Preface ...................................................... xvii
1 Introduction ................................................. 1
1.1 Organization of the ISM: Characteristic Phases .......... 4
1.2 Elemental Composition ................................... 9
1.3 Energy Densities ........................................ 9
2 Collisional Processes ....................................... 11
2.1 Collisional Rate Coefficients .......................... 11
2.2 Inverse-Square Law Forces: Elastic Scattering .......... 12
2.3 Electron-Ion Inelastic Scattering: Collision Strength
flut ................................................... 17
2.4 Ion-Neutral Collision Rates ............................ 17
2.5 Electron-Neutral Collision Rates ....................... 20
2.6 Neutral-Neutral Collision Rates ........................ 21
3 Statistical Mechanics and Thermodynamic Equilibrium ......... 22
3.1 Partition Functions .................................... 22
3.2 Detailed Balance: The Law of Mass Action ............... 23
3.3 Ionization and Recombination ........................... 24
3.4 Saha Equation .......................................... 25
3.5 Detailed Balance: Ratios of Rate Coefficients .......... 26
3.6 Detailed Balance: Ratios of Cross Sections ............. 26
3.7 Example: Three-Body Recombination ...................... 28
3.8 Departure Coefficients ................................. 30
4 Energy Levels of Atoms and Ions ............................. 32
4.1 Single-Electron Orbitals ............................... 32
4.2 Configurations ......................................... 32
4.3 Spectroscopic Terms .................................... 33
4.4 Fine Structure: Spin-Orbit Interaction ................. 34
4.5 Designation of Energy Levels for Atoms and Ions:
Spectroscopic Notation ................................. 34
4.6 Hyperfine Structure: Interaction with Nuclear Spin ..... 36
4.7 Zeeman Effect .......................................... 37
4.8 Further Reading ........................................ 37
5 Energy Levels of Molecules .................................. 38
5.1 Diatomic Molecules ..................................... 38
5.2 * Energy Levels of Nonlinear Molecules ................. 47
5.3 * Zeeman Splitting ..................................... 51
5.4 Further Reading ........................................ 52
6 Spontaneous Emission, Stimulated Emission, and Absorption ... 53
6.1 Emission and Absorption of Photons ..................... 53
6.2 Absorption Cross Section ............................... 55
6.3 Oscillator Strength .................................... 56
6.4 Intrinsic Line Profile ................................. 57
6.5 Doppler Broadening: The Voigt Line Profile ............. 58
6.6 Transition from Doppler Core to Damping Wings .......... 59
6.7 Selection Rules for Radiative Transitions .............. 60
7 Radiative Transfer .......................................... 63
7.1 Physical Quantities .................................... 63
7.2 Equation of Radiative Transfer ......................... 65
7.3 Emission and Absorption Coefficients ................... 66
7.4 Integration of the Equation of Radiative Transfer ...... 66
7.5 Maser Lines ............................................ 68
8 HI 21-cm Emission and Absorption ............................ 70
8.1 HI Emissivity and Absorption Coefficient ............... 70
8.2 Optically Thin Cloud ................................... 72
8.3 Spin Temperature Determination Using Background Radio
Sources ................................................ 73
9 Absorption Lines: The Curve of Growth ....................... 75
9.1 Absorption Lines ....................................... 75
9.2 Optically Thin Absorption, τ0  1 ...................... 77
9.3 Flat Portion of the Curve of Growth, 10 τ0
τdamp .................................................. 77
9.4 Damped Portion of the Curve of Growth, τ0  τdamp ...... 79
9.5 Approximation Formulae for W ........................... 81
9.6 Doublet Ratio .......................................... 81
9.7 Lyman Series of Hydrogen: Ly α, Ly β, Ly γ ............. 83
9.8 Lyman Limit ............................................ 84
9.9 H2: Lyman and Werner Bands ............................. 85
9.10 "Metal" Lines .......................................... 86
9.11 Abundances in H I Gas .................................. 90
10 Emission and Absorption by a Thermal Plasma ................. 92
10.1 Free-Free Emission (Bremsstrahlung) .................... 92
10.2 * Gaunt Factor ......................................... 93
10.3 Frequency-Averaged Gaunt Factor ........................ 95
10.4 Free-Free Absorption ................................... 95
10.5 Emission Measure ....................................... 96
10.6 Free-Bound Transitions: Recombination Continuum ........ 97
10.7 Radio Recombination Lines .............................. 97
11 Propagation of Radio Waves through the ISM ................. 101
11.1 Dispersion Relation for Cold Plasmas .................. 101
11.2 Dispersion ............................................ 102
11.3 Faraday Rotation ...................................... 105
11.4 * Refraction .......................................... 109
11.5 * Scintillation ....................................... 111
11.6 * Interstellar Electron Density Power Spectrum ........ 113
11.7 * Extreme Scattering Events ........................... 116
12 Interstellar Radiation Fields .............................. 119
12.1 Galactic Synchrotron Radiation ........................ 119
12.2 Cosmic Microwave Background Radiation ................. 120
12.3 Free-Free Emission and Recombination Continuum ........ 121
12.4 Infrared Emission from Dust ........................... 121
12.5 Starlight in an H I Region ............................ 123
12.6 X Rays from Hot Plasma ................................ 125
12.7 Radiation Field in a Photodissociation Region near
a Hot Star ............................................ 125
13 Ionization Processes ....................................... 127
13.1 Photoionization ....................................... 128
13.2 Auger Ionization and X-Ray Fluorescence ............... 131
13.3 Secondary Ionizations ................................. 132
13.4 Collisional Ionization ................................ 134
13.5 Cosmic Ray Ionization ................................. 134
14 Recombination of Ions with Electrons ....................... 137
14.1 Radiative Recombination ............................... 137
14.2 Radiative Recombination of Hydrogen ................... 138
14.3 * Radiative Recombination: Helium ..................... 146
14.4 Radiative Recombination: Heavy Elements ............... 150
14.5 Dielectronic Recombination ............................ 151
14.6 Dissociative Recombination ............................ 153
14.7 Charge Exchange ....................................... 154
14.8 Ion Neutralization by Dust Grains ..................... 157
14.9 Ionization Balance in Collisionally Ionized Gas ....... 159
15 Photoionized Gas ........................................... 162
15.1 H II Regions as Strömgren Spheres ..................... 162
15.2 Time Scales ........................................... 165
15.3 Neutral Fraction within an H II Region ................ 166
15.4 Dusty H II Regions with Radiation Pressure ............ 167
15.5 Ionization of Helium and Other Elements ............... 172
15.6 Planetary Nebulae ..................................... 175
15.7 * Escape of Lyman α ................................... 176
15.8 * Ionization by Power-Law Spectra ..................... 180
16 Ionization in Predominantly Neutral Regions ................ 182
16.1 H I Regions: Ionization of Metals ..................... 182
16.2 Cool H I Regions: Ionization of Hydrogen .............. 184
16.3 Warm H I Regions ...................................... 185
16.4 Diffuse Molecular Gas ................................. 186
16.5 Dense Molecular Gas: Dark Clouds ...................... 188
17 Collisional Excitation ..................................... 190
17.1 Two-Level Atom ........................................ 190
17.2 Critical Density ncrit,u .............................. 191
17.3 Example: H I Spin Temperature ......................... 192
17.4 Example: С II Fine Structure Excitation ............... 195
17.5 * Three-Level Atom .................................... 197
17.6 * Example: Fine Structure Excitation of С I and O I ... 198
17.7 * Measurement of Density and Pressure Using C I ....... 198
18 Nebular Diagnostics ........................................ 203
18.1 Temperature Diagnostics: Collisionally Excited
Optical/UV Lines ...................................... 204
18.2 Density Diagnostics: Collisionally Excited
Optical/UV Lines ...................................... 209
18.3 Density Diagnostics: Fine-Structure Emission Lines .... 210
18.4 * Other Diagnostic Methods ............................ 212
18.5 Abundance Determination from Collisionally Excited
Lines ................................................. 214
18.6 * Abundances from Optical Recombination Lines ......... 215
18.7 * Ionization/Excitation Diagnostics: The BPT
Diagram ............................................... 215
19 Radiative Trapping I ....................................... 219
19.1 Escape Probability Approximation ...................... 219
19.2 Homogeneous Static Spherical Cloud .................... 221
19.3 Example: CO J = 1-0 ................................... 222
19.4 * LVG Approximation: Hubble Flow ...................... 224
19.5 Escape Probability for Turbulent Clouds ............... 225
19.6 CO 1-0 Emission as a Tracer of H2 Mass: CO
"X-Factor" ............................................ 227
20 Optical Pumping ............................................ 229
20.1 UV Pumping by Continuum ............................... 229
20.2 * Infrared Pumping: OH ................................ 231
20.3 * UV Pumping by Line Coincidence: Bowen
Fluorescence .......................................... 232
21 Interstellar Dust: Observed Properties ..................... 235
21.1 Interstellar Extinction ............................... 236
21.2 Parametric Fits to the Extinction Curve ............... 239
21.3 Polarization by Interstellar Dust ..................... 240
21.4 Scattering of Starlight by Interstellar Dust .......... 242
21.5 Size Distribution of Interstellar Dust ................ 243
21.6 * Purcell Limit: Lower Limit on Dust Volume ........... 243
21.7 Infrared Emission ..................................... 246
21.8 * Luminescence ........................................ 247
22 Scattering and Absorption by Small Particles ............... 248
22.1 Cross Sections and Efficiency Factors ................. 248
22.2 Dielectric Function and Refractive Index .............. 249
22.3 Electric Dipole Limit: Size  λ ....................... 251
22.4 Limiting Behavior at Long Wavelengths ................. 252
22.5 Sizes Comparable to Wavelength: Mie Theory ............ 253
22.6 * Nonspherical Particles .............................. 256
22.7 Interstellar Grains ................................... 258
23 Composition of Interstellar Dust ........................... 263
23.1 Abundance Constraints ................................. 263
23.2 Presolar Grains in Meteorites ......................... 266
23.3 Observed Spectral Features of Dust .................... 267
23.4 Silicates ............................................. 271
23.5 Polycyclic Aromatic Hydrocarbons ...................... 274
23.6 * Graphite ............................................ 277
23.7 * Diamond ............................................. 278
23.8 * Amorphous Carbons, Including Hydrogenated
Amorphous Carbon ...................................... 278
23.9 * Fullerenes .......................................... 278
23.10 Models for Interstellar Dust ......................... 279
24 Temperatures of Interstellar Grains ........................ 285
24.1 Heating and Cooling of "Classical" Dust Grains ........ 285
24.2 Heating and Cooling of Ultrasmall Dust Grains:
Temperature Spikes .................................... 290
24.3 Infrared Emission from Grains ......................... 293
24.4 Collisionally Heated Dust ............................. 295
25 Grain Physics: Charging and Sputtering ..................... 296
25.1 Collisional Charging .................................. 296
25.2 Photoelectric Emission ................................ 297
25.3 Grain Charging in the Diffuse ISM ..................... 299
25.4 * Secondary Electron Emission ........................ 299
25.5 * Electron Field Emission ............................ 301
25.6 * Ion Field Emission and Coulomb Explosions .......... 302
25.7 Sputtering in Hot Gas ................................. 302
26 Grain Dynamics ............................................. 304
26.1 Translational Motion .................................. 304
26.2 Rotational Motion ..................................... 307
26.3 * Alignment of Interstellar Dust ..................... 310
27 Heating and Cooling of H II Regions ........................ 315
27.1 Heating by Photoionization ............................ 315
27.2 Other Heating Processes ............................... 317
27.3 Cooling Processes ..................................... 319
27.4 Thermal Equilibrium ................................... 322
27.5 Emission Spectrum of an H II Region ................... 324
27.6 Observed Temperatures in H II Regions ................. 325
28 The Orion H II Region ...................................... 326
28.1 Trapezium Stars ....................................... 326
28.2 Distribution of Ionized Gas ........................... 327
28.3 Orion Bar ............................................. 328
28.4 Gas Kinematics ........................................ 328
28.5 PIGS, Proplyds, and Shadows ........................... 330
29 H I Clouds: Observations ................................... 331
29.1 21-cm Line Observations ............................... 331
29.2 Distribution of the H I ............................... 332
29.3 Zeeman Effect ......................................... 333
29.4 Optical and UV Absorption Line Studies ................ 335
29.5 Infrared Emission ..................................... 335
30 HI Clouds: Heating and Cooling ............................. 337
30.1 Heating: Starlight, Cosmic Rays, X Rays, and MHD
Waves ................................................. 337
30.2 Photoelectric Heating by Dust ......................... 338
30.3 Cooling: [С II] 158 μm, [О I] 63 μm, and Other
Lines ................................................. 339
30.4 Two "Phases" for H I in the ISM ....................... 341
30.5 Emission Spectrum of an H I Cloud ..................... 343
31 Molecular Hydrogen ......................................... 344
31.1 Gas-Phase Formation of H2 ............................. 344
31.2 Grain Catalysis of H2 ................................. 345
31.3 Photodissociation of H2 ............................... 346
31.4 Self-Shielding ........................................ 348
31.5 * Excitation of Vibration and Rotation by UV
Pumping ............................................... 349
31.6 * Rotational Level Populations ........................ 350
31.7 * Structure of a Photodissociation Region ............. 352
31.8 Dense PDRs ............................................ 356
32 Molecular Clouds: Observations ............................. 357
32.1 Taxonomy and Astronomy ................................ 357
32.2 Star Counts ........................................... 362
32.3 Molecular Radio Lines ................................. 362
32.4 FIR Emission from Dust ................................ 363
32.5 γ rays ................................................ 364
32.6 * Compact, Ultracompact, and Hypercompact H II
Regions ............................................... 365
32.7 * IR Point Sources .................................... 366
32.8 * Masers .............................................. 366
32.9 Size-Linewidth Relation in Molecular Clouds ........... 366
32.10 Magnetic Fields in Molecular Clouds .................. 369
32.11 Energy Dissipation in Molecular Clouds ............... 371
33 Molecular Clouds: Chemistry and Ionization ................. 373
33.1 Photoionization and Photodissociation of Molecules .... 375
33.2 * Ion-Molecule Chemistry in Cold Gas .................. 376
33.3 * The CH+ Problem ..................................... 379
34 Physical Processes in Hot Gas .............................. 381
34.1 Radiative Cooling ..................................... 381
34.2 Radiative Cooling Time ................................ 384
34.3 * Thermal Conduction .................................. 385
34.4 * Cloud Evaporation in Hot Gas ........................ 386
34.5 * Conduction Fronts ................................... 387
35 Fluid Dynamics ............................................. 389
35.1 Mass Conservation ..................................... 389
35.2 Conservation of Momentum: MHD Navier-Stokes
Equation .............................................. 390
35.3 Heating and Cooling ................................... 392
35.4 Electrodynamics in a Conducting Fluid: Flux-
Freezing .............................................. 393
35.5 * Virial Theorem ...................................... 395
36 Shockwaves ................................................. 397
36.1 Sources of Interstellar Shocks ........................ 397
36.2 Jump Conditions: Rankine-Hugoniot Relations ........... 398
36.3 Cooling Time and Cooling Length ....................... 404
36.4 * Collisionless Shocks ................................ 404
36.5 * Electron Temperature ................................ 406
36.6 * Two-Fluid MHD Shocks in Low Fractional Ionization
Gas ................................................... 406
37 * lonization/Dissociation Fronts ........................... 412
37.1 * Ionization Fronts: R-Type and D-Type ................ 412
37.2 * Expansion of an H II Region in a Uniform Medium ..... 416
37.3 * Photodissociation Fronts ............................ 419
38 * Stellar Winds ............................................ 422
38.1 * Winds from Hot Stars: Stellar Wind Bubbles .......... 422
38.2 * Winds from Cool Stars ............................... 426
38.3 * Stellar Wind Bow-Shock .............................. 427
39 Effects of Supernovae on the ISM ........................... 429
39.1 Evolution of a Supernova Remnant in a Uniform ISM ..... 429
39.2 * Overlapping of SNRs ................................. 435
39.3 * Supernova Remnants in an Inhomogeneous Medium ....... 436
39.4 Three-Phase Model of the ISM .......................... 437
40 * Cosmic Rays and Gamma Rays ............................... 440
40.1 Cosmic Ray Energy Spectrum and Composition ............ 440
40.2 * Theory of Diffusive Shock Acceleration .............. 442
40.3 * Injection Problem ................................... 444
40.4 * Upper Limits on Cosmic Ray Energy ................... 446
40.5 * Cosmic Ray Propagation .............................. 447
40.6 * Synchrotron Emission and Supernova Remnants ......... 448
40.7 * Gamma Ray Emission from Interstellar Clouds ......... 448
40.8 * 26Al in the ISM ..................................... 449
40.9 * Positrons and Positronium in the ISM ................ 450
41 Gravitational Collapse and Star Formation: Theory .......... 451
41.1 Gravitational Instability: Jeans Instability .......... 451
41.2 * Parker Instability .................................. 453
41.3 Insights from the Virial Theorem ...................... 456
41.4 Magnetic Flux Problem: Ambipolar Diffusion ............ 459
41.5 Angular Momentum Problem .............................. 461
41.6 Accretion Disks ....................................... 463
41.7 Radiation Pressure .................................... 463
42 Star Formation: Observations ............................... 465
42.1 Collapse of Cores to form Stars ....................... 465
42.2 Class О, I, II, and III Protostars .................... 466
42.3 Initial Mass Function ................................. 468
42.4 Star Formation Rates .................................. 470
42.5 Schmidt-Kennicutt Law ................................. 471
Appendices
A. List of Symbols ......................................... 473
B. Physical Constants ...................................... 476
C. Summary of Radiative Processes .......................... 477
D. Ionization Potentials (eV) .............................. 481
E. Energy-Level Diagrams ................................... 482
F. Collisional Rate Coefficients ........................... 496
G. Semiclassical Atom ...................................... 503
H. Debye Length for a Plasma ............................... 505
I. Heuristic Model for Ion-Electron Inelastic Scattering ... 506
J. Virial Theorem .......................................... 508
Bibliography ............................................... 511
Index ......................................................... 529
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