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  • ASTROPHYSICS  (9)
  • ASTRONOMY  (1)
  • 1
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    Magnetic field evolution in white dwarfs: The hall effect and complexity of the field (1995)
    In:  Other Sources
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    Publikationsdatum: 2019-08-28
    Beschreibung: We calculate the evolution of the magnetic fields in white dwarfs, taking into account the Hall effect. Because this effect depends nonlinearly upon the magnetic field strength B, the time dependences of the various multipole field components are coupled. The evolution of the field is thus significantly more complicated than has been indicated by previous investigations. Our calculations employ recent white dwarf evolutionary sequences computed for stars with masses 0.4, 0.6, 0.8, and 1.0 solar mass. We show that in the presence of a strong (up to approximately 10(exp 9) G) internal toroidal magnetic field; the evolution of even the lowest order poloidal modes can be substantially changed by the Hall effect. As an example, we compute the evolution of an initially weak quadrupole component, which we take arbitrarily to be approximately 0.1%-1% of the strength of a dominant dipole field. We find that coupling provided by the Hall effect can produce growth of the ratio of the quadrupole to the dipole component of the surface value of the magnetic field strength by more than a factor of 10 over the 10(exp 9) to 10(exp 10) year cooling lifetime of the white dwarf. Some consequences of these results for the process of magnetic-field evolution in white dwarfs are briefly discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal (ISSN 0004-637X); 442; 2 Pa; p. 758-767
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  • 2
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    PG 1346+082 - An interacting binary white dwarf system (1987)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: PG 1346+082 is both a photometric and a spectroscopic variable, spanning the B-magnitude range 13.6-17.2. High-speed photometric data reveal rapid flickering in the low-state light curve. The system also shows spectroscopic variations, displaying broad shallow He I absorption lines at maximum light and a weak emission feature at He I (4471 A) at minimum light. Hydrogen lines are conspicuous by their absence. Is is concluded that PG 1346+082 is an interacting binary white dwarf system. Furthermore, because continuum fits to IUE high-state data suggest temperatures consistent with membership in the DB white dwarf instability strip, it is suggested that some of the photometric variations may arise from pulsations.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Stellar pulsation; Aug 11, 1986 - Aug 15, 1986; Los Alamos, NM; United States
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  • 3
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    Asteroseismology of the DOV star PG 1159 - 035 with the Whole Earth Telescope (1991)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: Results are reported from 264.1 hr of nearly continuous time-series photometry on the pulsating prewhite dwarf star (DPV) PG 1159 - 035. The power spectrum of the data set is completely resolved into 125 individual frequencies; 101 of them are identified with specific quantized pulsation modes, and the rest are completely consistent with such modal assignment. It is argued that the luminosity variations are certainly the result of g-mode pulsations. Although the amplitudes of some of the peaks exhibit significant variations on the time scales of a year or so, the underlying frequency structure of the pulsations is stable over much longer intervals. The existing linear theory is invoked to determine, or strongly constrain, many of the fundamental physical parameters describing this star. Its mass is found to be 0.586 solar mass, is rotation period 1.38 days, its magnetic field less than 6000 G, its pulsation and rotation axes to be aligned, and its outer layers to be compositionally stratified.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 378; 326-346
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  • 4
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    White dwarf stars and the age of the Galactic disk (1990)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: The history of the Galaxy is written in its oldest stars, the white dwarf (WD) stars. Significant limits can be placed on both the Galactic age and star formation history. A wide range of input WD model sequences is used to derive the current limits to the age estimates suggested by fitting to the observed falloff in the WD luminosity function. The results suggest that the star formation rate over the history of the Galaxy has been relatively constant, and that the disk age lies in the range 6-12 billion years, depending upon the assumed structure of WD stars, and in particular on the core composition and surface helium layer mass. Using plausible mixed C/O core input models, the estimates for the disk age range from 8-10.5 Gyr, i.e.,sustantially younger than most age estimates for the halo globular clusters. After speculating on the significance of the results, expected observational and theoretical refinements which will further enhance the reliability of the method are discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Royal Astronomical Society of Canada, Journal (ISSN 0035-872X); 84; 150-165
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  • 5
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    The white dwarf luminosity function - A possible probe of the galactic halo (1990)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: The dynamically inferred dark halo mass density, amounting to above 0.01 solar masses/cu pc at the sun's Galactocentric radius, can be composed of faint white dwarfs provided that the halo formed in a sufficiently early burst of star formation. The model is constrained by the observed disk white dwarf luminosity function which falls off below log (L/solar L) = -4.4, due to the onset of star formation in the disk. By using a narrow range for the initial mass function and an exponentially decaying halo star formation rate with an e-folding time equal to the free-fall time, all the halo dark matter is allowed to be in cool white dwarfs which lie beyond the falloff in the disk luminosity function. Although it is unlikely that all the dark matter is in these dim white dwarfs, a definite signature in the low-luminosity end of the white dwarf luminosity function is predicted even if they comprise only 1 percent of the dark matter. Current CCD surveys should answer the question of the existence of this population within the next few years.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 358; 164-169
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  • 6
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    Whole earth telescope observations of the white dwarf G29-38 - Phase variations of the 615 second period (1990)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: An extensive set of high-speed photometric observations obtained with the Whole Earth Telescope network is used to show that the complex light curve of the ZZ Zeti (DAV) star G29-38 is dominated by a single, constant amplitude period of 615 s during the time span of these observations. The pulse arrival times for this period exhibit a systematic variation in phase readily explained by light-travel time effects produced by reflex orbital motion about an unseen companion. The best-fit model to the observations indicates a highly eccentric orbit, a period of 109 + or - 13 days and a minimum mass of 0.5 solar mass for the companion.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 357; 630-637
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  • 7
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    Whole Earth Telescope observations of V471 Tauri - The nature of the white dwarf variations (1992)
    In:  Other Sources
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    Publikationsdatum: 2019-08-28
    Beschreibung: Time-series photometric observations of the binary star V471 Tauri were conducted using the Whole Earth Telescope observing network. The purpose was to determine the mechanism responsible for causing the 555 and 277 s periodic luminosity variations exhibited by the white dwarf in this binary. Previous observers have proposed that either g-mode pulsations or rotation of an accreting magnetic white dwarf could cause the variations, but were unable to decide which was the correct model. The present observations have answered this question. Learning the cause of the white dwarf variations has been possible because of the discovery of a periodic signal at 562 s in the Johnson U-band flux of the binary. By identifying this signal as reprocessed radiation and using its phase to infer the phase of the shorter wavelength radiation which produces it, made it possible to compare the phase of the 555 s U-band variations to the phase of the X-ray variations. It was found that U-band maximum coincides with X-ray minimum. From this result it was concluded that the magnetic rotator model accurately describes the variations observed, but that models involving g-mode pulsations do not.
    Schlagwort(e): ASTRONOMY
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 391; 2 Ju; 773-783
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  • 8
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    Constraints on the age and evolution of the Galaxy from the white dwarf luminosity function (1992)
    In:  Other Sources
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    Publikationsdatum: 2019-08-28
    Beschreibung: The white dwarf disk luminosity function is explored using observational results of Liebert et al. (1988, 1989) as a template for comparison, and the cooling curves of Wood (1990, 1991) as the input basis functions for the integration. The star formation rate over the history of the Galaxy is found to be constant to within an order of magnitude, and the disk age lies in the range 6-13.5 Gyr, where roughly 40 percent of the uncertainty is due to the observational uncertainties. Using the best current estimates as inputs to the integration, the disk ages range from 7.5 to 11 Gyr, i.e., they are substantially younger than most estimates for the halo globular clusters but in reasonable agreement with those for the disk globular clusters and open clusters. The ages of these differing populations, taken together, are consistent with the pressure-supported collapse models of early spiral Galactic evolution.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 386; 539-561
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  • 9
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    The exotic helium variable PG 1346 + 082 (1987)
    In:  Other Sources
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    Publikationsdatum: 2019-07-12
    Beschreibung: The results of an extensive study of the blue object PG 1346 + 082, which is both a photometric and a spectroscopic variable, are presented. The system spans the B magnitude range 13.6-17.2, is brighter than m(pg) of about 14.0 roughly 74 percent of the time, and has a photometric quasi-period of four to five days. The rapid photometric flickering at minimum light is identified as the signature of mass transfer in a binary sytsem, and the He I emission at minimum light as indicating the presence of an accretion disk. The inability to detect high-energy X-rays, the absence of the high-excitation lines of He II in the optical spectrum, and the complete absence of hydrogen in the spectrum show that PG 1346 + 082 is an interacting binary white dwarf system. This identification lends strong support to the interacting twin-degenerate interpretation for AM CVn. The outbursts of the system may be similar in physical origin to dwarf novae outbursts.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 313; 757-771
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  • 10
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    A variational approach to understanding white dwarf evolution (1989)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: A variational approach is used to map out the effects that uncertainties in the theoretical model parameters have upon the derived ages near the observed cutoff in the white dwarf luminosity function. Two representative sequences are assessed, simulating a white dwarf with a 50/50 C/O mixture in the core and an outer helium layer and a white dwarf with a C/O convective overshooting profile. The differential effects that the variation of selected model input quantities has on the ages are reported, and it is concluded that internal theoretical uncertainties are small and getting smaller with time, and the results underscore the power of using the observed white dwarf luminosity function for studying the history of star formation in the Galaxy.
    Schlagwort(e): ASTROPHYSICS
    Materialart: IAU Colloquium; Aug 15, 1988 - Aug 19, 1988; Hanover, NH; United States
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