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White Dwarves

Still under development, however changes are made regularly - check back by later.

White dwarfs are embers of dying stars shrunken to planetary dimensions. Only by accumulation of fresh Hydrogen from molecular cloud passages can an isolated dwarf hope to avoid a typical end. It has been found, however, that in close binary systems, white dwarfs may hope to escape thermal extinction, and may even be reincarnated briefly as a giant star.

When a massive star collapses, the outer parts of the star explode at speeds of up to 10,000 kilometers per second and more. A similar amount of energy is released when a white dwarf star undergoes complete disintegration in a thermonuclear detonation.¹³ Observations imply that a great deal more material is ejected in a nova explosion than was predicted by our calculations - that could make nova far more important in the evolution of the chemical elements in our galaxy than was previously believed.

Astronomers have released new Hubble Space Telescope image that show gas shells ejected into space at regular intervals by an unusual type of white dwarf star. The images may reveal important information about the role novae play in the evolution of our galaxy. first images of gas shells produced by novae that are members of a class of double star systems called Cataclysmic Variables.

These typically involve a white dwarf in very close proximity to a larger and cooler star. The strong gravity tides from the white dwarf drag hydrogen gas off the larger star. This gas spirals down onto the surface of the white dwarf and accumulates as a deeper and deeper hydrogen shell around the white dwarf core, which is primarily carbon, oxygen and other heavier elements.

When enough hydrogen has accumulated on the white dwarf's surface (after about 10,000 years) thermonuclear fusion reactions begin. These reactions gradually intensify until finally (much later) temperatures rise to the point where they cause a nova explosion in the hydrogen shell. blowing it and part of the white dwarf core into space, and then the process begins all over again.

White dwarfs in close binary systems that feed hydrogen-rich matter from a main-sequence type star filling its Roche lobe, and then transfers the gas with the proper angular momentum to form an accretion disk that enshrouds the white dwarf. During this time the dwarf experiences low ongoing accretion rates, which is interrupted every few weeks to months by intense accretion which lasts a few days to weeks (dwarf nova accretion event). It is thought that they undergo a thermonuclear-type explosion every few thousand years (classical nova), which also interrupts low accretion periods. Accretion heats the white dwarf, adds mass with processed or solar composition chemical abundances, and injects angular momentum into its envelope.

(CLICK on the PICTURE for a LARGER IMAGE)

Note - The pioneering detections were carried out by Panek & Holm (1984), Shafter, et al. (1985), Mateo and Szkody (1984), Cordova (1995), and Shara (1989)

The IUE archive offers a series of far-UV spectra of the ultrashort-period dwarf nova WZ Sge over a long time baseline and hence the opportunity to monitor the ongoing accretion, the cooling of the white dwarf, and temporal variations in the line spectra. We have carried out a quantitative analysis of these spectra including model atmosphere simulations. We find an indicated cooling of the white dwarf by 5140 K from 20,510 to 15,370 K with a thermal e-folding time of 690 days. This cooling rate is well represented by heating of the white dwarf by accretion of matter with angular momentum during the 1978 December outburst followed by subsequent cooling. We find that the abundance of carbon in the white dwarf photosphere is elevated above solar. This finding is consistent with the results of recent Hubble Space Telescope observations. We find marginal evidence that the C abundance was 6 times solar close to the outburst and 2 times solar in the most recent spectra obtained 13.7 years later. However, uncertainties in the abundances do not allow any definitive conclusion regarding temporal abundance variations. The origin of the elevated carbon abundance is discussed in terms of accretion and diffusion while arguments are presented which may rule out ordinary convective dredge-up and dredge-up due to forced convection associated with shear mixing.^84a

We consider the unusual evolutionary state of the secondary star in Cygnus X-2. Spectroscopic data give a low mass (M_2~=0.5-0.7M_solar) and yet a large radius (R_2~=7R_solar) and high luminosity (L_2~=150L_solar). We show that this star closely resembles a remnant of early massive Case B evolution, during which the neutron star ejected most of the ~3M_solar transferred from the donor (initial mass M_2i~3.6M_solar) on its thermal time-scale ~10^6yr. As the system is far too wide to result from common-envelope evolution, this strongly supports the idea that a neutron star efficiently ejects the excess inflow during super-Eddington mass transfer. Cygnus X-2 is unusual in having had an initial mass ratio q_iM_2iM_1 in a narrow critical range near q_i~=2.6. Smaller q_i lead to long-period systems with the former donor near the Hayashi line, and larger q_i to pulsar binaries with shorter periods and relatively massive white dwarf companions. The latter naturally explain the surprisingly large companion masses in several millisecond pulsar binaries. Systems like Cygnus X-2 may thus be an important channel for forming pulsar binaries.^84b

It has been suggested that the differences among the observational Type Ia supernovae (SNIa) set can be accounted for by invoking two regimes of propagation of combustion. Normal SNIa should be produced by rapid deflagrations that rapidly propagate across a white dwarf, while dim SNIa should be a consequence of a detonation issued during the contraction phase of a pulsation induced by a very slow conductive deflagration. In this paper, we explore the observational consequences of deflagrations, the properties of which are in between both behaviours. Using different laws for the flame velocity as a function of flame radius, a number of different outcomes were found, including direct explosions ejecting small quantities of ^56Ni, pulsations leading to recontraction and likely reignition of the flame, and a threshold explosion characterized by an extended gravitationally bound phase (several 10^3s), in which most of the white dwarf matter was ejected by the energy input of radioactive isotopes. Not one of these strange supernovae has been detected up to now. Nevertheless, since they are very dim and, for nucleosynthesis reasons, very rare, their existence cannot be excluded. Furthermore, the computed light curve shows that these events mimic the behaviour of peculiar Type II supernovae (SNII), for which reasons there is always the possibility that they have been misclassified as peculiar SNII whose spectrum is lacking.^84c

The inner region of the accretion disk onto a rotating magnetized central star (neutron star, white dwarf, or T Tauri star) is subjected to magnetic torques that induce warping and precession of the disk. The origin of these torques lies in the interaction between the surface current on the disk and the horizontal magnetic field (parallel to the disk) produced by the inclined magnetic dipole: the warping torque relies on the radial surface current generated by the twisting of the vertical field threading the disk, while the precessional torque relies on the azimuthal screening current due to the diamagnetic response of the disk. Under quite general conditions, there exists a magnetic warping instability in which the magnetic torque drives the disk plane away from the equatorial plane of the star toward a state in which the disk normal vector is perpendicular to the spin axis.^84d

Viscous stress tends to suppress the warping instability at large radii, but the magnetic torque always dominates as the disk approaches the magnetosphere boundary. The magnetic torque also drives the tilted inner disk into retrograde precession (opposite to the rotation of the disk) around the stellar spin axis. Moreover, resonant magnetic forcing on the disk can occur, which may affect the dynamics of the disk. The magnetically driven warping instability and precession may be related to a number of observational puzzles. Examples include (1) spin evolution of accreting X-ray pulsars: it is suggested that the observed torque reversal of the disk-fed magnetized neutron stars is associated with the wandering of the inner disk around the preferred perpendicular state; (2) quasi-periodic oscillations (QPOs) in low-mass X-ray binaries: the magnetic torque induces disk tilt, making it possible to explain the observed low-frequency QPOs using disk precession; (3) superorbital periods in a number of X-ray binaries as a result of warped, precessing disks; and (4) photometric period variations of T Tauri stars.^84d