The Astrophysics of Emission-Line Stars (eBook)

Stellar Atmospheres and Formation of Emission Lines.- Stellar Spectra and Radiation Fields.- Dynamic Processes in Stellar Atmospheres.- Formation of Emission Lines.- Emission-Line Stars.- Early-type Emission-line Stars.- Late-Type Stars and Close Binaries.- Pre-main Sequence Stars.

"Preface (p. viii-ix)

Many types of stars show conspicuous emission lines in their optical spectra. These stars are broadly referred to as emission-line stars, but, in the past, they were considered a type of peculiar stars, because emission lines were thought to be an indication of behaviors "peculiar" from the normal stellar atmospheres. Prior to 1950s, early-type emission-line stars such as Wolf-Rayet stars, Be stars, and P Cygni stars were called the early-type peculiar stars. With the advance in theories of emission-line formation, the name "emissionline stars" has been widely adopted to include both early- and late-type stars, and the name of "peculiar-stars" has been only used for chemically peculiar stars.

Some stars that have no particular names as emission-line stars, such as cataclysmic variables and Mira variables, are also included in the category of emission-line stars. In closer examination most of stars on the Hertzsprung-Russell (HR) diagram show somehow evidence of emission lines formed in a less-developed form as in case of the Sun. In this book, however, we confine our examination of emission-line stars to the stars having strong lines in the optical region. In the later half of the twentieth century, the physics of emission-line stars has been surprisingly developed under collaborations between ground-based and space observations.

Wide wavelength observations have opened a new era of understanding the active stellar envelopes in various forms, such as stellar winds, accretion flows, flare activities, and binary interaction. In this book an attempt is made to outline the physics of emission-line stars that are widely located on the HR diagram. Particular attention is paid to the spectral analysis of emission lines mainly in the optical region. Although intended mainly for the use of graduate student and teachers of stellar astronomy, the present work should also provide a useful reference for practicing astronomers, particularly, for small-telescope users in institutions and public or private observatories.

For these observers, emission-line stars may be an attractive choice to observe/monitor by their mysterious and often violent variable behaviors. This book consists of a brief historical review in Chapter 1 followed by two major parts. In Part I, first two chapters review the basic concepts on the spectroscopic processes (Chapter 2) and gas dynamical processes (Chapter 3) in stellar atmospheres. Chapter 4 is devoted to the mechanisms of emission-line formation in static and moving envelopes and in nonthermal atmospheres. Readers who are already familiar with basic astrophysics can skip Chapters 2 and 3 and move to Chapter 4.

In Part II, broad overviews of emission-line stars are given in three chapters. Chapter 5 deals with the early-type emission-line stars (WR, Of, Oe, Be, LBV), mostly forming developed expanding envelopes. Chapter 6 yields the late-type stars (dMe, flare stars, Mira variables) and close binary systems (Algol, RS CVn, cataclysmic variables, symbiotic stars), where the nonthermal processes are prevailing. In Chapter 7 the pre-main sequence stars (RES and TTS) are considered as activities in the contracting phase of stellar evolution. Thus Part II as a whole will show an amazing variety of emission-line stars.

Though we have confined the topics to the stars in this book, there are numerous objects showing strong and active emission-line phenomena in and out of the Galaxy. This book is expected to be useful for these related fields. Considering remarkable progress in the field of the physics of emission line stars since submission of the manuscript, we have added a self-contained Supplement at the end of the book to bring it up to 2006."