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Van Horn, H. M.

Publications and source records attributed to Van Horn, H. M..

17 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Magnetic field evolution in white dwarfs: The hall effect and complexity of the field

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.

Muslimov, A. G.↗
NASA NTRS1994

Dynamo generation of magnetic field in the white dwarf GD 358

On the basis of Whole Earth Telescope observations of the g-mode oscillation spectrum of the white dwarf GD 358, Winget et al. find evidence for significant differential rotation and for a time-varying magnetic field concentrated in the surface layers of this star. Here we argue on theoretical grounds that this magnetic field is produced by an alpha omega dynamo operating in the lower part of a surface convection zone in GD 358. Our argument is based on numerical solutions of the nonlinear, local dynamo equations of Robinson & Durney, with specific parameters based on our detailed models of white-dwarf convective envelopes, and universal constants determined by a calibration with the the Sun's dynamo. The calculations suggest a dynamo cycle period of about 6 years for the fundamental mode, and periods as short as 1 year for the higher-order modes that are expected to dominate in view of the large dynamo number we estimate for GD 358. These dynamo periods are consistent with the changes in the magnetic field of GD 358 over the span of 1 month inferred by Winget et. al. from their observations. Our calculations also suggest a peak dynamo magnetic field strength at the base of the surface convection zone of about 1800 G, which is consistent with the field strength inferred from the observations.

Markiel, J. Andrew↗
NASA NTRS1994

Global oscillation amplitudes excited by the Jupiter-comet collision

The energy released during the collision of fragments of comet Shoemaker-Levy 9 with Jupiter in 1994 July may excite a spectrum of global oscillation modes. We estimate the maximum amplitudes to which the p-modes, discontinuity modes, inertial modes, and r-modes can be excited by assuming that the full kinetic energy of the fragment, which we take to be 10(exp 30) ergs, is converted into the energy of each individual mode. We have used two realistics Jovian models as the basis for our estimates: one with and one without the predicted 'plasma phase transition' (PPT) of hydrogen. A density discontinuity in the planet's hydrogen-helium envelope is associated with the PPT. We find that high-frequency p-modes, with periods approximately less than 15 minutes, may be excited to sufficiently large amplitudes to be observable as Doppler shifts (velocity amplitudes approximately greater than serveral m/s) or temperature variations (delta(T) approximately greater than 0.01 K) at the planetary surface. Inertial modes may also be observable. If the PPT exists in Jupiter, inertial modes with periods approximately 8 hr or approximately 2.2 days trapped in the surface region of the planet, above the PPT, may be detectable as temperature fluctuations of order delta(T) approximately 0.01 K. Inertial modes with periods of order 8-8.5 hr appear to be particularly strongly excited if the PPT exists. If the PPT does not exist in Jupiter, intertial modes with periods approximately 8-8.5 hr have much lower amplitudes. In this case, inertial modes with periods longer than approximately 18 hr may produce temperature fluctuations of order delta(T) approximately 0.01 K. Discontinuity modes associated with the PPT and r-modes unfortunately may not reach observable amplitudes.

Lee, U.↗
NASA NTRS1992

Pycnonuclear reaction rates for binary ionic mixtures

Through a combination of compositional scaling arguments and examinations of Monte Carlo simulation results for the interparticle separations in binary-ionic mixture (BIM) solids, we have derived parameterized expressions for the BIM pycnonuclear rates as generalizations of those in one-component solids obtained previously by Salpeter and Van Horn and by Ogata et al. We have thereby discovered a catalyzing effect of the heavier elements, which enhances the rates of reactions among the lighter elements when the charge ratio exceeds a critical value of approximately 2.3.

Ichimaru, S.↗
NASA NTRS1992

Inertial mode oscillations of Jupiter

The properties of the inertial modes of Jupiter are investigated using several different models for that planet. The inertial modes are rotationally induced oscillation modes for which the kinetic energy generally dominates the potential energy of oscillation. The frequency spectrum of the inertial modes mainly depends on the density stratification of Jupiter and sensitively reflects the existence or nonexistence of density discontinuities in the interior. A particularly interesting consequence of the calculations is that observations of the inertial oscillations of Jupiter may enable us to determine whether or not the dense hydrogen in the envelope of this planet undergoes the plasma phase transition (PPT). This may provide an astrophysical test of current theories of the thermodynamic properties of dense hydrogen. It is also found that the discontinuity modes associated with the PPT have observable amplitdues at the surface. These modes may provide an independent test of the interior structure of Jupiter.

Lee, Umin↗
NASA NTRS1992

Determining the coherence of micropulses

A technique based on analysis of the cross spectra of single-use data from pulsars is described which can be used to measure the coherence properties of, and search for coherent periodicities in, pulsar signals. Several simulated pulsar models are constructed and analyzed to test the technique. From an analysis of 2000 consecutive pulses from PSR 2016 + 28 it is concluded that the micropulse coherence time of 430 MHz is less than the spin period for this object. This could could be consistent with a coherent model if the random phase jitter in such a model is 35 percent or more of the micropulse quasi-periodicity. The subpulse separation of roughly 10 ms is coherent across 4-5 pulse periods and is reasonably consistent with a model of subpulse drift that allows for variation about some mean drift rate. A two-component shot-noise model can reproduce much of the structure seen in the individual pulses as well as the spectra of PSR 2016 + 28.

Strohmayer, T. E.↗
NASA NTRS1992

Effects of Fe/C phase separation on the ages of white dwarfs

The energy release associated with the phase separation of Fe from C in a predominantly C white dwarf is calculated. The total gravitational-plus-internal energy differences between models of homogeneous compositions and those wth Fe-enriched cores are computed. In the unlikely case where the core is pure Fe, a substantial extension of the white dwarf cooling times is found, even with the small cosmic abundance of this element. For the more realistic core compositions that result if the Fe/C phase diagram is either of the spindle or of the azeotropic type, the energy release is still sufficient to prolong the cooling times by about 0.6 Gyr, comparable to that produced by C/O phase separation. Phase separation is found to produce an appreciable 'bump' in the luminosity function, although not one large enough to exceed the observational errors at low luminosities.

Xu, Z. W.↗
NASA NTRS1991

The shear modulus of the neutron star crust and nonradial oscillations of neutron stars

Shear moduli are calculated for bcc crystalline and rapidly quenched Coulomb solids produced by the Monte Carlo simulation method. The shear moduli are calculated up to the transition temperature and include the effects of thermal fluctuations. An effective shear modulus appropriate to an approximate 'isotropic' body is introduced. It is found that the values of the 'average shear modulus' for the quenched solids remain about the same as those for the corresponding bcc crystals, although the individual shear moduli of the former, disordered solids deviate considerably from the cubic symmetry of the latter. These results are applied to analyses of neutron star oscillations. It is found that the periods of the two interfacial modes are increased by about 10 percent compared to previous results, and that s-mode periods are increased by about 30 percent. The periods of the f and p modes are hardly affected at all. The surface g-mode periods are not greatly affected, while the t-mode periods are increased by 20-25 percent.

Strohmayer, T.↗
NASA NTRS1988

Nonradial oscillations of neutron stars

Linear, adiabatic, Newtonian, nonradial pulsation analyses have been performed for finite-temperature neutron star models with a fluid core, solid crust, and thin surface fluid 'ocean', including the effects of the neutron star crust. The pulsation equations are considered, including the spheroidal and toroidal modes. A local analysis is performed to provide information about the pulsation modes in the short-wavelength limit. Numerical calculations are made on the mode spectrum and systematic properties. Damping mechanisms are investigated, including gravitational radiation damping, neutrino emission damping, electromagnetic radiation from an oscillating stellar magnetic field, nonadiabatic effects, and internal friction and viscosity.

Mcdermott, P. N.↗
NASA NTRS1987

An independent method for determining the age of the universe

An age of 9.3 + or - 2.0 Gyr is derived for the Galactic disk on the basis of comparisons between the sudden drop in the observed luminosity distribution and theoretical evolutionary white dwarf models and allowance for a mean prewhite-dwarf lifetime of 0.3 Gyr. To obtain the age of the universe, the time between the big bang and the first appearance of stars in the Galactic disk is added. The age of the universe is estimated to be 10.3 + or - 2.2 Gyr.

Winget, D. E.↗
NASA NTRS1985

Accretion disk oscillations - A local analysis in a disk of finite thickness

Two types of oscillations are observed to occur in dwarf novae: 'coherent' and 'quasi-periodic' oscillations. These may be associated with the pulsation of the white dwarf or the accretion disk components of the dwarf nova. Here a local (short-wavelength) analysis is utilized to study the oscillation of a self-consistent, two-dimensional model of an accretion disk. The linearized equations describing adiabatic, inviscid, nonaxisymmetric oscillations are used to derive a fifth-order algebraic equation for the (complex) pulsation frequency of the disk. The solutions of this equation for various values of the wavevector k reveal that the disk is capable of supporting (1) a pair of high-frequency acoustic modes (p-modes); (2) a pair of intermediate-frequency modes which may share the characteristics of internal gravity waves (g-modes) and inertial waves; and (3) a mode associated with a dynamical instability (purely imaginary frequency). The role played by the shear in determining the stability or instability of these modes is also considered. Finally, the global oscillation frequencies of the disk are discussed.

Carroll, B. W.↗
NASA NTRS1984

Electromagnetic damping of neutron star oscillations

A simple model of magnetic field perturbations driven by neutron star oscillations is used to estimate the electromagnetic power radiated by g-modes and torsional oscillations. The calculation assumes that the neutron star has a frozen-in magnetic field which is perturbed by the oscillatory motions of the surface. The disturbances propagate into the vacuum as outgoing electromagnetic waves. The relative effectiveness of Joule heating of the neutron star crust by pulsation-induced electric currents is estimated. It is concluded that electromagnetic damping is the dominant energy dissipation mechanism for quadrupole g-mode oscillations of neutron stars. For dipole spheroidal modes, both electromagnetic radiation and Joule heating are important, and there is no gravitational radiation emitted by these modes.

Mcdermott, P. N.↗
NASA NTRS1982

Neutron star evolutionary sequences

Detailed numerical calculations which are solutions of the full set of general relativistic equations describing the evolution of a spherical star are presented, for the case of the evolution of neutron stars that are cooling over the central temperatures range of 10 to the 10th to 10 to the 7th K. The effects of nucleon superfluidity in the inner crust and core are included, and models are constructed with and without a pion condensate at high densities. It is found that the localized neutrino cooling which dominates the early evolution of neutron stars is so rapid that heat transport within the star cannot keep pace, and temperature distribution is not isothermal. The residual contraction of the neutron star during the early cooling phase contributes little to the heat budget of the star, and most of the gravitational energy released raises the Fermi energy of the degenerate nucleons. It is concluded that since calculations with and without pion condensate are consistent with the upper limits of current observations, these are not sufficient in distinguishing between the various models of neutron star cooling.

Richardson, M. B.↗
NASA NTRS1982

The Schweizer-Middleditch star - Not a stellar remnant of SN 1006

The blue star found by Schweizer and Middleditch (1980) near the center of SN1006 cannot be a stellar remnant of that supernova, unless it is considerably more exotic than is justified by existing observational data. Conventional stellar evolution theory shows that the time to cool to the observed effective temperature is approximately one million years, far in excess of the 975 year age of the supernova remnant.

Savedoff, M. P.↗
NASA NTRS1976

A single-star interpretation of Nova Cygni 1975

Observational evidence has revealed that Nova Cygni 1975 is not a typical nova event. We suggest that Nova Cygni is to be understood, not in terms of a canonical close binary system, but essentially as a single star undergoing a thin-shell thermonuclear explosion.

Starrfield, S.↗
NASA NTRS1976

On the white dwarf HZ 43 as an extreme-ultraviolet source

It is confirmed by atmospheric modeling that both the visual continuum and the XUV spectrum (60 to 600 A) of HZ 43 can be fitted self-consistently with the emergent flux from a stellar atmosphere having an effective temperature of about 125,000 K and log g of at least 7. An examination of theoretical white-dwarf cooling sequences and estimates of the stellar luminosity indicate that the parameters of HZ 43 are consistent with a hot white dwarf of 0.6 to 1.2 solar masses. The age is estimated to be as short as 100,000 years, raising the possibility that the star may have ejected a planetary nebula or may be surrounded by an expanding H II region. It is concluded that the detection of a planetary nebula shell or an H II region around HZ 43 may provide an astrophysical test of the direct neutrino-electron interaction.

Durisen, R. H.↗