Showing papers on "White dwarf published in 1971"

Origins of cosmic rays

Abstract: Cosmic rays origin, discussing nuclear, electron and electromagnetic components, supernovae, pulsars, white dwarfs and gas motions in Galactic Center

Detection of circular polarization in a second white dwarf

Abstract: White dwarf G195-19 circular polarization measurements of continuum radiation in broad band, using electro-optically switched polarimeter

Evolution in close binary systems

Abstract: 3. A hypothesis that this situation is the result of a changeover in the role caused by transfer of mass between them can be made compatible with the observed absence of a transient phase in which the more massive star has reached the Roche limit first only if a mass transfer at a rate of 10~5 to 10~4 O /yr is limited to KH-IO5 years, and occurs with velocities generally less than 102 km/sec. This would, however, require the density of such temporary gas streams to be of the order of IO^-IO21 particles per cm3 (i.e., 10"6 to 10"3 g/cm3 for hydrogen), and optically completely thick. 4. The gas streams actually observed by spectroscopic means in many semidetached systems fall short of these requirements by several orders of magnitude; and any mass transfer by these would become noticeable only on a long (nuclear) time scale. 5. The existence of binary systems like Sirius or Procyon (consisting of a main -sequence star attended by a white dwarf) - in which very wide separation of both components largely precludes mass transfer - suggests that a loss of mass of their white dwarfs must have occurred with velocities sufficient to remove it altogether from the system - as it happens in Wolf-Rayet stars or novae. β. Another significant fact concerning close binaries in their post -main -sequence evolutionary stage is a virtually complete absence of such binaries with mass ratios close to one - in contrast with their relatively high frequency encountered on the main sequence, and in spite of an increased probability of discovery. The reason why these should become extinct in the post -main -sequence stage is unknown. 7. Close binaries of the W UMa type are much too numerous in space to be regarded as evolutionary descendants of detached main -sequence systems of comparable masses and dimensions. A hypothesis is, therefore, advanced that their origin was by contraction towards two preexisting mass centers in close proximity of each other - a contraction which became arrested (by the onset of hydrogen burning) at, or even beyond, their respective Roche limits - thus resulting in the formation of systems with properties as we see them today. 8. The observed excesses of photometric over dynamical ellipticities of W UMa-type systems point to the conspicuous gravity -darkening operative over their surfaces - not smaller than that to be expected for the case of radiative equilibrium in subsurface layers.

Discovery of periodic variations in the circular polarization of the white dwarf g195--19.

Abstract: Periodic variations in circular polarization of continuum radiation of white dwarf G195-19 from 3800 to 5400 A

The chemical evolution of the galaxy

Abstract: Galaxy chemical evolution, discussing stellar mass loss, supernova explosions, white dwarf formations, nucleosynthesis, cosmochronology, etc

Origin of Magnetic Fields in White Dwarfs and Neutron Stars

Abstract: THE origin of magnetic fields in degenerate matter is an interesting question at present because of the discovery of large magnetic fields in some white dwarfs1, 2 and because of the even higher fields of the order of the critical magnetic field Bc≡(m2c3/eh)=4.4×l013 G that are thought to exist in pulsars (accepting the rotating neutron star theory3). An equally intriguing question is why some—but not all4—white dwarfs are magnetic. The fact that some supernovae do not have associated pulsars might also lead one to speculate that perhaps the collapse of all supernovae does indeed result in a neutron star, but that all neutron stars are not magnetic.

Thermonuclear Detonations in Collapsing White Dwarf Stars

Abstract: Collapsing white dwarf stars (or degenerate cores) may occur in binary systems, in the formation of Type I supernovae or in the formation of pulsars. These collapsing configurations may explode their nuclear fuel (12C or16O) by the detonation wave mechanism. A combination of analytical and numerical models is used to investigate the formation of detonation waves. The tentative conclusion is that a detonation wave will form which will lead to the ignition of esentially all the fuel in such a collapsing star. This potentially explosive configuration will be strongly affected, however, by rapid beta processes which occur in the detonated matter and which should cause a fraction of the stellar mass to collapse toward a neutron star state. The nature and effect of such beta processes, which have not yet been incorporated in the dynamical calculations, are discussed.

15. cooling of white dwarfs

Abstract: A knowledge of the precise relationship between the age and luminosity of a white dwarf can in principle be used to determine the compositions of the white dwarfs in galactic clusters. To this end the assumptions in Mestel's theory of white dwarf cooling are critically reviewed, and the results of recent work aimed at relaxing these restrictions are briefly summarized. It is concluded that on the basis of current knowledge an accuracy of the order of 10 or 20 % in the age­ luminosity relation should be attainable.

Cooling of White Dwarfs

Abstract: A knowledge of the precise relationship between the age and luminosity of a white dwarf can in principle be used to determine the compositions of the white dwarfs in galactic clusters. To this end the assumptions in Mestel’s theory of white dwarf cooling are critically reviewed, and the results of recent work aimed at relaxing these restrictions are briefly summarized. It is concluded that on the basis of current knowledge an accuracy of the order of 10 or 20% in the age-luminosity relation should be attainable.

Low-mass white dwarfs and the cooling sequences in the Hyades cluster

Abstract: Evolutionary cooling sequences and lifetimes for low mass white dwarfs in Hyades cluster, considering near surface convection in models with hydrogen envelopes

New Spectroscopic Results on Subluminous Stars, V

Abstract: Determination of temperature and surface gravity by colors and hydrogen-line profiles have been carried out for hot halo stars. A narrow horizontal branch is found stretching to above 40000 K; the hot O subdwarfs show a nearly vertical sequence, dropping towards the hot white dwarfs.

Further Evidence for Collapsed Objects in Binary Star Systems

Abstract: IT would be of great interest to discover a collapsed object (neutron star or black hole) as one component of a binary star system. Trimble and Thorne1 have compiled a list of fifty candidate systems from Batten's2 catalogue. These are non-eclipsing single-line spectroscopic binaries in which the unseen companion has a mass estimated to be greater than 1.4 M⊙. These unseen companions are thus too massive to be non-rotating white dwarfs (although the possibility that they are rapidly, differentially rotating white dwarfs cannot be completely excluded as Ostriker and Bodenheimer3 have shown). In principle, some of these unseen companions could be neutron stars or black holes. In most cases, including all those to be considered here, the unseen companion is less massive than the observed primary star and can be a normal main-sequence star, which is not observed simply because it is considerably less luminous than the primary. Recently, however, Gibbons and Hawking4 have presented evidence that suggests that one or more of the systems on the Trimble and Thorne list contain collapsed objects. Their argument is as follows. Let the initial binary system consist of two stars with masses M1 and M2. Let the more evolved star (M1) eject some of its mass and collapse to form a neutron star or black hole with gravitational mass MN1. The ejected mass-energy can be in the form of a shell of matter or gravitational radiation, for example. If the initial orbit is circular and the mass loss can be treated as instantaneous and spherically symmetric, then the resultant binary system (M2, MN1) will have an orbital eccentricity given by (Note: the notation for the star masses above is not the usual notation for spectroscopic binaries.) Three of the seven systems from the Trimble and Thorne list with P 0.08. By a comparison with short-period double-line binaries Gibbons and Hawking concluded that the probability of this occurring by chance is only 8%. This suggests that in one or more of the three systems, the eccentricity arises from a sudden mass loss and that the unseen companion is a collapsed object rather than a main-sequence star.

Polarization of Variable Stars

Abstract: The changes in stellar polarization with time were first discovered in 1958 by GRIGORYAN (1958) for the red supergiant µ Cephei. In subsequent year BEHR (1959) gave a hint in a footnote to his catalogue that the emission-line B-type star α Cas seems to have variable polarization. However, several years had to pass before these discoveries were confirmed and the changes in polarization with time for the red giants and supergiants and for the emission-line B-type stars were firmly established, first by the observations made by SHAKHOVSKOJ (1963, 1964) and then by COYNE, GEHRELS (1967) and the writer (SERKOWSKI 1966, 1968). The changes in polarization with time were also found for the RV Tauri and T Tauri stars, R Coronae Borealis stars, novae, Wolf-Rayet stars, and white dwarfs. The rapid changes in polarization with time indicate that the polarization is not produced in the interstellar medium but in the vicinity of a star; such polarization is called intrinsic.

The Subluminous B-Type Star CD - 42°14462

Abstract: Observations of the tenth-magnitude, newly discovered peculiar B-type star CD -42 14462 are described. Photometrically this object shows a large ultraviolet excess, and its spectrum contains only broad and shallow absorption lines of H and He I. It may be related to the white-dwarf stars, and certainly it is one of the brightest of the subluminous stars now known. Key words: subluminous stars - white dwarfs