Ekman layer damping of r modes revisited

We investigate the damping of neutron star r modes due to the presence of a viscous boundary (Ekman) layer at the interface between the crust and the core. Our study is motivated by the possibility that the gravitational wave driven instability of the inertial r modes may become active in rapidly spinning neutron stars, for example, in low-mass X-ray binaries, and the fact that a viscous Ekman layer at the core–crust interface provides an efficient damping mechanism for these oscillations. We review various approaches to the problem and carry out an analytic calculation of the effects due to the Ekman layer for a rigid crust. Our analytic estimates support previous numerical results, and provide further insight into the intricacies of the problem. We add to previous work by discussing the effect that compressibility and composition stratification have on the boundary-layer damping. We show that, while stratification is unimportant for the r-mode problem, composition suppresses the damping rate by about a factor of 2 (depending on the detailed equation of state).     

Report on the physical characteristics of Vaidya-Tikekar's exact relativistic model for a superdense star

We report here the results of our examination of the physical properties of Vaidya-Tikekar's model for a relativistic star. Full details will be published elsewhere. The analysis yields a strong indication that the model is stable with respect to infinitesimal radial oscillations. We find that the adiabatic speed of sound is smaller than the speed of light everywhere inside the fluid sphere if the radius of the sphere is larger than 1.46 times its Schwarzschild radius. We also find that the fluid must necessarily be supraluminal somewhere if the speed of sound is decreasing outwards close to the center. We further find that the strong energy condition is fulfilled everywhere if it is fulfilled at the center. Since the ratio of the pressure p and the density ⋅ is decreasing outwards, this indicates that the temperature gradient is negative. We also find that the relativistic adiabatic index is larger than two. Demanding the fluid to be causal, and taking the pressure and the density to be somewhere given by 7.4 ⋅ 10³³ dynes/cm³ and 5.1 ⋅ 10¹⁴ g/cm³, we calculate the maximum mass of the fluid sphere to be 3 solar masses.     

Spectral line profile variations in the γ Doradus variable HD 164615: non-radial pulsations versus star-spots

High-resolution spectral data of the Fe  II 5318 Å line in the γ Doradus star HD 164615 are presented. These show systematic changes in the spectral lineshapes with the photometric period of 0.8133 d which are modelled using either non-radial pulsations or cool star-spots. The non-radial modes that can fit the lineshape changes have m degree of 2–4. However, only the m = 2 mode seems to be consistent with the amplitude of the radial velocity variations measured for this star. The star-spot model, although it can qualitatively fit the lineshape changes, is excluded as a possible hypothesis on the basis of (1) poorer fits to the observed spectral line profiles, (2) an inability to account for the large changes in the spectral linewidth as a function of phase, (3) a predicted radial velocity curve that looks qualitatively different from the observed one, and (4) a predicted photometric curve that is a factor of 5 larger than the observed light curve (and with the wrong qualitative shape). Finally, a 'Doppler image' (assuming cool spots) derived from a sequence of synthetic line profiles having non-radial pulsations results in an image that is almost identical to the Doppler image derived for HD 164615. These results provide strong evidence that non-radial pulsations are indeed the explanation for the variability of HD 164615 as well as the other γ Dor variables.     

X-ray binary unified analysis: pulse/RV application to Vela X1: GP Velorum

A procedure for simultaneous analysis of multiple kinds of observations is developed for binaries that contain X-ray pulsars. A wide variety of observation types might be included, although we consider only velocity: pulse and light: velocity: pulse cases at present. The model operates with equipotentials and can accommodate non-synchronous rotation and eccentric orbits. The duration condition imposed by observed X-ray eclipses is incorporated as an embedded constraint, so that only solutions consistent with the eclipse duration are found. Relations needed for the method of differential corrections in least-squares solutions are specified, and we apply the Marquardt scheme for improvement of solution convergence. Parameters of Vela X1: GP Velorum are obtained from seven combinations of pulse and radial velocity data. The estimated masses are thereby put into some perspective, especially for the neutron star. The mass of the supergiant ( m  sin³  i ) now ranges only ±5 per cent from the mean of seven results, although the uncertainty in i makes the actual mass range larger. We discuss the determinacy of certain parameters and their historical consistency. For example, the systemic radial velocity differs among spectroscopic data sets by more than 20 km s⁻¹. We show that, contrary to published assertions, the orbital inclinations of high-mass X-ray binaries are essentially indeterminable by the usual methods, although lower limits to inclination may be meaningful for some examples.     

How can α 2-dynamos generate axisymmetric magnetic fields?

Inspired by a recently observed axisymmetric field in a fully convective star we investigate the influence of an anisotropic diffusivity on the dynamo. We find that with reasonable assumptions for the anisotropy of the diffusivity and the α -effect the preference of axisymmetric modes is achieved. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)