Astrophysics of the Diffuse Universe

445 Pages·2003·13.773 MB·English

by  Professor Michael A. Dopita, Dr. Ralph S. Sutherland (auth.)

ASTRO TOMY Ai D
\STRC)PlfYSICS LIBRARY
LIBRARY
Series Editors: I. Appenzeller, Heidelberg, Germany
G. Bomer, Garching, Gennany
A. Burkert, Miinchen, Gennany
M. A. Dopita, Canberra, Australia
A. Eckart, Koln, Germany
T. Encrenaz, Meudon, France
M. Harwit, Washington, DC, USA
R. Kippenhahn, Gottingen, Germany
J. Lequeux, Paris, France
A. Maeder, Sauvemy, Switzerland
V. Trimble, College Park, MD, and Irvine, CA, USA
M.A. Dopita
R. S. Sutherland
Astrophysics
of the
Diffuse Universe
With 70 Figures
~ Springer
Professor Michael A. Dopita
Dr. Ralph S. Sutherland
The Australian National University
Research School of Astronomy and Astrophysics
Weston Creek Post Office
Canberra, ACT 2611, Australia
Cover picture: The central regions of the Lagoon Nebula in the light ofHa and forbidden [0 III]'\ 5007 \lA. The false
colours are chosen to emphasise the differences in ionization conditions within the nebula as the ratio of the emission
lines change. Young hot stars are photoionizing the nebula in a Huorescent process which converts UV radiation
into visible light, mostly as emission lines of hydrogen and other light elements. Dark dust lanes and globules show
where dense cores of interstellar clouds are still condensing, and where new star formation may still be taking place.
(Credit: Sutherland, R.S. & Bessell, M. 1999, ANU Research School of Astronomy and Astrophysics, Mt Stromlo
Observatory. )
Library of Congress Cataloging-in-Publication Data.
Dopita, Michael A.
Astrophysics of the diffuse universelM. A. Dopita, R. S. Sutherland.
p.cm. - (Astronomy and astrophysics library, [SSN 0941-7834)
Includes bibliographical references and index.
1. Interstellar matter. 2. Astrophysics. I. Sutherland, R.S. (Ralph S.), 1964-. II. Title. III. Series
QB790.D67 2003 523.1' 135-<1c12 2002026830
First Edition 2003. Corrected Third Printing 2005
ISSN 0941-7834
ISBN 978-3-642-07771-5 ISBN 978-3-662-05866-4 (eBook)
DOI 10.1007/978-3-662-05866-4
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Originally published by Springer-Verlag Berlin Heidelberg New York in 2003.
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' ... Humanity needs practical men, who get the most out
of their work, and, without forgetting the general good,
safeguard their own interests. But humanity also needs
dreamers, for whom the disinterested development of
an enterprise is so captivating that it becomes impos
sible for them to devote their care to their own ma
terial pmjit. A well-organized society should assure to
such workers the efficient means of accomplishing their
task, in a life freed from material care and freely con
secrated to research. '
Marie (originally Manja Sklodowska) Curie (1867-1934)
Preface
Our purpose in writing this book is to show how physics has been applied to
developing our current understanding of the phase structure, physical condi
tions, chemical makeup and, evolution of the (thermal) interstellar medium.
We hope it provides an up-to-date overview which postgraduates, advanced
undergraduates, and professionals in astrophysics can use as a "reference
of first resort" before going on to read the more specialist monographs or
research literature. We have covered the exciting observational results, but
without consideration of the experimental techniques or instruments required.
An elementary understanding of mathematical physics and of quantum me
chanics has been assumed, and a knowledge of basic astrophysics would be
helpful.
Older textbooks on interstellar physics have tended to develop the subject
matter in an order which reflects the historical development of the field. Here
a more pedagological approach has been adopted, based on our lecture course
experience. We cover successively more complex physical systems found in the
diffuse universe. Detailed mathematical rigour is eschewed in favour of provid
ing the reader with a basic physical insight into these systems. Astrophysical
problems are treated as practical applications of the physics. In practice, the
material is generally ranked in order of decreasing entropy, since the hottest
and most diffuse phases tend to be physically less complex. We have also
tried to map out the breadth and richness of the field, which covers topics as
diverse as cosmology, hot galactic halos, shocked and photoionized objects,
cosmic dust, astrochemistry, dense molecular clouds, and the complex phase
structure of the interstellar medium.
Roughly a third of the text is used to extend the discussion of some topics.
These are either to develop greater rigour, or else provide a minireview of a
specific subject area, with many references to the key papers in the literature.
These sections, identified by (*) asterisks, can be skipped at the undergradu
ate level, but they should provide a useful introduction to potential research
topics for postgraduates or provide an initial source of information to active
researchers from other fields. The noteH provide recommended texts for fur
ther reading. Since these texts have been used by us in the development of
this book, these notes also serve as our "thank you" to these authors. Exer
cises and some worked examples have been provided. It is a good idea not to
VIn Preface
skip too many of these, since they help develop proficiency, competence and
a more intuitive understanding of the subject matter, and in some cases they
extend the material covered in the text. The exercises are chosen to provide
simple analytical solutions in the most part, but the parameters are appro
priate for real interstellar gas. The exercises use physical constants which are
given either in the text or in the Appendices.
Mike Dopita was able to develop much of this text while on study leave in
1998. He would like to take this opportunity to thank those institutions which
provided him hospitality and a pleasant working environment during that
period. In particular, he would like to extend his great thanks to Massimo
Capaccioli of the Osservatorio Astronomico di Capodimonte in Napoli for
his hospitality and for his encouragement and enthusiasm in supporting the
writing of this book, to Franco Pacini and Mario Perinotto of the Arcetri
Observatory for making his visit to Firenze so pleasant and productive, to
the Institute of Astrophysics at Cambridge for a Summer Visiting Fellowship,
and finally to Brigitte Rocca of the Institut d'Astrophysique of Paris; thank
you all.
In addition, we would like to thank all those who have provided encour
agement and assistance in checking the manuscript, advising us of omissions,
test-driving the text with their classes, and generally helping us to produce
a better quality product. These include Magda Arnaboldi, Bruce Balick, Ge
off Bicknell, Joel Bregman, Bruce Draine, Patricia Dopita, Bryan Gaensler,
Trung Hua, Kris Sellgren, Brent Groves, and a host of students in Australia
and the USA whose eyes proved to be sharper than ours in spotting the
typographical errors.
Canberra, Michael Dopita
August 2002 Ralph Sutherland
Contents
1. What Is the Diffuse Universe? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Phases................................................ 6
1.2 Observability.......................................... 8
2. Line Emission Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11
2.1 Atomic Spectra ........................................ 11
2.1.1 Resonance Lines ................................. 11
2.1.2 Pure Recombination Lines. . . . . . . . . . . . . . . . . . . . . . . .. 14
2.1.3 Spectroscopic Notation. . . . . . . . . . . . . . . . . . . . . . . . . . .. 20
2.1.4 Intercombination and Forbidden Lines .............. 22
2.2 Molecular Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 24
2.3 Rotating Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 25
2.3.1 The Rigid Linear Rotator. . . . . . . . . . . . . . . . . . . . . . . .. 26
*
2.3.2 The Nonrigid Linear Rotator .. , . . . . . . . . . . . . . . . . .. 28
*
2.3.3 Polyatomic Linear Rotators . . . . . . . . . . . . . . . . . . . . .. 29
2.4 Vibrating Molecules .................................... 31
2.5 Ro-Vibrational Spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33
2.6 Electronic Molecular Spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 35
2.6.1 The Franck-Condon Principle. . . . . . . . . . . . . . . . . . . . .. 36
2.6.2 Ro-Vib-Electronic Molecular Spectra. . . . . . . . . . . . . . .. 37
2.7 Exercises.............................................. 40
3. Collisional Excitation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 41
3.1 Collisional Excitation by Electron Impact ................. 42
3.1.1 Limiting Cases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 45
3.2 The Three-Level Atom. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 46
3.2.1 Low-Density Limit; E12 rv E23 ..................... 48
3.2.2 Ions in Which E23 «E12 ......................... 49
3.2.3 Infrared Line Diagnostics. . . . . . . . . . . . . . . . . . . . . . . . .. 53
3.3 The General Multilevel Atom. . . . . . . . . . . . . . . . . . . . . . . . . . .. 54
3.4 Exercises.............................................. 56
X Contents
4. Line Transfer Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 59
4.1 Resonance Line Transfer ................................ 59
4.1.1 Resonance Line Absorption by Heavy Elements. . . . .. 60
4.1.2 Absorption Line Studies of the ISM. . . . . . . . . . . . . . . .. 62
4.1.3 Line Transfer in Emission Resonance Lines. . . . . . . . .. 67
4.1.4 Escape Probabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 68
4.1.5 Mean Transmission Probabilities ................... 69
4.1.6 Mean Escape Probabilities - Emissivities . . . . . . . . . . .. 70
4.1.7 Nebula Line Transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 72
4.1.8 Line Transfer in the Lyman Series . . . . . . . . . . . . . . . . .. 73
4.2 The HI 21 cm Line ..................................... 75
4.2.1 H I Observations of Disk Galaxies. . . . . . . . . . . . . . . . . .. 77
4.3 Fluorescent Processes .................................. 82
*
4.3.1 The Bowen Mechanism. . . . . . . . . . . . . . . . . . . . . . . . .. 82
*
4.3.2 0 I Fluorescence with Lyj3 . . . . . . . . . . . . . . . . . . . . . .. 84
*
4.3.3 H2 Fluorescence with Lya ... . . . . . . . . . . . . . . . . . . .. 85
*
4.3.4 Raman Scattering Fluorescence. . . . . . . . . . . . . . . . . .. 86
4.4 Astrophysical Masers ................................... 88
4.4.1 Theory.......................................... 88
*
4.4.2 Observations of Galactic Masers. . . . . . . . . . . . . . . . .. 91
*
4.4.3 Observations of Extragalactic Masers. . . . . . . . . . . . .. 96
4.5 Exercises.............................................. 99
5. Collisional Ionization Equilibrium ........................ 101
5.1 Collisional Ionization ................................... 102
5.1.1 The Case of Hydrogen ............................ 104
5.2 Recombination ......................................... 105
5.2.1 Radiative Recombination Rates .................... 106
5.2.2 Dielectronic Recombination Rates .................. 107
5.3 Photoionization ........................................ 108
5.3.1 From Outer Shells ................................ 108
5.3.2 Inner Shell Photoionization ........................ 110
5.3.3 The Milne Relation ............................... 111
5.3.4 Photoionization Cross Sections ..................... 112
5.4 Charge-Exchange ....................................... 114
5.5 Coronal Equilibrium .................................... 118
5.5.1 The Case of a Pure Hydrogen Plasma ............... 119
5.5.2 Ionization Equilibrium of Heavy Elements ........... 119
5.6 Exercises .............................................. 122
6. Continuum and Recombination Line Processes ........... 125
6.1 Free-Free Continuum Emission ........................... 125
*
6.1.1 Free-Free Gaunt Factors ......................... 127
6.2 The Free-Bound Continuum ............................. 132
6.3 Continuum Emission Coefficients ......................... 133
Contents XI
6.4 The Two-Photon Process ................................ 135
6.5 Recombination Line Emission ............................ 138
6.5.1 Recombination Line Spectra ....................... 138
*
6.5.2 The Radio Recombination Lines .................. 140
7. Cooling Plasmas .......................................... 143
7.1 The Cooling Function ................................... 143
7.2 Conditions for Nonequilibrium Cooling .................... 146
*
7.3 Heat Transport ....................................... 151
*
7.3.1 Electron Conduction ............................ 151
*
7.3.2 Boundary Layer Mixing ......................... 153
*
7.4 Cold Clouds in Hot Gas ............................... 154
7.5 Thermal Instabilities .................................... 157
7.5.1 In a Stationary Medium ........................... 157
*
7.5.2 In an Expanding Medium ........................ 159
7.6 The Recombination Era of the Universe ................... 161
7.7 Hot Galactic Coronae ................................... 162
7.7.1 Early-Type Galaxies .............................. 162
7.7.2 Clusters of Galaxies .............................. 165
7.7.3 Disk Galaxies .................................... 168
7.8 Exercises .............................................. 173
8. Interstellar Shocks ........................................ 175
8.1 Why Do Shocks Exist? .................................. 175
8.2 J-Shocks .............................................. 180
8.2.1 The Rankine-Hugoniot Jump Conditions ............ 182
8.2.2 Radiationless Shocks .............................. 182
8.2.3 Isothermal Shocks ................................ 184
8.3 The Drivers of Interstellar Shocks ........................ 186
8.3.1 Supernova Explosions ............................. 187
8.3.2 Stellar Wind Bubbles ............................. 191
*
8.3.3 Galactic Jet-Driven Bubbles ..................... 193
8.4 The Radiative Properties of J-shocks ...................... 196
*
8.4.1 Radiation Properties of Shock Fronts .............. 196
8.4.2 The Structure of Radiative J-Shocks ................ 200
8.4.3 Spectra of Old Supernova Remnants ................ 204
*
8.4.4 Spectra of Herbig-Haro Objects .................. 206
8.5 C-Shocks .............................................. 209
*
8.5.1 The Structure and Spectrum of C-Shocks .......... 210
8.6 Exercises .............................................. 214