Showing papers on "White dwarf published in 1975"

Circumstellar Envelopes and Mass Loss of Red Giant Stars

Abstract: The necessary incidence of mass loss from stars in advanced stages of evolution was recognized many years ago. Stars with masses above the white-dwarf limit m = 1.4m⊙ cannot stabilize themselves after exhaustion of the various sources of nuclear fuel and will finally collapse, releasing an enormous amount of energy in a supernova explosion. Supernova rates, however, seem to be considerably lower than death rates of stars in the mass range above 1.4 m ⊙. On the other hand, the occurrence of nonbinary white dwarfs in the Hyades shows that stars with m ≈ 2.5 m ⊙ have succeeded in losing a large portion of their original masses during some phase of evolution. Furthermore, most field white dwarfs seem to have masses in the range 0.4 to 0.8m ⊙, whereas their parent stars must have had masses of more than one solar mass.

Problems in stellar atmospheres and envelopes

Abstract: 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.

Evolution of crystallizing pure $sup 12$C white dwarfs

Abstract: We describe the first results of a quantitative exploration of white dwarf evolution for models incorporating an accurate dense plasma equation of state and a full treatment of partial ionization and convection in the envelope. We discuss in detail the results for a 1 M/sub sun/, pure $sup 12$C star. The cooing curve and luminosity function deviate appreciably from the behavior predicted by Mestel's cooling theory above log (L/L/sub sun/) approx. =-0.5 due to neutrino energy losses and below log (L/L/sub sun/) approx. =-3.5 due to Debye cooling. Crystallization occurs at GAMMAapprox. =160, and the effects produced by the release of latent heat are evident. The combined effect of increased heat capacity due to Coulomb interactions and release of latent heat during crystallization increases the stellar lifetime by nearly a factor of 3 in the phases preceding Debye cooling, as expected. Deep convective cooling appears less important than previously thought. Agreement between the theoretical luminosity function and the observational functions derived by Weidemann from the data of Luyten and of Eggen and Greenstein is generally good, and white dwarf lifetimes determined from the cooling curve are consistent with the ages of white dwarfs in clusters. Resolution of differences between themore » theoretical discovery function and the number of known white dwarfs at very high and low luminosities may provide new information concerning the distribution and origin of the white dwarfs. The direct effects of crystallization are too small to permit observational detection at present. Debye cooling may offer an indirect test, however, because its effect is dramatic and roughly composition independent. For this, unfortunately, an accurate observational luminosity function down to at least log (L/L/sub sun/) approx. =-5 appears needed. (AIP)« less

Red giants and supergiants with degenerate neutron cores

Abstract: A new type of stellar model is constructed. It is related to neutron stars as ordinary red giants are related to white dwarfs. Its external appearance is similar to that of an ordinary M supergiant, but its evolutionary lifetime is 10 times longer. Our models are constrained to be relativistic but nonrotating, to constrain a degenerate neutron core of mass 1 M⊙ and radius 10 km, surrounded by a nondegenerate, massive, diffuse envelope. The core and envelope turn out to be separated by a thin (~40 m) energy-generation layer. The envelope convects from this layer all the way out to the photosphere. The effective temperatures and radii are ~2700 K and ~1000 R⊙. Within a fairly narrow range of effective temperatures and radii, two families of models were found: "red giants" and "red supergiants" with luminosities and masses less than and greater than ~65,000 L⊙ and ~10 M⊙, respectively. The luminosity of a giant comes 97 percent from gravitational contraction and 3 percent from nuclear burning. That of a supergiant is 5 percent from gravitational contraction and 95 percent from hydrogen burning by nonequilibrium, hot CNO reactions. The CNO reaction products are convected directly from the hydrogen-burning shell out to the photosphere of the supergiant, where they should be observable.

G 29-38 and G 38-39. Two new large-amplitude variable white dwarfs

Abstract: G28-38 and G38-29 are shown to be luminosity-variable white dwarfs whose characteristics are similar to those of HL Tau-76. Power spectrum analysis shows that the light curves of both stars have a complex low-frequency structure that is variable with time. It is suggested that many but not all of the features in the power spectra of these two stars could result from nonradial pulsations in a rotating white dwarf.

Upper mass limits for stable rotating white dwarfs.

Abstract: Models of nonmagnetic axisymmetric differentially rotating zero-temperature white dwarfs are constructed and tested for stability. Upper mass limits, beyond which models become secularly and dynamically unstable to growth of nonaxisymmetric perturbations, are found at 2.5 solar masses and 4.6 solar masses, respectively. The upper mass limit for secular stability is practically the same for the two angular momentum distributions considered. Crude growth rates due to gravitational radiation and degenerate-electron viscosity are calculated for the secularly unstable models. Except for a narrow parameter range, gravitational radiation is the dominant destabilizing mechanism. For masses between 3.5 and 4.6 solar masses, the secular instability grows on a time scale of about 10 to 1000 years; for masses exceeding about 4.6 solar masses, the dynamic instability grows on a time scale of seconds. Implications of these results for mass-transfer binary star systems and degenerate stellar cores are discussed.

Molecules in Astrophysics

Abstract: The total mass of our Galaxy is 2 × 1011 M⊙. In the Galaxy more than 90 % of this mass is in the form of stars, and less than 10% represents the tenuous gaseous material located between the stars: the interstellar matter (ISM). The principal constituents of interstellar matter are gas and fine dust particles. The qas consists mainly of hydrogen and helium, with an approximate ratio by mass of H: He: (all heavier elements) = 70:28:2. Dust accounts for about 1 % of the mass of the ISM. The raw material for the formation of young stars is supplied by the ISM. Conversely, stars can reach their final stable “white dwarf” stage only when their mass has decreased to less than 1.4 M⊙. During phases of extensive mass loss, through stellar winds, novae, super novae and planetary nebulae, stars expell mass into interstellar space. Thus, the ISM consists not only of matter left over from the formation of stars but also of matter that was processed through the interior of stars. Obviously stars and the ISM are not two separate entities but are very closely coupled through the cosmic recycling process of star formation and subsequent evolution.

What stars become supernovae

Abstract: This paper assembles a variety of empirical lines of evidence on the masses and stellar-population types of stars that trigger supernova (SN) explosions. The main theoretical motivations are to determine whether type I supernovae (SN I) can have massive precursors and whether there is an interval of stellar mass between the masses of precursors of pulsars and white dwarfs that is disrupted by carbon detonation. Statistics of stellar birthrates, SN, pulsars, and SN remnants in the Galaxy show that SN II (or all SN) could arise from stars with masses greater than about 12 to 49 solar masses. Several methods of estimating the masses of stars that become white dwarfs are consistent with a lower limit of about 5 solar masses, so carbon detonation may be avoided. Studies of the properties of galaxies in which SN occur, and their distributions within galaxies, support the usual views that SN I have low-mass precursors and SN II have massive precursors. The restriction of known SN II to Sc and Sb galaxies is shown to be statistically consistent with massive stars in other galaxies also dying as SN II. Possible implications of the peculiarities of some SN-producing galaxies are discussed.

White Dwarfs: Composition, Mass Budget and Galactic Evolution

Abstract: Considerable progress has been made in our understanding of white dwarfs during the past decade thanks to many new and excellent observations as well as to multiple efforts of theoretical interpretation.

Period variation in the white-dwarf eclipsing binary bd +16 516.

Abstract: Photometric observations of eclipse contacts over the past 4.7 years (3300 cycles) are discussed, revealing systematic variations in the period which suggest active mass transfer in progress. The peried is found to have increased and decreased over this interval, and current models are not adequate to account for such variations when one component is a white dwarf. Some new and improved data for the system are also discussed. Key words: eclipsing binary star - white dwarf- photometry

The detection of an Halpha Zeeman pattern in the cool magnetic white dwarf G 99-47.

Abstract: An apparent H$alpha$ absorption feature has been detected in image tube scans of the cool magnetic white dwarf G99-47, previously classified as DC (featureless). The broad approx.2.5 A EW line is centered some 21 A shortword from the normal H$alpha$ position. This lines is interpreted as the $pi$ component of H$alpha$-shifted by the quadratic Zeeman effect in a surface field of approx.15x10$sup 6$ gauss. This interpretation is supported by the detection of the characteristic circular polarization of the sigma components at +-350 A in data obtained with the multichannel spectrophotometer. Both results are fitted well by a simple, centered dipole model with a mean longitudinal field of 5.6x10$sup 6$ gauss. However, the longitudinal field strength indicated from the continuum circular dichroism theory is about a factor of four smaller. The strength of the H$alpha$ absorption and absence of other detectable Balmer and Ca ii lines are consistent with G99-47 being a very cool, metal-poor DA star. (AIP)

High-speed spectroscopy of the ultrashort-period variable AM Canum Venaticorum (=HZ 29)

Abstract: The ultrashort period variable AM CVn possesses a most unusual spectrum which may support an interpretation of the object as an accreting, semidetached binary white dwarf system.

Masses for Vela X-1 and other X-ray binaries.

Abstract: A detailed analysis of the optical light curve and other previously known observations of the X-ray binary Vela X-1 (3U 0900--40) is reported. All constraints are consistent only with a projected orbital velocity of the X-ray source of between 200 and 300 km s$sup -1$. The allowed ranges of the masses and other parameters is estimated. If the mass of the X-ray source is less than the Chandrasekhar limit for white dwarfs, then the optical star must have an unusually small mass-to-radius ratio for its spectral type. The importance of the X-ray eclipse duration in determining binary parameters is emphasized, and some new results are presented from analyses of the observations of 3U 1700--37 and other X-ray binaries. (AIP)

Strong magnetic fields in white dwarfs

Abstract: In the past year, our understanding of the strength and structure of magnetic fields in white dwarfs has been considerably advanced by studies of three different examples. GD 90 is a white dwarf with a hydrogen atmosphere that clearly shows the Zeeman structure characteristic of a field of 5 X l o 6 G.' HP is resolved into a triplet with Ah= k60 A, whereas at H6, the quadratic Zeeman interaction is dominant and gives a doublet a t 4091 and 4064 A. The u components of the Zeeman patterns in HP and Hy are found t o be circularly polarized, which indicates that the field has a component directed along the line of sight. There is evidence for an additional stronger and less homogeneous field, as would be expected if the dominant 5 X lo6 G region were at the equator of a dipolar field. Another magnetic white dwarf that exhibits Zeeman structure, though not resolved into individual components, is G 99-37.z In the polarization spectrum of this star, there is a prominent feature of alternate positive and negative circular polarizations centered on the molecular CH band at h4300 A. The magnetic circular dichroism of this band, X2n A Z A, has been calculated, taking into account the magnetic perturbation to first order of both energy eigenvalues and eigenfunctions of the CH molecule. From these results, stellar profiles of absorption and polarization across the band are obtained for different fields, taking into account the effect of radiative transfer. A good fit t o the observations is obtained, with an average longitudinal field I I = 3.6 X 1 O6 G. An estimate of the strength and wavelength dependence of continuous circular polarization made for this field strength, assuming Heas the dominant opacity, is in agreement with the observations. The third object in which the field strength can be deduced is Grw + 70'8247. This was the first magnetic white dwarf t o be discovered3 and now appears t o have the strongest magnetic field.4 The spectrum of circular polarization is interpreted in terms of a helium atmosphere. The continuous Heopacity gives circular polarization that decreases monotonically t o the red, whereas boundfree opacity of He gives a strong broad feature centered at 3400 A. A mean longitudinal field of 4 X lo7 G gives the general features of the observed wavelength dependence. The absorption feature a t 5855 A in this star is interpreted as the 71 component of the 5876 line of He, quadratically shifted in a field region of 1.5 X lo7 G . The observed characteristics of this star thus imply a complex field structure, with surface fields that probably reach in excess of los G. Five other magnetic white dwarfs are known, but accurate field determinations have not been made in them, because spectral features are either unidentified or absent.

Spectra of white dwarfs.

Abstract: Results of a spectroscopic survey of white dwarf candidates from the Lowell Observatory lists are reported. UBV photometry is shown to be effective in enhancing the detection rate of DB white dwarfs. The spectrum of GD 40 contains lines of Ca ii H and K in addition to those of He i; it is the first DB to show lines of any element other than helium. (AIP)

Novae, supernovae, and neutron sources

Abstract: The evolution of thermonuclear runaways is examined in two models of white dwarfs with extreme enhancements of C-12 in their envelopes to test the predictions of Hoyle and Clayton (1974) that novae will result from such stars and a large neutron flux will be produced. In agreement with these predictions, it is assumed that the large amount of C-12 is due to the accretion of hydrogen-rich material from a disk surrounding a carbon-oxygen white dwarf. The evolution of the two models is described in detail, and the results suggest that accretion of hydrogen-rich material will always result in a thermonuclear runaway, although mass ejection will not occur unless CNO nuclei are enhanced. It is noted that one model produces a substantial neutron flux for a short time which is sufficient to drive an intermediate neutron-capture process.

Harmonic analysis of the light of DQ Herculis

Abstract: SEVERAL classes of interacting binary stars, such as the dwarf novae, nova-like variable stars and X-ray stars, present many examples of rapid periodic optical variations. DQ Herculis (nova 1934) is, however, the only nova known to show such variations. They are of low amplitude, generally less than 0.04 mag semi-amplitude, and have a period of 71.065459 s (refs 1–3). We report here a study of the harmonic components of the 71-s periodicity, and show that the results place severe limits on the available models of the variation.

Hot vibrating white dwarf models of pulsating X-ray sources

Abstract: The hypothesis that pulsating X-ray sources could be vibrating white dwarfs is investigated. The masses are calculated for several white-dwarf models having various modes of vibration which have the observed eigenfrequencies of the Centaurus X-3 and the Hercules X-1 sources. If the hypothesis is correct, Cen X-3 is vibrating radially in the fundamental mode or not higher than the first two overtones, and its mass is in the range 0.7-1.2 solar masses. The Her X-1 source would be vibrating in a quadrupole P mode, and would have a mass in the range 1.1-1.25 solar masses.

Tidal Flows in Binary Systems

Abstract: The expressions of the tidal velocity in not very close binaries (double stars, the Sun and a planet, a planet and a satellite) are derived and applied in particular to white dwarfs and the giant planets of the solar system. The magnitude of the velocity on the surface of Jupiter is estimated to be about 0.5 cm s−1. In white dwarfs the velocities of the order of tens m s−1 may be encountered, and they can influence their evolution. The symmetry of the tidal flows is noted to be suitable for the magnetic field generation.