scattering Models of Reflection Nebulae 87
4.6. Diffuse Galactic Light 90
5. Interactions between Dust, Gas and Radiation
5.1. Introductory Remarks 94
5.2. Grain Temperatures for Standard Grains 96
5.3. Temperature Spikes in Very Small Grains 100
5.4. Electrostatic Gharge on Grains 102
5.5. Rotation of Grains 104
5.6. Radio Waves from Grains 105
5.7. Molecule Formation 106
vi CONTENTS
5.8. Growth and Destruction of Grains 109
5.9. Effects of Radiation Pressure 112
5.10. Gyration About the Magnetic Field 119
5.11. Alignment of Grains 119
5.12. Depletion of Elements from the Gas Phase 124
6. Inorganic Theories of Grain Formation
6.1. Interstellar Condensation 128
6.2. Condensation of Graphite Grains 133
6.3. Condensation of Grains in Cool Oxygen-rich Giant
Stars 143
6.4. Core-mantle Grains 148
7. The Organic Grain Model
7.1. Introductory Remarks 152
7.2. Polymerisation of Formaldehyde 153
7.3. From Formaldehyde to Polysacharides 157
7.4. Polysacharide Formation in Stellar Mass Flows 158
7.5. HAC, PAH and QCC Models 163
7.6. Fischer Tropsch Reactions in the Gas Phase 166
7.7. The Biological Grain Model 169
8. Models of the Extinction and Polarisation of Starlight
8 .1. Introduction 178
8.2. The Visual Extinction Curve 180
8.3. The Ultraviolet Extinction Curve: Extinction Curves
for Graphite Grains 183
8.4. Polarisation Constraints 191
8.5. An Organic/Biologic Grain Model 191
8.6. Analysis of a Biological Grain Model 197
8.7. Refinements to Biological Extinction Model 210
9. Spectroscopic Identifications
9 .1. Introduction 216
9.2. The 8 - 13flm Features in Astronomy 219
9.3. The 8 - 40flm Flux from the Trapezium Nebula 222
9.4. The 3.4flm Band: Proof that Grains are Mainly Organic 230
9.5. Modelling the 2.9 - 4flm IR Data for GC-IRS7 233
9.6. How Much Water - Ice? 246
9.7. Sources with Spectra in the 2 - 14flm Waveband 250
9.8. Evidence for P AH 258
9.9. Aromatic Molecules and the Diffuse Optical Bands 268
10. Dust in External Galaxies
10.1. Introduction 276
10.2. The Magellanic Clouds: LMC and SMC 276
10.3. M82 and Other Galaxies 281
10.4. Particles of High Infrared Emissivity 285
10.5. The Ejection of Iron Whiskers from Galaxies 288
10.6. The Microwave Background 289
ACKNOWLEDGEMENT
We are indebted to Mr. G.H. Weston for his unstinting support of this work over
many years.
PREFACE
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.
Certain metal oxides under special conditions show absorptions at 2200A but only
weakly. On the other hand, suites of organic materials such as are actually found in
nature have integrated extinction spectra remarkably similar in the ultraviolet to
interstellar grains. Indeed on a wide range of counts, organic grains fit data in both
visual and ultraviolet wavelengths ranges far better than the inorganic alternatives,
data that is both extensive and complex, and which agreement is again confirmed
extensively in the infrared.
Organic materials are far more efficiently synthesized biologically than
abiologically. The antecedents of modern astronomers existed precariously as minor
court officials who were strongly motivated to keep their atronomical studies and
discoveries rigorously separated from the world of everyday events. This long
history prejudices astronomers today against ideas that might suggest their work
could be of practical relevance, or even of great importance in the circumstances
that it has to do with the origin of life. Our aim is to take the reader through all the
details which in the end support such a view, in the hope that at least some may be
able to shake off an intellectually inhibiting attitude from the past.
Life on the Earth is almost as old as the oldest known rocks which are thought to
have been too strongly heated for fossil evidence of life to have been retained at the
beginning. On this evidence alone, therefore, life could as well be older than the
Earth as younger. To the unbiased, the one should be just as much a fit subject of
study as the other, taking the exploring mind into the astronomical field considered
in this book.
Cardiff, June 1991 F. Hoyle
N.C. Wickramasinghe
1. Introduction
1.1. EARLY IDEAS
Amongst the most startling pictures of the night sky are those that involve
interstellar dust clouds. They show up as dark patches and striations against more
or less uniform starfields, or as bright nebulosity around individual stars or groups
of stars. The Trifid Nebula seen in Fig. 1.1 is an example of a region of the Milky
Way where dark lanes are superposed on an irregular-shaped complex of clouds of
hot emitting gas and dust. These patches are not recesses through the nebula as
they were once thought, but rather are caused by the very effective obscuration of
background optical radiation by much cooler clouds containing small solid particles.
The term 'interstellar' grains was given to these particles by Lyman Spitzer Jr., one
of the great pioneers of interstellar astronomy.
Investigations relating to the nature of interstellar grains have been going on
since the early years of the present century. These studies have gathered momentum
over the years with approximately 450 research papers appearing annually at the
present time that are in some way connected with the properties of grains.
Nevertheless, the problem of interstellar grain composition has not yet been finally
resolved. It remains one of the most fascinating unsolved problems of modern
astronomy.
As a matter of historical interest we note that visual recordings of "dark
nebulae" preceded the advent of photography. In 1784 William Herschel catalogued
visual sitin&s of thousands of nebulae - dark nebulae (which we now know to be
dust clouds) and bright nebulae, many of which later turned out to be external
galaxies. Herschel noted that the latter tended to avoid the plane of the Milky Way.
This observation was interpreted by Herschel and most of his successors until about
1910 to imply that the nebulae were a truly galactic population, thus defining a
plane of avoidance related to the Galaxy itself. The misconception implied here led
to a failure to recognise both the existence of external galaxies and also the presence
of interstellar dust in the Galaxy.
Since we are now so accustomed to take the presence of interstellar dust for
granted, it is worth reflecting briefly on the early difficulties that were encountered
in recognising that any interstellar obscuration could be present at all. Even as late
as 1927, after Barnard (1919, 1927) had published a comprehensive atlas of dark
clouds, their true nature remained an enigma. It was very much an open question as
to whether the markings (seen for instance in Fig. 1.1) were clouds of opaque matter
in front of the stars or, as was often suggested, they actually represented holes
through the distribution of stars.
2 CHAPTER I
Fig. 1.1 Photograph o/the Trifid Nebula showing conspicuous dust lanes
One of the earliest attempts to resolve this problem was made by F.G.W. Struve
as early as 1847. By counting the surface density of stars up to varying limiting
values of the apparent magnitude in different regions of the sky, he argued that
there may be an extinction of order 1 mag/kpc at the visual wavelength. This
conclusion however was considered by later workers to be insecure on account of
uncertainties in the stellar density distribution.
1.2. TRUMPLER'S METHOD OF ESTIMATING INTERSTELLAR
EXTINCTION
One of the most convincing demonstrations of the existence of a general interstellar
extinction was made by Trumpler (1930). In addition to showing that extinction
must be present, he was also able to estimate its average amount.
The crucial step in proving the existence of interstellar extinction was to devise
a distance scale that was independent of photometric measurements. Trumpler's
method was based on measurements of the angular diameters of open galactic
clusters. These are groups, each containing of the order of 102 to 103 individual
stars, distributed more or less uniformly in regions close to the galactic plane. The
Pleiades and Praesepe are typical examples of such open galactic clusters.
If m and M are the apparent and absolute photographic magnitudes of a
particular star in a cluster, the apparent distance r' in parsecs (in the absence of
any extinction) is given by