Physics and Chemistry of the Interstellar Medium
Sun Kwok
University Science Books, 2006
Table of Contents
page
1 The interstellar medium 1
1.1 States of Matter in the ISM 2
1.2 Interactions between stars and the ISM 4
1.3 Chemical abundances in the ISM 6
1.4 The coupling between interstellar matter and radiation 7
1.5 Emission nebulae and the ionized component of the ISM 10
1.6 Reflection Nebulae  13
1.7 Dark Clouds and Molecular Clouds 15
1.8 Diffuse interstellar clouds and infrared cirrus 15
1.9 Hot gas and galactic corona  18
1.10 Multiple phases of the ISM 19
1.11 Summary  20
2 Fundamental concepts of radiation 22
2.1 Intensity  23
2.2 Flux  25
2.2.1  Power and luminosity 29
2.3 Moments of intensity 29
2.4 Thermodynamic equilibrium vs. steady state 31
2.5 Blackbody radiation  35
2.6 The Equation of Transfer  37
2.7 Solutions to the Equation of Transfer  41
2.7.1 Absorption only 43
2.7.2 Constant source function 44
2.8 Numerical solution to the equation of transfer  46
2.9 Scattering  47
2.9.1 Angular dependence 49
2.9.2 Refraction  50
2.9.3 Multiple scattering  51
2.10 Polarization  51
2.11  Summary  51
2.12  Further reading  52
3 Measurements of radiation 56
3.1 Flux measurements  57
3.1.1 The magnitude system  57
3.1.2 Aperture response and the telescope beam  62
3.2 Measurement of intensity  64
3.2.1 Direct imaging 64
3.2.2 Interferometry 66
3.2.3 Multi-element array 74
3.2.4 Aperture synthesis 77
3.3 Spectroscopy  78
3.4 Summary  80
3.5 Further reading  80
4 Photoionization and recombination  83
4.1 The hydrogen atom  84
4.2 Spectroscopic notation  86
4.3 Bound-free transition  90
4.4 Bound-free absorption in a stellar atmosphere  94
4.5 Recombination  96
4.6 Ionization structure of a static nebula  99
4.7 Diffuse interstellar radiation field  102
4.8 Ionization of complex atoms  102
4.9 Dielectronic recombination  105
4.10  Charge exchange reactions  105
4.11  Summary  107
4.12  Further readings  107
4.13  Resource materials 107
5 Line radiation from atoms and ions  110
5.1 Permitted and forbidden transitions 111
5.2 Transitions within multiplets  119
5.3 Fine-structure lines  120
5.4 Hyperfine lines  128
5.4.1 21 cm H line  128
5.4.2 Intercombination lines from hyperfine states  131
5.5 Absorption and emission  133
5.6 Spectral line formation  136
5.7 Scattering cross sections  141
5.8 Line broadening  144
5.8.1 Natural broadening 145
5.8.2 Doppler broadening 145
5.9 The Voigt profile  147
5.10 Equivalent width and the curve of growth 150
5.11 Recombination lines  151
5.11.1 Optical depth of Lyman �\ 154
5.11.2 Population distribution of the H atom  155
5.11.3 Recombination spectrum of H  156
5.12 Collisionally excited lines 161
5.12.1 The 21 cm HI line  165
5.12.2 Forbidden lines of metals  167
5.13 Resonance fluorescence  170
5.14 Recombination lines of metals  170
5.15 Statistical equilibrium and population distribution  171
5.16 Determination of nebular density and temperature by diagnostic diagrams  173
5.17 Atomic parameters  175
5.18 Abundance determination by absorption spectroscopy 176
5.19 Abundance determination in emission nebulae          176
5.20 Summary  178
5.21 Further reading  179
5.22 Resource materials 179
6 Continuum radiation in the gas phase  182
6.1 Free-bound continuum radiation  182
6.2 2-photon Radiation 184
6.3 Free-free continuum emission  187
6.3.1 Total Energy Loss  192
6.3.2 Derivation of cloud mass from the free-free flux  192
6.4 Electron scattering  193
6.5 Heating and cooling or photoionized regions  194
6.6 Determination of the temperature of the central star by nebular properties  196
6.6.1 Zanstra Temperature  198
6.6.2 The energy-balance method  198
6.7 Summary  199
7 Interstellar molecules  201
7.1 Molecular transitions  204
7.2 Electronic structures of molecules  205
7.3 Molecular orbitals and hybridization  207
7.4 Rotational transitions  209
7.5 Vibrational transitions of diatomic molecules  213
7.6 Electronic transitions 217
7.7 Effects of nuclear spins on rotational spectra  218
7.7.1 Molecular hydrogen 220
7.7.2 The oxygen molecule 226
7.7.3 Hyperfine lines in molecules 228
7.7.4 Isotopologues of CO 229
7.8 Rotational spectra of symmetric tops  230
7.8.1 Ammonia and inversion transitions 233
7.8.2 Centrifugal distortion  236
7.9 Asymmetric rotators  238
7.9.1 Formaldehyde  238
7.9.2 The water molecule 238
7.9.3 Methanol: an organic asymmetric rotator  242
7.9.4 Cyclic molecules  247
7.10 Radicals  249
7.10.1 OH  250
7.10.2 CH 253
7.10.3 SO 255
7.10.4 Methylene 257
7.11 Molecular ions  259
7.12 Molecules containing metals  262
7.12.1 Halides and cyanides  262
7.12.2 Calcium carbide  263
7.12.3 Molecules containing phosphorus 263
7.12.4 Metal hydrides  265
7.13 Raman scattering  267
7.14 Summary  267
7.15 Further readings  269
7.16 Resource materials  269
8 Vibrational spectroscopy of polyatomic molecules 271
8.1 Linear molecules  272
8.2 Symmetric tops  276
8.2.1 H3+ 276
8.2.2 Hydrocarbon radicals 281
8.3 Asymmetric tops  282
8.4 Carbon chains 283
8.5 Polycyclic Aromatic Hydrocarbons 289
8.6 Summary 291
8.7 Further readings 293
9 Molecular excitation and abundance determination 294
9.1 Partition functions and thermal distribution 294
9.2 Deriving the rotational excitation temperature 296
9.3 Excitation of molecules 297
9.4 Deriving molecular abundance from line observations 300
9.5 A two-level molecule under collisional excitation 303
9.6 A two-level molecule under collisional and radiative excitation 305
9.7 Optically thick lines and non-LTE population distribution 306
9.8 Molecular abundance by absorption spectroscopy  308
9.9 ortho para ratios 310
9.10 Isotopic abundance 312
9.11 Isomers 313
9.12 Conformers 317
9.13 Summary 318
10 Interstellar Grains: Physical Processes  320
10.1 Interstellar extinction  322
10.2 Dust absorption and scattering  325
10.3 Dust emission  328
10.3.1 Uniform temperature case  329
10.3.2 Centrally heated dust cloud  331
10.4 Thermal coupling between the dust and gas  335
10.5 Reflection and scattering  338
10.6 Dust absorption in the X-ray  339
10.7 Stochastic heating  339
10.8 Interstellar polarization  340
10.9 Photoelectric effect  342
10.10 Grain processing  342
10.11 Further reading  344
11 The Chemical Composition of Interstellar and Circumstellar Grains 346
11.1 Optical properties of solids  347
11.1.1 UV and visible absorption  348
11.1.2 Infrared absorption  349
11.2 Inorganic compounds  349
11.2.1 Amorphous silicates  349
11.2.2 Crystalline silicates  350
11.2.3 Refractory oxides  352
11.2.4 Silicon carbide  355
11.2.5 Carbonates  357
11.2.6 Ices  358
11.2.7 Luminescence  361
11.3 Organic compounds  363
11.3.1 Crystalline forms of carbon  366
11.3.2 Amorphous forms of carbon  370
11.4 Summary 375
11.5 Further readings 375
12 Carbonaceous grains 376
12.1 The aromatic infrared bands 377
12.1.1 Side groups 384
12.2 The 2175 �XA absorption feature  389
12.3 The diffuse interstellar bands 392
12.4 Extended red emission 393
12.5 The 21 µm feature  395
12.6  The 30 µm feature  396
12.7 Plateau features 397
12.8 Photochemistry 399
12.9 Summary 401
12.10 Further readings 402
13 The origin of interstellar dust 404
13.1 The formation of dust in the atmospheres of AGB stars 405
13.2 Models of carbonaceous dust 407
13.3 Laboratory simulations of cosmic dust 407
13.3.1 Silicates 408
13.3.2 Carbon grains 409
13.3.3 Nanoparticles 410
13.4 UV processing of ices 413
13.5 Interstellar dust and the solar system 415
13.5.1 Comets 416
13.5.2 Interplanetary dust particles 417
13.5.3 Planetary atmospheres and surfaces 419
13.5.4 Meteorites 419
13.6 Possible enrichment of the solar system by interstellar grains 423
13.7 Summary 424
14 Chemical Reactions in the ISM (by Tatsuhiko Hasegawa)  426
14.1 Example of Chemistry  426
14.2 Photodissociation  430
14.3 Attenuation of the radiation field  435
14.4 Cosmic Ray Ionization  436
14.5 Gas Phase Reactions  438
14.5.1 Ion-neutral collisions  439
14.5.2 Neutral-neutral collisions  440
14.5.3 Exit processes  440
14.5.4 Ion-molecule reactions  442
14.5.5 Neutral-Neutral Reactions (Multiple products)  443
14.5.6 Radiative Recombination  443
14.5.7 Dissociative Recombination  444
14.6 Rate Equation  445
14.7 Photon dominated regions  446
14.8 Grain surface chemistry  450
14.8.1 Adsorption and evaporation  451
14.8.2 Formation of molecular hydrogen on grains  453
14.8.3 Migration and evaporation  453
14.9 Summary 456
14.10 Further reading  457
15 Gas Dynamics, gravitational collapse and Stellar Winds  458
15.1 Hydrodynamical equations  458
15.2 Self-gravitating hydrostatic spheres  462
15.3 The virial theorem  468
15.4 Pressureless gravitational collapse  470
15.5 Gravitational instability  472
15.6 Stellar winds from hot stars  473
15.6.1 Radiation pressure on resonance lines as a mechanism of 475
driving hot star winds 
15.6.2 Derivation of mass loss rates from line profiles  478
15.6.3 Derivation of mass loss rates from free-free radiation  481
15.7 Winds from AGB stars  482
15.7.1 Radiation pressure on grains as a mechanism of driving AGB winds 484
15.7.2 Molecular Line Profile in an Expanding Envelope  485
15.8 Systematic motions in star formation regions 488
15.9 Summary  491
15.10 Further reading  492
16 Interaction between stars and the interstellar medium  495
16.1 Supersonic dynamics  495
16.2 Supernova remnants  500
16.2.1 Momentum conserving phase  503
16.3 Interstellar bubbles  505
. 16.3.1 Temperature and density of the bubble  509
16.3.2 Energy efficiency of the bubble expansion  510
16.3.3 Density of the swept-up shell  511
16.4 Interacting stellar winds  512
16.4.1 Momentum-conserving case  513
16.4.2 Energy conserving case  515
16.4.3 Transition from ionization bounded to density bounded  520
16.5 The momentum paradox  521
16.6 Summary  525
16.7 Further reading  525
17 Beyond the Galaxy  526
17.1 Overall structure of the ISM  526
17.2 Radiative interactions between different states of matter  528
17.3 Application to extragalactic astronomy  529
References  544
Appendix 1 Vector formulas in different coordinate systems 559
Appendix 2  Ionization pontentials for atoms and molecules  561
Appendix 3  List of interstellar molecules  567
Appendix 4  Symbols and abbreviations  570
Appendix 5  Glossary  575
Appendix 6  Solutions to problems 578
A6. 1   Chapter 2 578
A6. 2   Chapter 3 581
A6. 3   Chapter 4 583
A6. 4   Chapter 5 585
A6. 5   Chapter 6 587
A6. 6   Chapter 7 587
A6. 7   Chapter 8 589
A6. 8   Chapter 9 589
A6. 9   Chapter 10 590
A6. 10 Chapter 15 591
A6. 11 Chapter 16 592
Appendix 7  About the author 593
Index  594