W. Huebner – Opacity (2014)

Автор: W. Huebner
Название книги: Opacity
Формат: PDF
Жанр: Астрономия
Страницы: 582
Качество: Изначально компьютерное, E-book

“The essential principle of the fallen world appears to be discreteness or opacity.”
This quote is from an essay by Northrope Frye on “[Wm.] Blake’s Treatment of
the Archetype.” The essay appeared in English Institute Essays, ed. A. S. Downer,
Columbia U. Press (1951). However, this book on opacity is a scientific treatise of
a property of matter that describes its resistance (attenuation, i.e., absorption and
scattering also known as extinction) to transmission of electromagnetic radiation
(or, more explicitly, resistance to energy transport by photons). It is written with a
positive disposition and is of a quantitative nature. We stress modern methods as
well as historical development for calculating and measuring opacities.
The book was developed over many years. We follow the advances of opacity
calculations and measurements over more than half a century. Many new techniques,
particularly in the area of non-local thermodynamic equilibrium (non-LTE) opacity,
are being developed that will revolutionize radiative transfer in the foreseeable
future. We only briefly introduce this rapidly expanding field. Another area of
relevant research not covered in this book is based on quantum molecular dynamics
(QMD) for dense plasmas. QMD automatically includes electron degeneracy and
collective effects.
In some applications, opacities are used for mixtures involving chemical compounds
undergoing phase changes as the temperature and density or pressure
change with time. For example, opacities have been used inconsistently from
various different sources for changing density – temperature regions, ignoring
that they represent different elemental compositions, i.e., heterogeneous chemical
equilibrium between gas and condensed phases has been ignored. As a typical
example, the opacity of a dust-containing interstellar molecular cloud may be used
in modeling early stages of star formation while the opacity used for the evolution
of that star at a later stage comes from a different source or model and has a
different elemental (chemical) composition. In other applications, phase changes
may involve formation (or destruction) of liquid droplets or dust particles containing
layered materials with different heats of melting, vaporization, or sublimation. To
circumvent such problems, we discuss minimization of the Gibbs free energy as
a tool for applying the applicable phase transformations from solid to liquid to gas to plasma (or the reverse sequence) in calculating the equation of state and
the associated opacity, thus preserving the basic elemental (chemical) composition
during phase changes.
The nature of opacities requires us to apply many different disciplines in which
the same well-established symbol is used to refer to very different quantities,
even though symbols to represent many commonly used quantities have been
recommended by several international organizations. Unfortunately, the alphabet
does not provide the large number of options to uniquely define by a single letter
all quantities we encounter in a multidisciplinary treatise. We tried to be creative
and modified symbols with subscripts, superscripts, and various accent designations
to make our symbols distinct but similar to established usage. There is room for
further improvement in this area. For example, we use ˛ for the chemical potential,
˛o for the fine structure constant, ˛p for the polarizability, ˛e for the molecular
vibration-rotation coupling constant, etc. When we use ˛ for other purposes locally,
its meaning is defined at that time. When we use it as a dummy variable, its use will
be apparent. A list of symbols is provided in Appendix A.
The meanings of most commonly encountered basic quantities in radiation
processes have also been standardized by international agreements, e.g., the International
Union of Pure and Applied Physics, the International Union of Pure
and Applied Chemistry, the International Commission on Radiation Units and
Measurements, the International Standards Organization, the American Illuminating
Engineering Society, and the Royal Society of London. However, several basic and
internationally defined quantities have conflicting names in astronomy. Examples
include radiance, irradiance, exitance, flux, and intensity. Making matters worse,
many books use different definitions for various physical and chemical quantities.
Even though many books use the MKS system of units and SI units are
based on the MKS system, the definition of physical quantities are not always
consistent. Since this book deals with subjects of interest to plasma physics and
astronomy, and astronomers and astrophysicists regrettably do not always abide
by these conventions, we will use the terminology and definitions endorsed by
many international bodies, but occasionally point out astronomical usage. We will
quote units associated with these quantities to dispel any confusion. Appendix B,
which contains also a glossary of terms and a list of commonly used abbreviations,
should be consulted in case of any remaining confusion. Appendix C provides some
mathematical functions useful for opacity calculations.
Another area of confusion widely encountered in the literature is the terminology
for some definitions such as line strength. The definition for line strength is exactly
the same for atoms and molecules. However, for molecules (see, e.g., the section on
the just-overlapping lines model), the line strength is often defined by the frequency
(or wavenumber) integrated absorption cross section of a line. This is incorrect.
We have tried to avoid confusions of this type, but again, there is room for further
improvements.
We also try to use SI units consistently throughout the book. However, atomic
units are widely used in calculating atomic and molecular structure and closely
related quantities such as cross sections, electrical and thermal conductivities, etc.
Thus, in some cases we also provide equations using atomic units. Since many
different disciplines are involved in opacity and equation of state calculations,
we provide Appendix D: Units, Conversion Factors, and Fundamental Physical
Constants to ease the burden for readers from different fields. Finally, Appendix E
provides some relevant websites