Dust in the Galactic Environment Second Edition Series in Astronomy and Astrophysics Series Editors: M Birkinshaw, University of Bristol, UK M Elvis, Harvard–Smithsonian Center for Astrophysics, USA J Silk, University of Oxford, UK The Series in Astronomy and Astrophysics includes books on all aspects of theoretical and experimental astronomy and astrophysics. Books in the series range in level from textbooks and handbooks to more advanced expositions of current research. Other books in the series An Introduction to the Science of Cosmology D J Raine and E G Thomas The Origin and Evolution of the Solar System M M Woolfson The Physics of the Interstellar Medium J E Dyson and D A Williams Dust and Chemistry in Astronomy T J Millar and D A Williams (eds) Observational Astrophysics R E White (ed) Stellar Astrophysics R J Tayler (ed) Forthcoming titles The Physics of Interstellar Dust EKr¨ugel Very High Energy Gamma Ray Astronomy T Weekes Dark Sky, Dark Matter P Wesson and J Overduin Series in Astronomy and Astrophysics Dust in the Galactic Environment Second Edition D C B Whittet Professor of Physics, Rensselaer Polytechnic Institute, Troy, New York, USA Institute of Physics Publishing Bristol and Philadelphia c IOP Publishing Ltd 2003 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher. Multiple copying is permitted in accordance with the terms of licences issued by the Copyright Licensing Agency under the terms of its agreement with Universities UK (UUK). British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. ISBN 0 7503 0624 6 Library of Congress Cataloging-in-Publication Data are available First Edition published 1992 Series Editors: M Birkinshaw, University of Bristol, UK M Elvis, Harvard–Smithsonian Center for Astrophysics J Silk, University of Oxford, UK Commissioning Editor: John Navas Production Editor: Simon Laurenson Production Control: Sarah Plenty Cover Design: Victoria Le Billon Marketing: Nicola Newey and Verity Cooke Published by Institute of Physics Publishing, wholly owned by The Institute of Physics, London Institute of Physics Publishing, Dirac House, Temple Back, Bristol BS1 6BE, UK US Office: Institute of Physics Publishing, The Public Ledger Building, Suite 929, 150 South Independence Mall West, Philadelphia, PA 19106, USA Typeset in LATEX2ε by Text 2 Text, Torquay, Devon Printed in the UK by J W Arrowsmith Ltd, Bristol Contents Preface to the second edition xiii 1 Dust in the Galaxy: Our view from within 1 1.1 Introduction 1 1.2 Historical perspective: Discovery and assimilation 2 1.3 The distribution of dust and gas 8 1.3.1 Overview8 1.3.2 The galactic disc 10 1.3.3 High galactic latitudes 12 1.3.4 Diffuse galactic background radiation 13 1.4 Interstellar environments and physical processes 15 1.4.1 Overview15 1.4.2 The physical state of the interstellar medium 15 1.4.3 Interstellar clouds 18 1.4.4 H II regions 21 1.4.5 The interstellar environment of the Solar System 22 1.5 The significance of dust in modern astrophysics 24 1.5.1 From Cinderella to the search for origins 24 1.5.2 Interstellar processes and chemistry 24 1.5.3 Stars, nebulae and galaxies 26 1.5.4 Back to basics 27 1.6 A brief history of models for interstellar dust 28 1.6.1 Dirty ices, metals and Platt particles 29 1.6.2 Graphite and silicates 30 1.6.3 Unmantled refractory and core/mantle models 33 1.6.4 Biota 35 Recommended reading 35 Problems 36 2 Abundances and depletions 38 2.1 The origins of the condensible elements 39 2.1.1 The cosmic cycle: an overview39 2.1.2 Nucleogenesis 41 viii Contents 2.1.3 Stellar nucleosynthesis 41 2.1.4 Enrichment of the interstellar medium 44 2.2 The Solar System abundances 45 2.2.1 Significance and methodology 45 2.2.2 Results 46 2.3 Abundance trends in the Galaxy 50 2.3.1 Temporal variation 50 2.3.2 Spatial variation 51 2.3.3 Solar abundances in space and time 53 2.4 The observed depletions 54 2.4.1 Methods 54 2.4.2 Average depletions in diffuse clouds 56 2.4.3 Dependence on environment 59 2.4.4 Overview60 2.5 Implications for grain models 61 Recommended reading 64 Problems 64 3 Extinction and scattering 66 3.1 Theoretical methods 67 3.1.1 Extinction by spherical particles 67 3.1.2 Small-particle approximations 69 3.1.3 Albedo, scattering function and asymmetry parameter 70 3.1.4 Composite grains 71 3.2 Observational technique 72 3.3 The average extinction curve and albedo 75 3.3.1 The average extinction curve 75 3.3.2 Scattering characteristics 77 3.3.3 Lonwavelength extinction and evaluation of RV 80 3.3.4 Neutral extinction 82 3.3.5 Dust density and dust-to-gas ratio 83 3.4 Spatial variations 84 3.4.1 The blue–ultraviolet 84 3.4.2 The red–infrared 88 3.4.3 Order from chaos? 91 3.5 The 2175 A˚ absorption feature 91 3.5.1 Observed properties 92 3.5.2 Implications for the identity of the carrier 97 3.6 Structure in the visible 102 3.7 Modelling the interstellar extinction curve 106 Recommended reading 109 Problems 109 Contents ix 4 Polarization and grain alignment 112 4.1 Extinction by anisotropic particles 113 4.2 Polarimetry and the structure of the galactic magnetic field 115 4.2.1 Basics 115 4.2.2 Macroscopic structure 117 4.2.3 Polarization efficiency 120 4.2.4 Small-scale structure 122 4.2.5 Dense clouds and the skin-depth effect 123 4.3 The spectral dependence of polarization 125 4.3.1 The Serkowski law 125 4.3.2 Power-law behaviour in the infrared 127 4.3.3 Polarization and extinction 128 4.3.4 Regional variations 132 4.3.5 Circular polarization 137 4.4 Polarization and grain models 138 4.5 Alignment mechanisms 141 4.5.1 Grain spin and rotational dissipation 142 4.5.2 Paramagnetic relaxation: the DG mechanism 145 4.5.3 Superparamagnetic alignment 147 4.5.4 Suprathermal spin 148 4.5.5 Radiative toues 149 4.5.6 Mechanical alignment 150 4.5.7 Alignment in dense clouds 151 Recommended reading 152 Problems 152 5 Infrared absorption features 154 5.1 Basics of infrared spectroscopy 155 5.1.1 Vibrational modes in solids 155 5.1.2 Intrinsic strengths 159 5.1.3 Observational approach 160 5.2 The diffuse ISM 162 5.2.1 The spectra 162 5.2.2 Silicates 165 5.2.3 Silicon carbide 169 5.2.4 Hydrocarbons and organic residues 170 5.3 The dense ISM 174 5.3.1 An inventory of ices 174 5.3.2 The threshold effect 176 5.3.3 H2ice: the 3 µm profile 178 5.3.4 Solid CO: polar and apolar mantles 181 5.3.5 Other carbobearing ices 182 5.3.6 Nitrogen and sulphur-bearing ices 185 5.3.7 Refractory dust 187 x Contents 5.3.8 Spectropolarimetry and alignment of core–mantle grains 190 Recommended reading 193 Problems 193 6 Continuum and line emission 195 6.1 Theoretical considerations 196 6.1.1 Equilibrium dust temperatures 196 6.1.2 FIR continuum emission from an interstellar cloud 198 6.1.3 Effect of grain shape 200 6.1.4 Effect of grain size 202 6.2 Galactic continuum emission 204 6.2.1 Morphology 204 6.2.2 Spectral energy distribution 204 6.2.3 Dust and gas 207 6.2.4 The ‘cold dust problem’ 210 6.2.5 Polarization and grain alignment 210 6.3 Spectral emission features 212 6.3.1 Silicates 212 6.3.2 Polycyclic aromatic hydrocarbons 214 6.4 Extended red emission 222 Recommended reading 224 Problems 225 7 Dust in stellar ejecta 226 7.1 The formation of dust in stellar outflows 227 7.1.1 Theoretical considerations 227 7.1.2 The circumstellar environment 229 7.1.3 O-rich stars 230 7.1.4 Carbon stars 232 7.1.5 Late stages of stellar evolution 234 7.2 Observational constraints on stardust 235 7.2.1 Infrared continuum emission 235 7.2.2 Infrared spectral features 238 7.2.3 Circumstellar extinction 245 7.2.4 Stardust in meteorites 247 7.3 Evolved stars as sources of interstellar grains 252 7.3.1 Mass-loss 252 7.3.2 Grain-size distribution 255 7.3.3 Dust-to-gas ratio 256 7.3.4 Composition 257 7.3.5 Injection rate 257 Recommended reading 261 Problems 261 Contents xi 8Evolution in the interstellar medium 263 8.1 Grain surface reactions and the origin of molecular hydrogen 264 8.2 Gas-phase chemistry 268 8.3 Mechanisms for growth 271 8.3.1 Coagulation 272 8.3.2 Mantle growth 273 8.4 Ice mantles: deposition and evolution 275 8.4.1 Surface chemistry and hierarchical growth 275 8.4.2 Depletion timescales and limits to growth 277 8.4.3 Thermal and radiative processing 279 8.5 Refractory dust 287 8.5.1 Destruction 287 8.5.2 Size distribution 290 8.5.3 Metamorphosis 291 8.5.4 Dust in galactic nuclei 292 Recommended reading 293 Problems 293 9 Dust in the envelopes of young stars 295 9.1 The early phases of stellar evolution 296 9.1.1 Overview296 9.1.2 Infrared emission from dusty envelopes 298 9.1.3 Polarization and scattering 302 9.1.4 Ice sublimation in hot cores 304 9.2 Protoplanetary discs 306 9.2.1 T Tauri discs 307 9.2.2 Vega discs 308 9.2.3 The solar nebula 310 9.3 Clues from the early Solar System 312 9.3.1 Comets 313 9.3.2 Interplanetary dust 318 9.3.3 Meteorites 320 9.4 Ingredients for life 322 9.4.1 Motivation 322 9.4.2 The deuterium diagnostic 323 9.4.3 Amino acids and chirality 325 9.4.4 Did life start with RNA? 328 9.4.5 Delivery to Earth 329 Recommended reading 331 Problems 331 xii Contents 10 Toward a unified model for interstellar dust 332 10.1 Areas of consensus 333 10.1.1 A generic grain model 333 10.1.2 Silicates 335 10.1.3 Carbon 335 10.1.4 Ices 336 10.1.5 Alignment 336 10.2 Open questions 337 A Glossary 340 A.1 Units and constants 340 A.2 Physical, chemical and astrophysical terms 341 A.3 Acronyms 346 References 348 Index 378 Preface to the second edition Dust is a ubiquitous feature of the cosmos, impinging directly or indirectly on most fields of modern astronomy.