Hugh M. Van Horn

Dynamos in asymptotic-giant-branch stars as the origin of magnetic fields shaping planetary nebulae

Planetary nebulae are thought to be formed when a slow wind from the progenitor giant star is overtaken by a subsequent fast wind generated as the star enters its white dwarf stage. A shock forms near the boundary between the winds, creating the relatively dense shell characteristic of a planetary nebula. A spherically symmetric wind will produce a spherically symmetric shell, yet over half of known planetary nebulae are not spherical; rather, they are elliptical or bipolar in shape. A magnetic field could launch and collimate a bipolar outflow, but the origin of such a field has hitherto been unclear, and some previous work has even suggested that a field could not be generated. Here we show that an asymptotic-giant-branch (AGB) star can indeed generate a strong magnetic field, having as its origin a dynamo at the interface between the rapidly rotating core and the more slowly rotating envelope of the star. The fields are strong enough to shape the bipolar outflows that produce the observed bipolar planetary nebulae. Magnetic braking of the stellar core during this process may also explain the puzzlingly slow rotation of most white dwarf stars.

The role of the molecular-metallic transition of hydrogen in the evolution of Jupiter, Saturn, and brown dwarfs

An equation of state for hydrogen which predicts a molecular-metallic phase transition at finite temperatures has become available recently. A companion paper addresses the issue of the internal structures of Jupiter and Saturn, as derived with this new equation of state. Here we study the effect of this phase transition on the cooling histories of these two giant planets and of substellar brown dwarfs. The phase transition alters the present age of Jupiter and of Saturn by a few percent. Interestingly, the cooling of brown dwarfs is most strongly affected at the time when the interior adiabat crosses the critical point of the phase transition

Further Monte Carlo calculations for the classical one-component plasma in the range 100 ⩽ Γ ⩽ 160: The FCC lattice

Very accurate Monte Carlo calculations for the one-component plasma (OCP) have been compared with the results of Slattery, Doolen, and DeWitt.(1,2) We confirm their results and also find a slight dependence of the calculation of the internal energy per particle uponN, the number of particles. A detailed investigation forN=108 permits us to evaluate the Helmholtz free energy for an OCP fcc lattice. As is usually believed, we find that the bcc lattice is more stable than the fcc lattice. The transition from the liquid to the fcc lattice phase occurs whenΓfcc = 196 ± 1. A three-dimensional modified cubic procedure, capable of achieving high accuracy in using tables of two-particle interaction potentials, is described in Appendix B.

Thermonuclear reaction rates for dense binary-ionic mixtures

Enhancement factors for the thermonuclear reaction rates in dense binary-ionic mixtures (BIMs) with charge ratio R Z ≤5 are calculated through detailed examination of the screening potentials at short ranges obtained by the Monte Carlo simulation method. Small but nonnegligible deviations of the screening distances from the constant electron-density, ion-sphere scaling are discovered at R Z ∼5 for the lighter elements; the deviations act further to enhance the reaction rates. The triple-α reactions in He-C mixtures are treated as an example

Fuel-Supply-limited Stellar Relaxation Oscillations: Application to Multiple Rings around Asymptotic Giant Branch Stars and Planetary Nebulae

We describe a new mechanism for pulsations in evolved stars: relaxation oscillations driven by a coupling between the luminosity-dependent mass-loss rate and the H fuel abundance in a nuclear-burning shell. When mass loss is included, the outward flow of matter can modulate the flow of fuel into the shell when the stellar luminosity is close to the Eddington luminosity LEdd. When the luminosity drops below LEdd, the mass outflow declines and the shell is resupplied with fuel. This process can be repetitive. We demonstrate the existence of such oscillations and discuss the dependence of the results on the stellar parameters. In particular, we show that the oscillation period scales specifically with the mass of the H-burning relaxation shell (HBRS), defined as the part of the H-burning shell above the minimum radius at which the luminosity from below first exceeds the Eddington threshold at the onset of the mass-loss phase. For a stellar mass M* ~ 0.7 M☉, luminosity L* ~ 104 L☉, and mass-loss rate || ~ 10-5 M☉ yr-1, the oscillations have a recurrence time of ~1400 yr ~ 57τfsm, where τfsm is the timescale for modulation of the fuel-supply in the HBRS by the varying mass-loss rate. This period agrees very well with the ~1400 yr period inferred for the spacings between the shells surrounding some planetary nebulae. We also find the half-width of the luminosity peak to be ~0.39 times the oscillation period; for comparison, the observational shell thickness of ~1000 AU corresponds to ~0.36 of the spacing between pulses. We find oscillations only for models in which the luminosity of the relaxation shell is ~10%-15% of the total stellar luminosity and for which energy generation occurs through the p-p chain. We suggest this mechanism as a natural explanation for the circumnebular shells surrounding some planetary nebulae, which appear only at the end of the AGB phase.

Toward an Improved Pure Hydrogen EOS for Astrophysical Applications

The motivation for our equation of state (EOS) work derives from our interest in the structure and evolution of substellar “brown dwarf” stars. To construct an evolutionary model requires the solution of the differential equations governing stellar structure and evolution, which in turn requires knowledge of the EOS of stellar matter.

White Dwarf/Accretion Disk Interactions: Instabilities of Infinite, Differentially Rotating Cylinders

We have initiated an investigation of the coherent and quasiperiodic oscillations observed in some dwarf novae. The periods and relatively high Q associated with the coherent oscillations suggest a nonradial pulsation of the white dwarf primary for these variations, while the longer periods and lower Q of the quasi-periodic oscillations indicate a pulsation of the accretion disk. Guided by a terrestrial analogy, we have made a preliminary exploration of the shear layer instability which may drive the observed oscillations.

Nova Cygni 1975 as a Classical Thermonuclear Runaway Event

We discuss the various features of the optical and spectral developments of Nova Cygni 1975 (V1500 Cygni). The range of more than 19 magnitudes, the rounded maximum and steep decline and the absence of post-maximum absorption systems establish this as a unique object. We are able to explain these features if we assume that a thermonuclear runaway has taken place in the carbon-enhanced, hydrogen-rich, envelope of a single white dwarf where the hydrogen has been accreted by passage of the white dwarf through an interstellar cloud. This would be possible for Nova Cyg since it lies directly in the plane of the galaxy. The carbon enhancement, necessary for ejection, has occurred through the action of a surface convection zone which has mixed a fraction of the accreted hydrogen into the edge of the carbon core. We shall also discuss the observed oscillations and present alternative models for the object.