ASTRONOMY AND ASTROPHYSICS LIBRARY Series Editors: G. B¨orner, Garching, Germany A. Burkert, M¨unchen, Germany W. B. Burton, Charlottesville, VA, USA and Leiden, The Netherlands M.A. Dopita, Canberra, Australia A. Eckart, K¨oln, Germany T. Encrenaz, Meudon, France M. Harwit, Washington, DC, USA R. Kippenhahn, G¨ottingen, Germany B. Leibundgut, Garching, Germany J. Lequeux, Paris, France A. Maeder, Sauverny, Switzerland V. Trimble, College Park, MD, and Irvine, CA, USA R. E. Gershberg Solar-Type Activity in Main-Sequence Stars Translated by S. Knyazeva With 69 Figures and 17 Tables 123 Professor Roald E. Gershberg Crimean Astrophysical Observatory Crimea Nauchny 98409, Ukraine Dr. Svetlana Knyazeva Department of International Programmes Siberian Branch of RAS 17, Prosp. Akademika Lavrentieva Novosibirsk 630090, Russia Cover picture: Flare on AD Leo of 18 May 1965. (Gershberg and Chugainov, 1966) Library of Congress Control Number: 2005926088 ISSN 0941-7834 ISBN-10 3-540-21244-2 Springer Berlin Heidelberg New York ISBN-13 978-3-540-21244-7 Springer Berlin Heidelberg New York This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on mi- crofilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. 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Typesetting by the authors Final Layout: TechBooks India Cover design: design & production GmbH, Heidelberg Printed on acid-free paper SPIN: 10992593 55/3141/BM-543210 In memory of Solomon Borisovich Pikelner, a marvelous person and eminent astrophysicist Preface Over the last decades our insight into the realm of stars was deepened consid- erably by many outstanding discoveries: pulsars, neutron stars predicted in the 1930s; discrete X-ray sources, stellar systems with much stronger X-ray emission than optical luminosity; maser emission related to the early stage of stellar evolution and to some oscillating supergiants that have reached an advanced evolutionary stage; intense outflow of matter from hot stars, etc. Parallel to these discoveries, which are mainly due to progressing methods of astrophysical observations, the comprehension of many stellar events im- proved markedly. The spectra of supernovae were successfully interpreted and the hydrodynamic theory of flares on them was elaborated. The theory of stellar evolution in binary systems yielded the concept of mass exchange as a decisive factor of their evolution and gave a key to understanding the activity of a wide family of novae, symbiotic and cataclysmic systems. As a result of the development of the evolution theory of early spectral-type stars, oscillat- ing cepheids – the lighthouses of the Universe that were used to determine distances to different stellar systems – had become space-time references of the development of such systems. As compared to these impressive findings for extremely interesting but rather rare types of stars, which attract the attention of astronomers of all specialties and not only astronomers, the study of very faint and cool vari- able dwarfs – UV Cet-type flare stars – might seem too narrowly specialized. However, successful investigation of these stars changes essentially the gen- eral concepts of the “stellar kingdom”. During the last decades it became clear that, first, flare dwarfs make up the most abundant type of stars and, secondly, their activity, being of the same physical nature as solar activity, plays a major role among lower main-sequence stars. Among several tens of thousands of variable stars known today, only sev- eral hundreds are classified as UV Cet-type variables in the solar vicinity. However, the relatively low number is due to low luminosities, which prevent detection of the stars at large distances from the Sun. Indeed, variables of this type total about half of all stars within a distance of several parsecs from VIII Preface the Sun. One cannot be sure that all flare stars have been detected in this vicinity, only the lower limit has been estimated so far. Moreover, the study of the nearest stellar clusters has shown that flare stars form an overwhelming majority of populations of these systems. Finally, according to the spectral classification of all field red dwarfs, a significant part of M4 and later stars have strong chromospheres, which suggests their flare activity. Thus, one can state that 40 to 90% of all stars are UV Cet-type objects. In other words, if there was a detector capable of recording stars “apiece” without regard to their luminosity, the image of the Galaxy would essentially differ from nor- mal photos of extragalactic systems. In addition to hundreds of millions of practically constant stars of middle-spectral types, which determine the inte- gral luminosity of galaxies, tens of thousands of regularly oscillating cepheids and semiregular late supergiants, hundreds of novae stars that appear every year and grandiose flares of supernovae occurring once a decade, the detector first of all would find billions of UV Cet-type stars flaring irregularly with characteristic time intervals between flares of about one or several hours. The prevalence of flare stars in the stellar population not only raises general in- terest in this basic form of stellar matter, but also necessitates the revision of conventional notions about the dynamics of the interstellar medium, the ways of formation of its chemical composition, and the sources of high-energy particles. The similarity of the solar and UV Cet-type activity was noted in the early 1950s, but even in the late 1960s this fact was only an occasion for discussing one of many physical models of such variable stars. This situation was thoroughly considered in my first monograph “Flares on Red Dwarf Stars” (Gershberg, 1970a). However, further intense photometric, spectral, and radio- astronomical observations of UV Cet-type stars and impressive advances in the study of solar activity supported the idea on the identity of physical nature of active red dwarfs and the Sun, as well as an approach to these variables as objects with maximum surface magnetic activity (Gershberg and Pikelner, 1972; Mullan, 1975a). Many researchers of flare stars could not immediately accept the analogy with solar activity. But now that X-ray emission of strong stellar coronae and ultraviolet emission line spectra of transition regions have been directly detected, ultraviolet and X-ray radiation of stellar flares has been recorded, statistical studies of several thousand stellar flares have proved that energy and time characteristics of solar flares are commensurable with those of stellar flares, and the spectra of solar and stellar chromospheres have been theoretically calculated using identical programs, qualitative similarity of the general picture of solar and stellar spottedness has been detected, long-term cycles of stellar activity similar to the 11-year solar cycle have been detected in observations of stellar chromospheres, and very young and very old G dwarfs have enabled reconstruction of the evolution of solar activity in the past and predictions for the future billions of years – the thesis about physical identity between the solar activity and that of red dwarf stars sounds trivial. However, this “trivial statement” acquires increasingly constructive substance. Preface IX Indeed, the results of high time, spectral, and spatial resolution observa- tions of active processes on the solar surface are initial data for the working models of transient flares, dark spots, active regions, coronal loops, and other local stellar structures. On the other hand, flares on UV Cet-type stars are accompanied by an energy release sometimes exceeding that of the strongest solar flares by 100–1000 times. This ensures a strict fidelity criterion of the- oretical constructs for solar flares: a theory of solar flares is correct only if, under the appropriate change of essential parameters, it can present 2–3 or- ders of magnitude stronger and shorter flares than the strongest and shortest flares on the Sun. Finally, the dependences of the intensity of permanent chromospheric emission and thermal radiation from the corona on rotational velocity and stellar age are initial experimental data for constructing the the- ory of the magnetism of stars with convective envelopes, in general, and the evolutionary theory of solar activity in particular. In essence, the statement about physical identity between the activity of the Sun and red dwarf stars underlies quickly developing topics of solar-stellar physics. By now, general outlines of evolutionary and physical bases of the activity of medium- and low-mass stars have been clarified: the initial angular momen- tum of a protostellar cloud produces a rotating star → in a gravitationally bound gaseous sphere, rotation becomes differential and