Jianke Li

Planets around White Dwarfs

The fate of a planetary system like our own, as the parent star expands through the red giant phase and becomes a white dwarf (WD), has been a topic of some discussion. For an Earth-like inner planet, the conducting core may remain intact, even though severe ablation occurs of the outer layers. We argue that a planetary core in orbit around a WD may reveal its presence through its interaction with the magnetosphere of the WD. As the planet moves through the magnetosphere, electrical currents will be generated, which will heat the atmosphere of the WD near its magnetic poles. The results of such a heating may be detected in the optical as Hα emission. Ohmic dissipation will result in the slow decay of the planetary orbit, and such a planet will merge with the WD in less than a Hubble time, unless the initial orbital separation is greater than about 10 solar radii. We propose that the peculiar emission-line WD GD 356 may be a system in the process of such a merger.

Accretion Disk Reversal and the Spin-up/Spin-down of Accreting Pulsars

We numerically investigate the hydrodynamics of accretion disk reversal and relate our findings to the observed spin-rate changes in the accreting X-ray pulsar GX 1+4. In this system, which accretes from a slow wind, the accretion disk contains two dynamically distinct regions. In the inner part viscous forces are dominant, and disk evolution occurs on a viscous timescale. In the outer part dynamical mixing of material with opposite angular momentum is more important, and the externally imposed angular momentum reversal timescale governs the flow. In this outer region the disk is split into concentric rings of material with opposite senses of rotation that do not mix completely but instead remain distinct, with a clear gap between them. We thus predict that torque reversals resulting from accretion disk reversals will be accompanied by minima in accretion luminosity.

Effects of Magnetized Winds on Advective Disks. I. A Self-similar Solution

In this paper we investigate the effects of a large-scale magnetic field (with open field lines) on the structure of an advective accretion disk. We find self-similar solutions to the MHD equations describing the disk/magnetic field system; these equations reduce to the case studied by Narayan & Yi (1994) in the absence of an external, macroscopic magnetic field. Our main assumptions are the existence of a hot, tenuous corona above and below the disk, and the presence of a wind starting from the base of the corona and centrifugally accelerated along the field lines up to the Alfven surface. The wind appears to be the most efficient mechanism for extracting angular momentum from the inflowing gas, even when the mass lost in the wind is negligible with respect to the mass accreted; other notable effects of the interplay between the disk structure and the magnetic field with its associated wind include a bending of the field lines toward the surface of the disk (dragging of the field by the accreting matter), a squeezing effect (the disk scale height is reduced because of a magnetic pressure gradient), an increased radial infall velocity (consequence of the quicker loss of angular momentum), and a decrease of the gas temperature in the disk. We can equivalently describe the loss of angular momentum in the wind by introducing an effective viscosity parameter αeff, which can become greater than 1 regardless of the true viscosity parameter α.

Torque Reversals in Disk Accreting Pulsars

X-ray binaries in which the accreting component is a neutron star commonly exhibit significant changes in their spin. In the system Cen X-3, a disk accreting binary system, the pulsar was observed to spin up at a rate f = 8 x 10^-13 Hz s^-1 when averaged over the past twenty years, but significant fluctuations were observed above this mean. Recent BASTE observations have disclosed that these fluctuations are much larger than previously noted, and appeared to be a system characteristic. The change in the spin state from spin-up to spin-down or vice-versa occurs on a time scale that is much shorter than the instrument can resolve (<= 1 d), but appears always to be a similar amplitude, and to occur stochastically. These observations have posed a problem for the conventional torque{mass accretion relation for accreting pulsars, because in this model the spin rate is closely related to the accretion rate, and the latter needs to be finely tuned and to change abruptly to explain the observations Here we review recent work in this direction and present a coherent picture that explains these observations. We also draw attention to some outstanding problems for future studies.

Magnetic Braking and Its Implication for CV Evolution

We review the models of magnetic braking for synchronously rotating magnetic cataclysmic variables, and discuss the implications of magnetic braking for orbital evolution and the upper limit to the magnetic fields (about 70 MG) of the observed AM Herculis systems.