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MOLECULARCOLLISIONSINTHEINTERSTELLARMEDIUM
SecondEdition
Intheinterstellarmedium–whichoccupiesthespacebetweenthestarsingalaxies–new
starsarebornfrommaterialthatisreplenishedbythedebrisejectedbystarswhentheydie.
This book presents a detailed account of the atomic and molecular processes which give
risetotheradiationweobservefromtheinterstellarmedium,knowledgethatisessentialto
understandingstarformationinourownandothergalaxies.
This Second Edition has been thoroughly updated and extended to cover related topics in
radiation theory. It considers the chemistry of the interstellar medium, both in the present
epochandtheearlyUniverse.Thebookdiscussesthephysicsandchemistryofshockwaves,
whichareproducedbythejetsofmattergeneratedasaconsequenceofstarformation.The
methodsforcalculatingratesofcollisionalexcitationofinterstellarmoleculesandatomsare
explained,withemphasisonthequantummechanicalmethod.
Acomprehensivemanualforstudyingcollisionalandradiativeprocessesintheinterstellar
medium, this book will be ideal for researchers involved in calculating the rates of such
processes,forthosestudyingtheinterstellarmediumandstarformation,aswellasforphysical
chemists specializing in collision theory or in the measurement of the rates of collision
processes.
davidflowerisaprofessorofphysicsattheUniversityofDurham,UK.Heisafellowof
theRoyalAstronomicalSocietyandaneditoroftheSociety’sastronomyresearchjournal,
MonthlyNotices.Hisresearchinterestsincludeatomicandmolecularphysicsinastrophysical
environments,andthephysicsoftheinterstellarmedium.
CambridgeAstrophysicsSeries
Serieseditors
AndrewKing,DouglasLin,StephenMaran,JimPringleandMartinWard
Titlesavailableintheseries
7. SpectroscopyofAstrophysicalPlasmas
editedbyA.DalgarnoandD.Layzer
10. QuasarAstronomy
byDanielW.Weedman
18. PlasmaLoopsintheSolarCorona
byR.J.Bray,L.E.Cram,C.Durrant,R.E.Loughhead
19. BeamsandJetsinAstrophysics
editedbyP.A.Hughes
22. Gamma-rayAstronomy2ndEdition
byP.V.RamanaMurthy,A.W.Wolfendale
24. SolarandStellarActivityCycles
byPeterR.Wilson
26. X-rayBinaries
byWalterH.G.Lewin,JanvanParadijs,EdwardP.J.vandenHeuvel
27. RRLyraeStars
byHoraceA.Smith
28. CataclysmicVariableStars
byBrianWarner
29. TheMagellanicClouds
byBengtE.Westerlund
32. AccretionProcessesinStarFormation
byLeeHartmann
33. TheOriginandEvolutionofPlanetaryNebulae
bySunKwok
34. SolarandStellarMagneticActivity
byCarolusJ.Schrijver,CornelisZwaan
35. TheGalaxiesoftheLocalGroup
bySidneyvandenBergh
36. StellarRotation
byJean-LouisTassoul
37. ExtremeUltravioletAstronomy
byMartinA.Barstow,JayB.Holberg
38. PulsarAstronomy3rdEdition
byAndrewG.Lyne,FrancisGraham-Smith
39. CompactStellarX-raySources
editedbyWalterH.G.Lewin,MichielvanderKlis
40. EvolutionaryProcessesinBinaryandMultipleStars
byPeterEggleton
41. ThePhysicsoftheCosmicMicrowaveBackground
byPavelD.Naselsky,DmitryI.Novikov,IgorD.Novikov
42. MolecularCollisionsintheInterstellarMedium2ndEdition
byDavidFlower
MOLECULAR COLLISIONS IN THE
INTERSTELLAR MEDIUM
Second Edition
DAVID FLOWER
UniversityofDurham
CAMBRIDGE UNIVERSITY PRESS
Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo
Cambridge University Press
The Edinburgh Building, Cambridge CB2 8RU, UK
Published in the United States of America by Cambridge University Press, New York
www.cambridge.org
Information on this title: www.cambridge.org/9780521844833
© D. Flower 2007
This publication is in copyright. Subject to statutory exception and to the provision of
relevant collective licensing agreements, no reproduction of any part may take place
without the written permission of Cambridge University Press.
First published in print format 2007
ISBN-13 978-0-511-27516-6 eBook (NetLibrary)
ISBN-10 0-511-27516-1 eBook (NetLibrary)
ISBN-13 978-0-521-84483-3 hardback
ISBN-10 0-521-84483-5 hardback
Cambridge University Press has no responsibility for the persistence or accuracy of urls
for external or third-party internet websites referred to in this publication, and does not
guaranteethatanycontentonsuchwebsitesis,orwillremain,accurateorappropriate.
ToMichaelSeaton,mentorandfriend
Contents
1Interstellarmolecules 1
1.1 Introduction 1
1.2 Chemistryininterstellarclouds 3
1.3 Chemicalbistabilityindenseclouds 9
2Interstellarshocksandchemistry 12
2.1 Introduction 12
2.2 TheMHDconservationequations 13
2.3 Thestructureofinterstellarshockwaves 21
2.4 Shockwavesindarkclouds 29
2.5 Shockwavesindiffuseclouds 33
3 Theprimordialgas 36
3.1 Introduction 36
3.2 Thegoverningequations 36
3.3Theroleofmolecules 39
3.4 Chemistry 42
3.5 Gravitationalcollapse 44
4 Therotationalexcitationofmolecules 49
4.1 Introduction 49
4.2 TheBorn–Oppenheimerapproximation 49
4.3 Thescatteringofanatombyarigidrotator 52
4.4 Therotationalexcitationofnon-linearmolecules 69
5 Thevibrationalexcitationoflinearmolecules 82
5.1 Introduction 82
5.2 Thescatteringofanatombyavibratingrotor 82
5.3 ExcitationofH andHDincollisionswithH molecules 92
2 2
5.4 Coolingfunctions 93
6 Theexcitationoffinestructuretransitions 98
6.1 Introduction 98
6.2 Theoryoffinestructureexcitationprocesses 99
vii
viii Contents
7 Radiativetransferinmolecularlines 118
7.1 Introduction 118
7.2 Theradiativetransferequation 119
7.3 TheOHradical 124
7.4 Producingpopulationinversion 128
7.5 RotationalexcitationofOHbyH 129
2
8 Chargetransferprocesses 139
8.1 Introduction 139
8.2 TheLandau–Zenermodel 140
8.3 The‘orbiting’model 143
8.4 Thequantummechanicalmodel 145
8.5 Selectivepopulationofexcitedstates 151
9 Electroncollisions 153
9.1 Introduction 153
9.2 SelectionrulesandLS-coupling 154
9.3 Electroncollisionalexcitation 156
9.4 Resonances 158
9.5 ForbiddenlineemissionfromHerbig–Haroobjects 161
10 Photoncollisions 163
10.1 Introduction 163
10.2 Theoscillatorstrength 163
10.3 Thetransitionprobability 165
10.4 Photoionizationandradiativerecombination 166
10.5 Radiativetransitionsinmolecules 169
Appendix1 Theatomicsystemofunits 172
Appendix2 Reactionratecoefficients 173
References 177
Index 185
