Dust in the Galactic Environment

402 Pages·2002·3.617 MB·English

by  Whittet, D C B

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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FirstEditionpublished1992
SeriesEditors: MBirkinshaw,UniversityofBristol,UK
MElvis,Harvard–SmithsonianCenterforAstrophysics
JSilk,UniversityofOxford,UK
CommissioningEditor:JohnNavas
ProductionEditor:SimonLaurenson
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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
xii Contents
10 Towardaunifiedmodelforinterstellardust 332
10.1 Areasofconsensus 333
10.1.1 Agenericgrainmodel 333
10.1.2 Silicates 335
10.1.3 Carbon 335
10.1.4 Ices 336
10.1.5 Alignment 336
10.2 Openquestions 337
A Glossary 340
A.1 Unitsandconstants 340
A.2 Physical,chemicalandastrophysicalterms 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. Dust grains composed of small (submicron-
sized) solid particles pervade interstellar space in the Milky Way and other
galaxies:theyoccurinawidevarietyofastrophysicalenvironments,rangingfrom
comets to giant molecular clouds, from circumstellar shells to galactic nuclei.
The study of this phenomenon is a highly active and topical area of current
research. Thisbookaimstoprovideanoverviewofthesubject,coveringgeneral
concepts, methods of investigation, important results and their significance,
relevantliteratureandsomesuggestionsforpromisingavenuesoffutureresearch.
It is aimed at a level suitable for those embarking upon postgraduate research
but will also be of more general interest to researchers, teachers and students
as a review of a significant area of astrophysics. As a formal text for taught
courses, it will be particularly useful to advanced undergraduateand beginning
postgraduatestudentsstudyingtheinterstellarmedium. Myaimthroughoutisto
create a compact, coherent text that will stimulate the reader to investigate the
subjectfurther.
Ourconceptofinterstellarspacehaschangedovertheyears,fromapassive,
static‘medium’toanactive‘environment’.Forthisreason,theunderlyingtheme
ofthebookisthesignificanceofdustininterstellarastrophysics,withparticular
reference to the interaction of the solid particles with their environment. The
discussionis focusedon interstellardustinthe solar neighbourhoodofourown
Galaxy,theMilkyWay:ourGalaxyisboththeenvironmentofplanetarysystems
andthemostaccessibleexampleofthebuildingblocksoftheUniverse.Ifwecan
better understandthe nature and evolution of dust in our local Galaxy, this will
greatlyaidusinourquesttocomprehendbothitsroleintheoriginsofstarsand
planetarysystemssuchasourownanditsinfluenceontheobservedpropertiesof
distantgalaxies.
Manyimportantnewdiscoverieshavebeenmadeinthefieldofcosmicdust
sincethefirsteditionofDustintheGalacticEnvironmentwascompletedinmid-
1991. The Astrophysical Journal alone typically publishes a hundred or more
research papersper year on interstellar dust andrelated topics. Major advances
havebeenmadeinfieldsasdiverseasmeteoritics,infraredastronomyandfractal
graintheory.Aneweditionthusseemstimelyfortwoprimaryreasons:
xiii
xiv Prefacetothesecondedition
(1) Tobringthetextuptodate. Thisisespeciallyurgentinthelightofexciting
new results from space missions such as the Hubble Space Telescope, the
CosmicBackgroundExplorerandtheInfraredSpaceObservatory,together
with the latest developments in ground-based observational astronomy,
laboratoryastrophysicsandtheoreticalmodelling.
(2) To expand the scope of the text to provide a context for future research
opportunities. In thefirstdecadeofthenewmillennium,we cananticipate
discoverieslinkedtomissionssuchasSIRTF(theSpaceInfraredTelescope
Facility)andSTARDUST(acometary/interstellardustcollectionandreturn
mission). Keygoalsforthesemissionsincludethestudyofdustboth‘near’
and ‘far’, in our own Solar System, in protoplanetary discs around other
nearbystars,andindistantgalaxies.
The new edition places greater emphasis on these topics and has increased in
overalllengthbymorethan30%. Thetextisdividedintotenchapters. Thefirst
providesahistoricalperspectiveforcurrentresearch,togetherwithanoverviewof
interstellarenvironmentsandtheroleofdustinastrophysicalprocesses.Chapter2
discusses the cosmichistory ofthe chemicalelementsexpectedto be presentin
dust and examinesthe effectof gas–dustinteractions on gas phase abundances.
Chapters 3–6 describe the observed properties of interstellar grains, i.e. their
extinction,polarization,absorptionandemissioncharacteristics,respectively. In
chapters7–9,wediscusstheoriginandevolutionofthedust,tracingitslifecycle
in a sequence of environments from the circumstellar envelopes of old stars to
diffuseinterstellarclouds,molecularclouds,protostarsandprotoplanetarydiscs.
Thefinalchaptersummarizesprogresstowarda unifiedmodelforgalacticdust.
Dustinothergalaxiesisdiscussedasanintegralpartofthetextratherthanasa
distincttopicrequiringseparatechapters.
It is assumed throughoutthat the reader is familiar with basic concepts in
stellarandgalacticastronomy,suchasstellarmagnitudeanddistancescalesand
thespectralclassificationsequenceandhasaqualitativefamiliaritywithgalactic
structure and stellar evolution according to current models. The reader with
little or no backgroundin astronomy will find many suitable introductorytexts
available: Foundations of Astronomy by Michael A Seeds (Wadsworth 1997)
wouldbeanexcellentchoice.
Syste`me Internationale (SI) units are used in addition to the units of
astronomybuttheunsuspectingreadershouldbeawarethatthecgssystemisstill
widespread in the astronomical literature. There are a few isolated exceptions
to SI in the present text. For example, it is convenient to use microgauss
(1µG=10−10T) to specify interstellar magneticflux densitiesand A˚ngstroms
(1A˚ =10−10m)todenotethewavelengthsofspectralfeaturesinthevisibleand
ultravioletregionsofthespectrum.
The authorhasfoundastrophysicaldustto be a challengingandrewarding
topic of study. An important reason for this is the wide variety of techniques
involved,embracingobservationalastronomyovermuchof the electromagnetic