Bodo Baschek

Problems in Stellar Atmospheres and Envelopes

The Energy Flux of the Sun A Critical Discussion of Standard Values for the Solar Irradiance.- 1. Introduction.- 2. High Altitude Experiments.- 3. Discussion of the Results.- 4. Results of Measurements in the Far Ultraviolet.- References.- Model Stellar Atmospheres and Heavy Element Abundances.- 1. Introduction.- 2. The Temperature Stratification.- 3. The Gas and Electron Pressures.- 4. The Energy Distribution in the Continuum.- 4.1. The Balmer Discontinuity.- 4.2. The Ultraviolet Continuum.- 5. The Line Absorption.- 5.1. The Total Line Blanketing.- 5.2. The Metallic Line Absorption.- 5.3. Molecular Lines.- 5.4. The Hydrogen Lines.- 6. The UBV Colors.- 7. The Temperature Calibrations.- 8. The Bolometric Correction.- 9. Convection and Metal Abundances.- 9.1. Convective Instability.- 9.2. Convection Velocities, Microturbulence and Chromospheres.- 9.3. Influence of Convection on the Observed Energy Distribution of Stellar Spectra.- References.- Properties and Problems of Helium Stars.- 1. Introduction.- 1.1. Definitions.- 2. General Properties.- 2.1. List of Objects.- 2.2. Distribution on the Sphere and Velocities.- 2.3. H/He Ratio.- 3. Spectrum.- 3.1. Visual Spectrum.- 3.2. UV-Spectrum.- 4. Atmospheric Structure.- 4.1. Model Atmospheres.- 4.2. Synthetic Spectra and Atmospheric Parameters.- 4.3. Non-LTE Effects.- 5. Individual Obj ects.- 5.1. Extreme Helium Stars.- 5.2. Intermediate Helium Stars.- 5.3. O-Subdwarfs.- 6. Abundances.- 7. Evolution of Model Helium Stars and the (g, Teff) -Diagram.- 7.1. Main Sequences.- 7.2. Evolutionary Tracks.- 7.3. Lifetimes.- 8. Empirical (g, Teff) -Diagram.- 8.1. (g, Teff)-Classification and Masses.- 8.2. Observed Objects.- 9. Variability and Atmospheric Motions.- 10. Conclusion.- References.- Abundance Anomalies in Early-Type Stars.- 1. Introduction.- 2. Problems Related to the Determination of Abundance Anomalies.- 2.1. Definition of Abundances.- 2.2. Relative and Normal Abundances.- 2.3. Model Atmospheres of Peculiar Stars Compared to Normal Stars.- 3. The Population I Peculiar B Stars.- 3.1. The Major Groups of the Ap Stars.- 3.2. The Weak-Helium-Line Stars.- 3.3. Relationship of the Weak-Helium-Line Stars to the Silicon and Manganese Stars.- 3.4. Peculiar Early-B Stars.- 4. The CNO Stars.- 4.1. Properties of the CNO Stars.- 4.2. Element Abundances.- 4.3. Nature of the CNO Anomalies.- 5. The Population II B Stars.- 5.1. Classification.- 5.2. Evolutionary Status.- 5.3. Element Abundances.- 5.4. Discussion of the Abundance Anomalies.- 6. On the Origin of the Ap Phenomenon.- 6.1. Nuclear Processes.- 6.2. Non-Nuclear Processes.- 6.3. Removal of Surface Abundance Anomalies.- 6.4. Inferences from the Early-Type Peculiar Stars.- References.- A-Type Horizontal-Branch Stars.- 1. Introduction.- 2. Characteristics of A-Type Atmospheres.- 3. Observational Quantities.- 4. Field Horizontal-Branch Stars.- 5. Horizontal-Branch Stars in Globular Clusters.- 5.1. NGC 6397.- 5.2. NGC 6121 (M4).- 6. Chemical Composition and Mass-Luminosity Relation.- 6.1. Chemical Composition.- 6.2. Mass-Luminosity Ratio.- References.- White Dwarfs: Composition, Mass Budget and Galactic Evolution.- 1. Introduction.- 2. The Atmospheres of White Dwarfs.- 2.1. White Dwarfs with Hydrogen-Rich Atmospheres.- 2.2. White Dwarfs with Hydrogen-Deficient Atmospheres.- 3. Composition of Interiors and Envelopes. White-Dwarf Formation.- 4. Interpretation of Atmospheric Composition Differences DA vs. Non-DA Stars.- 5. White Dwarfs: Mass Budget and Galactic Evolution.- References.- Herbig-Haro Objects and T Tauri Nebulae.- 1. Introduction.- 2. Observation of Herbig-Haro Obj ects.- 2.1. Occurrence and Apparent Structure.- 2.2. Variability.- 2.3. Reddening and Interstellar (Circumstellar?) Absorption.- 2.4. Spectra.- 2.5. Polarization and Relation of Herbig-Haro Objects to Infrared Sources.- 3. Theory and Theoretical Deductions from the Observations.- 3.1. Direct Interpretation of the Spectra.- 3.2. Theoretical Interpretation of the Observed Ionization and Excitation.- 3.3. Evolutionary Significance of Herbig-Haro Objects.- 4. The T Tauri Emission Nebula.- References.- Circumstellar Envelopes and Mass Loss of Red Giant Stars.- 1. Introduction.- 2. Circumstellar Absorption Lines.- 3. Dust and Molecules in the Circumstellar Envelopes of Red Giants.- 3.1. The Infrared Silicate Excess.- 3.2. Polarization.- 3.3. Microwave Emission from Molecules.- 4. The Dependence of Mass Loss on Basic Stellar Parameters.- 5. Consequences for Stellar Evolution.- References.- Cosmic Masers.- 1. Introduction.- 2. Observational Characteristics.- 2.1. OH Sources.- 2.2. H2O Sources.- 3. Radiative Transfer.- 3.1. General Relations.- 3.2. Unsaturated Masers.- 3.3. Saturation Effects.- 3.4. The Influence of the Infrared Lines.- 3.5. The Influence of Continuous Absorption.- 3.6. Velocity Fields.- 4. Polarization.- 5. Pumping Mechanisms.- 5.1. General Considerations.- 5.2. The OH Molecule.- 5.3. The H2O Molecule.- 6. Models.- 7. Discussion and Conclusion.- References.- Radio Emission from Stellar and Circumstellar Atmospheres.- 1. Introduction.- 2. Stellar Chromospheres and Coronas.- 3. Solar-Type Activity on Stars.- 4. Flare Stars: UV Cet et al.- 5. Radio Emission from Close Binaries.- 6. Radio Emission from Objects with Circumstellar Envelopes.- 7. X-Ray Stars as Radio Emitters.- 8. The Remaining Stellar Observations.- References.- Line Formation in Turbulent Media.- 1. Introduction.- 2. General Structure of the Problem.- 3. Line Formation in Discontinuous Velocity Fields.- 4. Line Formation in Media with Continuous Velocity Fields.- 5. Solution of the Generalized Transfer Equation.- 6. An Approach to NLTE Line Formation in Turbulent Media.- 7. Concluding Remarks.- References.- Index of Astronomical Objects.

Analysis of the 3d64s(6D)6d subconfiguration of Fe I by laser-enhanced ionisation and emission spectroscopy

Laser-enhanced ionisation spectrometry has been used to confirm 36 levels within the highly-excited 3d64s(6D)6d subconfiguration in Fe I that have been established by grating spectrometry. The new levels are used to identify ~ 200 lines in the UV grating and Fourier transform spectra, most of which are also observed in the solar spectrum.

The Structure and Evolution of Stars

The significance of his diagram for investigating stellar evolution was clearly realized by H.N. Russell as early as 1913. But its deeper understanding, and thus a theory of stellar evolution founded on observational facts, became possible only in connection with the study of the innerstructure of stars. The older works oft. H. Lane (1870), A. Ritter (1878–89), R. Emden (the “Spheres of Gas” appeared in 1907) and others could be based only on classical thermodynamics. A.S. Eddington succeeded in combining these approaches with the theory of radiation equilibrium and with Bohr’s theory of atomic structure, which had in the meantime been formulated; his book “The Internal Constitution of Stars” (1926) gave the start signal for the whole development of modern astrophysics.

Classical Astronomy. The Solar System

We shall begin our treatment of astronomy with a historical overview of classical astronomy (Sect. 2.1) from ancient times up through the introduction of the heliocentric system by Nicholas Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei and their contemporaries, and the founding of celestial mechanics by Isaac Newton at the close of the 17th century. We then turn in Sect. 2.2 to the description of motions on the celestial sphere and the coordinate systems used to describe the positions of objects in the sky. In Sect. 2.3, we treat the motions of the Earth its rotation around its own axis and its revolution around the Sun, which make themselves evident as motions on the celestial sphere; in this section, we also take up the sidereal time scale. Following these preparatory remarks, we make the acquaintance of the objects of our Solar System beginning in Sect. 2.4 with the Moon, its motions, its phases, and with lunar and solar eclipses. In Sect. 2.5, we survey the motions of the planets, the comets, and other bodies in the Solar System, and the methods for determining distances between them. After reviewing the fundamentals of mechanics and the theory of gravitation, we discuss in Sect. 2.6 some of their applications to celestial mechanics and, in Sect. 2.7, to the calculation of the orbits of artificial satellites and space probes. Section 2.7 also contains a brief chronology of space-research missions. Finally, we treat the physical structure of the objects in the Solar System: the planets, their moons, and the asteroids in Sect. 2.8; and the comets, meteors, and meteorites in Sect. 2.9.

Interstellare Materie und Sternentstehung

Zwischen den Sternen des Milchstrasensystems fein verteilte Materie trat in den Gesichtskreis der Astronomen zuerst in Gestalt der Dunkelwolken, welche das Licht der hinter ihnen befindlichen Sterne durch Absorption schwachen und roten. Aber erst 1930 konnte R. J. Trumpler zeigen, das auch auserhalb der erkennbaren Dunkelwolken interstellare Extinktion und Verfarbung in der ganzen Milchstrase bei der photometrischen Messung von Entfernungen uber wenige hundert parsec keineswegs zu vernachlassigen sind. Schon 1922 hatte E. Hubble erkannt, das die galaktischen (diffusen) Reflexionsnebel (wie sie z. B. die Plejaden umgeben) durch Streuung des Lichtes relativ kuhler Sterne an kosmischen Staubwolken entstehen, wahrend in den galaktischen (diffusen) Emissionsnebeln interstellares Gas durch die Strahlung heiser Sterne zur Emission eines Linienspektrums angeregt wird. Daraufhin kam in den Jahren 1926/27 die Erforschung des interstellaren Gases rasch in Gang. Zwar hatte schon 1904 J. Hartmann die „stationaren“ Ca lI-Linien entdeckt, welche in den Spektren von Doppelsternen die Bahnbewegung nicht mitmachen, aber erst 1926 entwickelten A.S. Eddington von, der Theorie, O. Struve, J. S. Plaskett u. a. von der Beobachtung her die Vorstellung, das die interstellaren Ca II-, Na I-,... Linien in einer durch die Strahlung der Sterne teilweise ionisierten Gasschicht entstehen, welche die ganze Scheibe der Milchstrase erfullt und auch an deren Rotation teilnimmt. Auf der anderen Seite gelang 1927 I.S. Bowen die lange gesuchte Identifikation der „Nebuliumlinien” in den Spektren der Gasnebel als verbotene Ubergange in den Spektren von [011], [0111], [N II],..., und H. Zanstra entwickelte die Theorie des Nebelleuchtens. Erst etwa zehn Jahre spater erkannte man, das auch im interstellaren Gas — wie in den Sternatmospharen — der Wasserstoff das weitaus uber-wiegende Element ist. O. Struve und seine Mitarbeiter entdeckten mit Hilfe ihres sehr lichtstarken Nebelspek-trographen, das viele O- und B-Sterne von einer ziemlich scharf begrenzten Region umgeben sind, die in der roten Rekombinationslinie Hα des Wasserstoffs leuchtet. Hier mus der interstellare Wasserstoff also ionisiert sein. Die Theorie dieser H II-Regionen hat dann 1938 B. Stromgren entwickelt.

Entfernungen und Zustandsgrößen der Sterne

Wir beginnen mit der Diskussion des uns nachsten Sterns, der Sonne. In Abschn. 6.1 stellen wir ihre Daten zusammen, die haufig fur die Sterne als Einheiten verwendet werden, und lernen die Energieverteilung ihrer Strahlung aus der Photosphare kennen: ein Kontinuum mit zahlreichen Absorptionslinien.

The Cosmogony of the Solar System

We turn back now from the depths of interstellar space to our own Solar System, and the old question of how it came into existence. The daring thought that this question cannot be answered by the handing-down of ancient myths, but only through our own probing, was proposed in France as early as 1644 by Rene Descartes in his whirlpool theory. In Germany, still by 1755, Immanuel Kant had to publish the first edition of his “Allgemeine Naturgeschichte and Theorie des Himmels” anonymously, for fear of the (protestant) theologians; in it, he treated the origin of the Solar System for the first time “according to Newtonian principles”. Kant assumed a rotating, flattened primordial nebula, from which the planets and later their satellites were formed. The description offered independently somewhat later by S. Laplace in 1796 in his popular “Exposition du Systeme du Monde” was based on a similar hypothesis.

The Distances and Fundamental Properties of the Stars

We shall begin with a discussion of our nearest star, the Sun. In Sect. 6.1, we collect the data concerning the Sun, which are often used to define units for describing the properties of other stars, and introduce the energy distribution of the solar radiation from the photosphere: a continuum with numerous absorption lines.

The Physical Structure of the Objects in the Solar System

The study of the planets and their satellites, exploiting the possibilities of space research, has developed in recent years into one of the most interesting but also most difficult branches of astrophysics. In particular, understanding the observations has required all the available resources of physical chemistry and the Earth sciences (geology, mineralology, etc.).

Interstellar Matter and Star Formation

The matter which is finely distributed between the stars of the Milky Way at first came to the attention of astronomers in the form of dark clouds, which weaken and redden the light of those stars which are behind them, due to absorption and scattering. But it was only in 1930 that R. J. Trumpler was able to show that even outside the recognizable dark clouds, interstellar extinction and reddening are by no means negligible in the photometric determination of distances of a few hundred parsec throughout the Milky Way Galaxy. Already in 1922, E. Hubble had recognized that the galactic (diffuse) reflection nebulae (like the one which surrounds the Pleiades, for example) are due to scattering of the light from relatively cool stars in cosmic dust clouds, while in the galactic (diffuse) emission nebulae, interstellar gas is excited bythe radiation from hot stars and therefore emits line spectra. Following his observations, the investigation of the interstellar gas quickly gained momentum in the years 1926/27. The “stationary” Call lines had already been discovered in 1904 by J. Hartmann; they occur in the spectra of binary stars but do not show Doppler shifts corresponding to the orbital motion. Only in 1926 was an explanation developed, theoretically by A.S. Eddington, and based on observations by O. Struve, J. S. Plaskett, and others: the interstellar Ca II, Na I,... lines are produced in a gas layer which is partially ionized by the stellar radiation. This gas layer fills the entire disk of the Milky Way and participates in its rotation. About the same time, in 1927, I. S. Bowen succeeded in making the long-sought identification of the “nebulium lines” in the spectra of gaseous nebulae, finding that they are due to forbidden transitions in the spectra of [O II], [0 III], [N II],...; and H. Zanstra developed the theory of nebular luminescence. Only about ten years later was it recognized that in the interstellar gas, as in stellar atmospheres, hydrogen is the strongly predominant constituent. O. Struve and his coworkers discovered with the aid of their nebula spectrograph, which had great light-gathering power, that many O and B stars are surrounded by well-defined regions which fluoresce in the red hydrogen recombination line, Hα. Here, the interstellar hydrogen must thus be ionized. The theory of these H II regions was formulated in 1938 by B. Stromgren.