M. P. Savedoff

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.

Electromagnetic damping of neutron star oscillations

Nonradial pulsations of a neutron star with a strong dipole magnetic field cause emission of electromagnetic radiation. Here we compute the power radiated to vacuum by neutron star g-mode pulsations and by torsional oscillations of the neutron star crust. For the low-order quadrupole fluid g-modes we have considered, we find electromagnetic damping to be considerably more effective than gravitational radiation. For example, a 0.5 M/sub sun/ neutron star with a core temperature approx.10/sup 7/ K has a g/sub 1/-mode period of 371 ms; for this mode were find the electromagnetic damping time to be tau/sub FM/approx.0.3 s, assuming the surface magnetic field strength of the neutron star to be B/sub 0/approx.10/sup 12/ gauss. This is considerably less than the corresponding gravitational radiation time tau/sub GR/approx.3 x 10/sup 17/ yr. For dipole g-mode oscillations, there is no gravitational radiation, but electromagnetic damping and ohmic dissipation are efficient damping mechanisms. For dipole torsional oscillations, we find that electromagnetic damping again dominates, with tau/sub EM/approx.5 yr. Among the cases we have studied, quadrupole torsional oscillations appear to be dominated by gravitational radiation damping, with tau/sub GR/approx.10/sup 4/ yr, as compared with tau/sub EM/approx.2 x 10/sup 7/ yr.

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

The visual continuum and the recently observed 60--600 A extreme-ultraviolet spectrum of the white dwarf HZ 43 can be fitted self-consistently with the emergent flux from a hot (T/sub eff/approx.125,000 K), high-gravity (log gapproximately-greater-than7) stellar atmosphere. If this interpretation is correct, then comparison with cooling sequences suggests that the mass of the white dwarf must exceed 0.6 M/sub sun/., and the age may be as short as 10/sup 5/ years. A planetary nebula or H ii region surrounding the star may thus be detectable. Detection of an ionized region around HZ 43 or extreme-ultraviolet observations of known planetary nuclei may afford a definite astrophysical test of the direct neutrino-electron weak interaction theory. Alternatively, the presence of hydrogen absorption lines at this high temperature suggests that differentially rotating models may have to be considered. (AIP)

Binary pulsar PSR 1913 + 16: model for its origin

The existing observational data for the binary pulsar PSR 1913 + 16 are sufficient to give a rather well-defined model for the system. On the basis of evolutionary considerations, the pulsar must be a neutron star near the upper mass limit of 1.2 solar masses (M�). The orbital inclination is probably high, i≥ 700, and the mass of the unseen companion probably lies close to the upper limit of the range 0.25 M� to 1.0 M�. The secondary cannot be a main sequence star and is probably a degenerate helium dwarf. At the 5.6-kiloparsec distance indicated by the dispersion measure, the magnetic dipole model gives an age of ∼4 x 104 years, a rate of change of the pulsar period of P ∼2 nanoseconds per day, and a surface magnetic field strength ∼⅓ that of the Crab pulsar. The pulsar is fainter than an apparent magnitude V∼+ 26.5 and is at least ∼80 times fainter than the Crab pulsar in the x-ray band. The companion star should be fainter than V ∼+ 30, and a radio supernova remnant may be detectable near the position of the pulsar at a flux level of ≤10 janskys. Important tests of this model will be provided by more accurate measurement of P and by a careful search for a faint supernova remnant.

Code for general relativistic stellar evolution

We describe a numerical method for the automatic computation of evolutionary sequences of general relativistic, spherically symmetric, quasi-static stellar models. Thornes six partial differential equations governing the time-dependent structure of relativistic stars are solved by a Newton-Raphson technique closely analogous to that employed in conventional Newtonian evolutionary calculations. Sample calculations of an /sup 56/Fe white dwarf and of a simplified neutron star model are presented as illustrations of the method.

Some Effects of the UV Radiation From White Dwarfs on the Accretion of Interstellar Hydrogen

We have carried out preliminary hydrodynamic calculations of the accretion of interstellar hydrogen by white dwarfs, and we are beginning to study the effects of the ionizing UV radiation from the star upon the flow. Even for velocities as low as ~2.82 km s -1, with a stellar temperature T eff = 10,000 K, the parameter values we have selected for the trial calculations discussed here, the HII region around the star is highly distorted. For this case, we find that the ionizing radiation does reduce the accretion rate of hydrogen, by about a factor of three. Further work is in progress and will be reported elsewhere.

Production of Planetary Nebulae

For various reasons, we have been studying evolution in the pre-white dwarf phase at Rochester. Our attention to the relevance of this work to planetary nebulae came as a result of calculations of the evolution of a one solar-mass iron star carried out at Rochester by S. Vila. Here the neutrino processes drive the peak luminosity to log L/L ⊙ = 4.26, at an effective temperature logT eff = 5.53. Although these models are brighter than 100 L ⊙ for 500000 years, they are brighter than 1000 L ⊙ for 4000 years, and exceed 10000 L ⊙ for 900 years. We are therefore near the luminosity of the planetary-nebula nuclei, but considerably hotter, for a period of the order of the planetary lifetimes. Except for the temperature discrepancy, these models are in rough agreement with the observationally determined evolutionary sequence found by O’Dell (1963) and by Harman and Seaton (1964) and Seaton (1966).