Planet formation and dynamics

Abstract:

We examine the effect of dynamical tides raised by a companion on a solar-type star. In these binaries, gravity or g mode oscillations are excited by the companion in the radiative region beneath the convective envelope of the star. They become evanescent in the convection zone. This is of particular interest in connection with the newly discovered planets, some of which are found to orbit around solar-type stars with a period comparable to that of the high order g modes of the star. One such example is 51 Pegasi. Here, we determine the magnitude of the perturbed velocity induced by the tides at the stellar surface. We show that, in the case of 51 Pegasi, this velocity is too small to be observed. This result is insensitive to the magnitude of the stellar turbulent viscosity assumed and is not affected by the possibility of resonance, which occurs when the frequency of the tidal disturbance is close to that of some normal mode of the star. We also discuss the orbital evolution and synchronization timescales associated with the tidal interaction.

Abstract:

We describe some recent nonlinear three dimensional hydrodynamic simulations of accretion discs in binary systems where the orbit is circular and not necessarily coplanar with the disc midplane. The calculations are relevant to a number of observed astrophysical phenomena, including the precession of jets associated with young stars, the high spectral index of some T Tauri stars, and the light curves of X-ray binaries such as Hercules X-1 which suggest the presence of precessing accretion discs.

Abstract:

We study the global bending modes of a thin annular disc subject to both an internally generated magnetic field and a magnetic field due to a dipole embedded in the central star with axis aligned with the disc rotation axis. When there is a significant inner region of the disc corotating with the star, we find spectra of unstable bending modes. These may lead to elevation of the disc above the original symmetry plane facilitating accretion along the magnetospheric field lines. The resulting non-axisymmetric disc configuration may result in the creation of hotspots on the stellar surface and the periodic photometric variations observed in many classical T Tauri stars (CTTSs). Time-dependent behaviour may occur including the shadowing of the central source in magnetic accretors even when the dipole and rotation axes are aligned. © 1997 RAS.

Abstract:

We extend a previous study of the global stability of a stratified differentially rotating disc containing a toroidal magnetic field to include the effect of a non-zero resistivity η. We consider the situation when the disc is stable to convection in the absence of the magnetic field. The most robust buoyancy driven unstable modes, which occur when the field is strong enough, have low azimuthal mode number m. They grow exponentially, apparently belonging to a discrete spectrum. They exist for the dimensionless ratio η/(H2Ω) smaller than ∼ 10-2, where Ω is the angular velocity and H is the disc semithickness. In contrast the magnetorotational modes develop arbitrarily small radial scale and show transient amplification as expected from a shearing sheet analysis. The most robust modes of this type are local in all directions. Because of their more global character, the buoyancy driven modes may be important for the generation of large-scale fields and outflows. © 1997 RAS.