H. M. Van Horn

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. 39 references.

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. 82 references.

Dense astrophysical plasmas.

Degenerate bodies composed primarily of dense hydrogen and helium plasmas range from giant planets to the so far hypothetical brown dwarfs. More massive objects begin their lives as nondegenerate stars and may end as white dwarfs, composed primarily of carbon and oxygen, or as neutron stars, with solid crusts of iron or heavier elements and cores of neutron matter. The physical properties of dense plasmas that are necessary to construct theoretical models of such degenerate stars include the equation of state, transport properties, and nuclear reaction rates.

Neutron star evolutionary sequences

We present detailed numerical calculations of the evolution of neutron stars that are cooling through the range of central temperatures from about 10/sup 10/ to 10/sup 7/ K. The calculations are solutions of the full set of central relativistic equations that describe the evolution of a spherical star. We have employed the best current expressions for transport processes and neutrino emission rates and have been particularly careful in our treatment of the thermal properties of neutron star matter. Specifically, we have included the effects of nucleon superfluidity in the inner crust and core, and we have constructed models with and without a pion condensate at high densities. We have also included a variety of thermal contributions due to nuclei in the inner crust, and in some sequences we have followed the crystallization of the crust in order to investigate the consequences of this process for the evolution.

Micropulses, drifting subpulses, and nonradial oscillations of neutron stars

Rotating, magnetized neutron stars can support a rich variety of oscillation modes. The periods of the modes are estimated and compared with periodicities found in the pulsars. The millisecond time scale quasi-periodic micropulsations observed in PSR 2016+28 and PSR 1133+16 can be identified as nonradial l=0 or 1 p-mode oscillations, if mode-beating or internal noise broadening can account for the apparent rapid variability of amplitude and frequency. Torsional oscillations of the neutron star crusts have periods in the range of the pulsar subpulses; and rotational splitting of modes with l=2, 3, or 4 can account qualitatively for subpulse drift. Long-period Alfven modes, as well as Tkachenko oscillations, should be excited by pulsar glitches, and postglitch data may contain direct evidence of these modes.

Oscillation spectra of neutron stars with strong magnetic fields

The effects of strong frozen-in vertical magnetic fields on nonradial oscillation spectra in neutron stars are investigated theoretically, focusing on the surface layers near the polar cap of a cylindrically symmetric neutron-star model with shear-supporting crust and molten-crust oceans. The pulsation equations are derived; analytical estimates are obtained; and the results of numerical experiments are presented in tables and graphs. Significant modifications in the frequencies and displacements of the modes are found when a magnetic field is present: Alfven-like g modes (designated magneto-gravity), pseudotoroidal a modes with periods less than 100 ns for a 1-TG field, p-mode displacements almost totally parallel to the field, and a mode spectrum for periods of 100 microsec or more comprising only t, s, and p modes at 1 TG. 42 references.

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.

Magnetic field evolution in white dwarfs

The evolution of magnetic fields in a star experiencing radial contraction and cooling into a 0.6 solar mass white dwarf phase is examined analytically. An MHD equation is defined for a radially contracting star with a weak poloidal field that does not disturb the structure or evolution of the star. The distribution of conductivity within the star is approximated. A Newton-Raphson finite difference code is used to track the evolution of the star by iterative solution of the radial equation. A deconvolution algorithm is defined to track the decay eigenmodes. The central conductivity of a star increases rapidly, decreasing the decay time scale beyond the age of the star. Dominance of successively lower eigenmodes can cause multiple reversals in the surface field. 43 references.

Crystallization of a classical, one-component coulomb plasma

Abstract A calculation of the melting parameter φm introduced by Brush et al. is given which is exact within the uncertainties in the Lindemann constant. The calculated result is φm = 170 ± 10.

The pulsation properties of DB white dwarfs - A preliminary analysis

We report preliminary results of a numerical investigation of the nonradial g-mode pulsation properties of evolutionary DB white dwarf models. We have solved the fully nonadiabatic equations for modes corresponding to spherical harmonic index l = 1 through 3. For each of the sequences of models we have examined (M/sub asterisk/ = 0.6 M/sub sun/; and helium layer masses of 10/sup -6/ M/sub asterisk/ and 10/sup -4/ M/sub asterisk/), we find a nonradial g-mode instability strip about 3000 K wide. For models with standard ML1 convection, this strip lies in the effective temperature range 19,000 K> or approx. =T/sub e/> or approx. =16,000 K. The boundaries of the instability strip are extremely sensitive to the assumed efficiency of convection, however, and for sequences with more efficient (ML3) convection, we find the instability strip to be in the range 29,000 K> or approx. =T/sub e/> or approx. = 26,000 K. Extrapolation of our calculations to 0.4 M/sub sun/ and 0.9 M/sub sun/ indicates that that the instability strip boundaries are insensitive to uncertainties in the total stellar mass. The most unstable modes have e-folding times of the order of days.