Astronomical Masers
One of the most spectacular discoveries of molecular astronomy has been the detection of maser emission. The same radiation that is generated in the laboratory only with elaborate, special equipment occurs naturally in interstellar space. This intense rad
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ASTRONOMICAL MASERS by
MOSHE ELITZUR Department of Physics and Astronomy, University of Kentucky, Lexington, U.S.A.
SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.
Library ofCongress Cataloging-in-Publication Data El1tzur, Moshe. Astronomical masers I Moshe Elltzur. p. cm. -- (Astropnyslcs and space science Includes blbllographlcal references and index.
libr~ry
v. 17G;
ISBN 978-0-7923-1217-8 ISBN 978-94-011-2394-5 (eBook) DOI 10.1007/978-94-011-2394-5
1. Astronomlcal masers. CB790.E45 1991 523. l' 12--dc20
1. Tltle.
II. Serles.
91-11947 91-11947
ISBN 978-0-7923-1217-8
Printed on acid-free paper
All Rights Reserved © 1992 Springer-Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1992 Softcover reprint ofthe hardcover Ist edition 1992 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright OW.1er.
To my wife, Shlomit, for her understanding and companionship, and to my sons, Ofer, Haggai, and Ben.
CONTENTS Chapter 1: Introduction Chapter 2: Basic Background Concepts 2.1. Thermodynamic Equilibrium and Detailed Balance 2.2. Thermal Motions and Collisions 2.2.1 The Maxwellian Distribution; Kinetic Temperature 2.2.2 Inelastic Collisions; Excitation Temperature 2.2.3 Derivation of the Detailed Balance Relation 2.3. Radiation - Generalities 2.3.1 Radiation Intensity; Flux; Measured Quantities 2.3.2 Radiative Transfer; Optical Depth; Source Function 2.3.3 Photon State Density; Black Body Radiation 2.3.4 Brightness Temperature 2.4. Interaction Between Matter and Line Radiation 2.4.1 The Line Profile 2.4.2 Spontaneous Emission; Line Emission Coefficient 2.4.3 Absorption 2.4.4 Induced Emission 2.4.5 Detailed Balance Relations 2.4.6 The Line Absorption Coefficient; Source Function 2.5. The Two-Level Model 2.6. The Escape Probability Method 2.6.1 Intuitive Arguments 2.6.2 The Homogeneous Slab Model 2.6.3 Large Velocity Gradients 2.6.4 The Two-Level Model Revised 2.7. The Level Population Equations Chapter 3: Astronomical Maser Radiation 3.1. The Maser Molecules 3.1.1 OH
3.1.2 H20 3.1.3 SiO 3.1.4 Other Masers
3.2. Evidence for Maser Action 3.3. Characteristics of the Radiation Chapter 4: Maser Theory 4.1. Formal Backgroundt
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t Sections marked with dagger present material that is mostly of formal nature, and may be omitted by readers primarily interested in practical applications.
CONTENTS
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4.1.1 The Radiation Field 4.1.2 The Atomic System 4.1.3 The Radiative Transfer Equation 4.1.4 Maser Statistics 4.2. Phenomenological Maser Theory 4.2.1 Population Inversion 4.2.2 Maser Amplification 4.2.3 Saturation 4.2.4 Maser Pump Terms 4.2.5 Radiative Transfer 4.3. The Linear Maser
4.3.1 Unsaturated Masers 4.3.2 Saturated Masers; the Dominant Stream 4.3.3 Th
