Dust in the Galactic Environment

405 Pages·2003·3.564 MB·English

by  D.C.B Whittet

Dust in the Galactic Environment
Second Edition
Series in Astronomy and Astrophysics
SeriesEditors: MBirkinshaw,UniversityofBristol,UK
MElvis,Harvard–SmithsonianCenterforAstrophysics,USA
JSilk,UniversityofOxford,UK
The Series in Astronomy and Astrophysics includes books on all aspects of
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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
(cid:1)c IOPPublishingLtd2003
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AcataloguerecordforthisbookisavailablefromtheBritishLibrary.
ISBN0750306246
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FirstEditionpublished1992
SeriesEditors: MBirkinshaw,UniversityofBristol,UK
MElvis,Harvard–SmithsonianCenterforAstrophysics
JSilk,UniversityofOxford,UK
CommissioningEditor:JohnNavas
ProductionEditor:SimonLaurenson
ProductionControl:SarahPlenty
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Published by Institute of Physics Publishing, wholly owned by The Institute of
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Contents
Prefacetothesecondedition xiii
1 DustintheGalaxy:Ourviewfromwithin 1
1.1 Introduction 1
1.2 Historicalperspective:Discoveryandassimilation 2
1.3 Thedistributionofdustandgas 8
1.3.1 Overview 8
1.3.2 Thegalacticdisc 10
1.3.3 Highgalacticlatitudes 12
1.3.4 Diffusegalacticbackgroundradiation 13
1.4 Interstellarenvironmentsandphysicalprocesses 15
1.4.1 Overview 15
1.4.2 Thephysicalstateoftheinterstellarmedium 15
1.4.3 Interstellarclouds 18
1.4.4 HIIregions 21
1.4.5 TheinterstellarenvironmentoftheSolarSystem 22
1.5 Thesignificanceofdustinmodernastrophysics 24
1.5.1 FromCinderellatothesearchfororigins 24
1.5.2 Interstellarprocessesandchemistry 24
1.5.3 Stars,nebulaeandgalaxies 26
1.5.4 Backtobasics 27
1.6 Abriefhistoryofmodelsforinterstellardust 28
1.6.1 Dirtyices,metalsandPlattparticles 29
1.6.2 Graphiteandsilicates 30
1.6.3 Unmantledrefractoryandcore/mantlemodels 33
1.6.4 Biota 35
Recommendedreading 35
Problems 36
2 Abundancesanddepletions 38
2.1 Theoriginsofthecondensibleelements 39
2.1.1 Thecosmiccycle:anoverview 39
2.1.2 Nucleogenesis 41
viii Contents
2.1.3 Stellarnucleosynthesis 41
2.1.4 Enrichmentoftheinterstellarmedium 44
2.2 TheSolarSystemabundances 45
2.2.1 Significanceandmethodology 45
2.2.2 Results 46
2.3 AbundancetrendsintheGalaxy 50
2.3.1 Temporalvariation 50
2.3.2 Spatialvariation 51
2.3.3 Solarabundancesinspaceandtime 53
2.4 Theobserveddepletions 54
2.4.1 Methods 54
2.4.2 Averagedepletionsindiffuseclouds 56
2.4.3 Dependenceonenvironment 59
2.4.4 Overview 60
2.5 Implicationsforgrainmodels 61
Recommendedreading 64
Problems 64
3 Extinctionandscattering 66
3.1 Theoreticalmethods 67
3.1.1 Extinctionbysphericalparticles 67
3.1.2 Small-particleapproximations 69
3.1.3 Albedo,scatteringfunctionandasymmetryparameter 70
3.1.4 Compositegrains 71
3.2 Observationaltechnique 72
3.3 Theaverageextinctioncurveandalbedo 75
3.3.1 Theaverageextinctioncurve 75
3.3.2 Scatteringcharacteristics 77
3.3.3 Long-wavelengthextinctionandevaluationof R 80
V
3.3.4 Neutralextinction 82
3.3.5 Dustdensityanddust-to-gasratio 83
3.4 Spatialvariations 84
3.4.1 Theblue–ultraviolet 84
3.4.2 Thered–infrared 88
3.4.3 Orderfromchaos? 91
3.5 The2175A˚ absorptionfeature 91
3.5.1 Observedproperties 92
3.5.2 Implicationsfortheidentityofthecarrier 97
3.6 Structureinthevisible 102
3.7 Modellingtheinterstellarextinctioncurve 106
Recommendedreading 109
Problems 109
Contents ix
4 Polarizationandgrainalignment 112
4.1 Extinctionbyanisotropicparticles 113
4.2 Polarimetryandthestructureofthegalacticmagneticfield 115
4.2.1 Basics 115
4.2.2 Macroscopicstructure 117
4.2.3 Polarizationefficiency 120
4.2.4 Small-scalestructure 122
4.2.5 Densecloudsandtheskin-deptheffect 123
4.3 Thespectraldependenceofpolarization 125
4.3.1 TheSerkowskilaw 125
4.3.2 Power-lawbehaviourintheinfrared 127
4.3.3 Polarizationandextinction 128
4.3.4 Regionalvariations 132
4.3.5 Circularpolarization 137
4.4 Polarizationandgrainmodels 138
4.5 Alignmentmechanisms 141
4.5.1 Grainspinandrotationaldissipation 142
4.5.2 Paramagneticrelaxation:theDGmechanism 145
4.5.3 Superparamagneticalignment 147
4.5.4 Suprathermalspin 148
4.5.5 Radiativetorques 149
4.5.6 Mechanicalalignment 150
4.5.7 Alignmentindenseclouds 151
Recommendedreading 152
Problems 152
5 Infraredabsorptionfeatures 154
5.1 Basicsofinfraredspectroscopy 155
5.1.1 Vibrationalmodesinsolids 155
5.1.2 Intrinsicstrengths 159
5.1.3 Observationalapproach 160
5.2 ThediffuseISM 162
5.2.1 Thespectra 162
5.2.2 Silicates 165
5.2.3 Siliconcarbide 169
5.2.4 Hydrocarbonsandorganicresidues 170
5.3 ThedenseISM 174
5.3.1 Aninventoryofices 174
5.3.2 Thethresholdeffect 176
5.3.3 H O-ice:the3µmprofile 178
2
5.3.4 SolidCO:polarandapolarmantles 181
5.3.5 Othercarbon-bearingices 182
5.3.6 Nitrogenandsulphur-bearingices 185
5.3.7 Refractorydust 187
x Contents
5.3.8 Spectropolarimetryandalignmentofcore–mantlegrains 190
Recommendedreading 193
Problems 193
6 Continuumandlineemission 195
6.1 Theoreticalconsiderations 196
6.1.1 Equilibriumdusttemperatures 196
6.1.2 FIRcontinuumemissionfromaninterstellarcloud 198
6.1.3 Effectofgrainshape 200
6.1.4 Effectofgrainsize 202
6.2 Galacticcontinuumemission 204
6.2.1 Morphology 204
6.2.2 Spectralenergydistribution 204
6.2.3 Dustandgas 207
6.2.4 The‘colddustproblem’ 210
6.2.5 Polarizationandgrainalignment 210
6.3 Spectralemissionfeatures 212
6.3.1 Silicates 212
6.3.2 Polycyclicaromatichydrocarbons 214
6.4 Extendedredemission 222
Recommendedreading 224
Problems 225
7 Dustinstellarejecta 226
7.1 Theformationofdustinstellaroutflows 227
7.1.1 Theoreticalconsiderations 227
7.1.2 Thecircumstellarenvironment 229
7.1.3 O-richstars 230
7.1.4 Carbonstars 232
7.1.5 Latestagesofstellarevolution 234
7.2 Observationalconstraintsonstardust 235
7.2.1 Infraredcontinuumemission 235
7.2.2 Infraredspectralfeatures 238
7.2.3 Circumstellarextinction 245
7.2.4 Stardustinmeteorites 247
7.3 Evolvedstarsassourcesofinterstellargrains 252
7.3.1 Mass-loss 252
7.3.2 Grain-sizedistribution 255
7.3.3 Dust-to-gasratio 256
7.3.4 Composition 257
7.3.5 Injectionrate 257
Recommendedreading 261
Problems 261
Contents xi
8 Evolutionintheinterstellarmedium 263
8.1 Grainsurfacereactionsandtheoriginofmolecularhydrogen 264
8.2 Gas-phasechemistry 268
8.3 Mechanismsforgrowth 271
8.3.1 Coagulation 272
8.3.2 Mantlegrowth 273
8.4 Icemantles:depositionandevolution 275
8.4.1 Surfacechemistryandhierarchicalgrowth 275
8.4.2 Depletiontimescalesandlimitstogrowth 277
8.4.3 Thermalandradiativeprocessing 279
8.5 Refractorydust 287
8.5.1 Destruction 287
8.5.2 Sizedistribution 290
8.5.3 Metamorphosis 291
8.5.4 Dustingalacticnuclei 292
Recommendedreading 293
Problems 293
9 Dustintheenvelopesofyoungstars 295
9.1 Theearlyphasesofstellarevolution 296
9.1.1 Overview 296
9.1.2 Infraredemissionfromdustyenvelopes 298
9.1.3 Polarizationandscattering 302
9.1.4 Icesublimationinhotcores 304
9.2 Protoplanetarydiscs 306
9.2.1 TTauridiscs 307
9.2.2 Vegadiscs 308
9.2.3 Thesolarnebula 310
9.3 CluesfromtheearlySolarSystem 312
9.3.1 Comets 313
9.3.2 Interplanetarydust 318
9.3.3 Meteorites 320
9.4 Ingredientsforlife 322
9.4.1 Motivation 322
9.4.2 Thedeuteriumdiagnostic 323
9.4.3 Aminoacidsandchirality 325
9.4.4 DidlifestartwithRNA? 328
9.4.5 DeliverytoEarth 329
Recommendedreading 331
Problems 331