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This page intentionally left blank 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