Braneworlds with timelike extra-dimension

In this work, we consider a braneworld model with a timelike extra-dimension. There are strong constraints to the parameter values of such a model resulting from the claim that there must be a physical solution to the Friedmann equation at least between now and the time of recombination. We fitted the model to supernova type Ia data and checked the consistency of the result with other observations. For parameter values that are consistent with observations, the braneworld model is indistinguishable from a LambdaCDM universe as far as the considered cosmological tests are concerned.Comment: 7 pages, 8 figures, matches version accepted by Phys. Rev.

Collimation of Highly Variable Magnetohydrodynamic Disturbances around a Rotating Black Hole

We have studied non-stationary and non-axisymmetric perturbations of a magnetohydrodynamic accretion onto a rotating (Kerr) black hole. Assuming that the magnetic field dominates the plasma accretion, we find that the accretion suffers a large radial acceleration resulting from the Lorentz force, and becomes highly variable compared with the electromagnetic field there. In fact, we further find an interesting perturbed structure of the plasma velocity with a large peak in some narrow region located slightly inside of the fast-magnetosonic surface. This is due to the concentrated propagation of the fluid disturbances in the form of fast-magnetosonic waves along the separatrix surface. If the fast-magnetosonic speed is smaller in the polar regions than in the equatorial regions, the critical surface has a prolate shape for radial poloidal field lines. In this case, only the waves that propagate towards the equator can escape from the super-fast-magnetosonic region and collimate polewards as they propagate outwards in the sub-fast-magnetosonic regions. We further discuss the capabilities of such collimated waves in accelerating particles due to cyclotron resonance in an electron-positron plasma.Comment: 15 pages, 6 postscript figures, LaTe

Magnetospheric Gap and Accumulation of Giant Planets Close to the Star

The bunching of giant planets at a distance of several stellar radii may be explained by the disruption of the inner part of the disk by the magnetosphere of the star during the T Tauri stage of evolution. The rotating magnetic field of the star gives rise to a low density magnetospheric gap where stellar migration is strongly suppressed. We performed full 3D magnetohydrodynamic simulations of the disk-magnetosphere interaction and examined conditions for which the magnetospheric gap is "empty", by changing the misalignment angle between magnetic and rotational axes of the star, Theta, and by lowering the adiabatic index gamma, which mocks up the effect of heat conductivity and cooling. Our simulations show that for a wide range of plausible conditions the gap is essentially empty. However, in the case of large misalignment angles Theta, part of the funnel stream is located in the equatorial plane and the gap is not empty. Furthermore, if the adiabatic index is small (gamma=1.1) and the rotational and magnetic axes are almost aligned, then matter penetrates through the magnetosphere due to 3D instabilities forming high-density equatorial funnels. For these two limits there is appreciable matter density in the equatorial plane of the disk so that a planet may migrate into the star.Comment: 6 pages, 6 figures, Accepted to the ApJ Letters. See version of the paper with higher resolution plots at http://astrosun2.astro.cornell.edu/us-rus/planets.ht

Atomic alignment and Diagnostics of Magnetic Fields in Diffuse Media

We continue our studies of atomic alignment in diffuse media, in particularly, in interstellar and circumstellar media, with the goal of developing new diagnostics of magnetic fields in these environments. We understand atomic alignment as alignment of atoms or ions in their ground state. Such atoms are sensitive to weak magnetic fields. In particular, we provide predictions of the polarization that arises from astrophysically important aligned atoms (ions) with fine structure of the ground level, namely, OI and SII and Ti II. Unlike our earlier papers which dealt with weak fields only, a substantial part of our current paper is devoted to the studies of atomic alignment when magnetic fields get strong enough to affect the emission from the excited level, i.e. with the regime when the magnetic splitting is comparable to the line-width. This is a regime of Hanle effect modified by the atomic alignment. Using an example of emission and absorption lines of SII ion we demonstrate how polarimetric studies can probe magnetic fields in circumstellar regions and accretion disks. In addition, we show that atomic alignment induced by anisotropic radiation can induce substantial variations of magnetic dipole transitions within the ground state, thus affecting abundance studies based on this emission. Moreover, the radio emission is polarized, provides a new way to study magnetic fields, e.g. at the epoch of Universe reionization.Comment: Minor changes, accepted to Ap

The Axisymmetric Pulsar Magnetosphere

We present, for the first time, the structure of the axisymmetric force-free magnetosphere of an aligned rotating magnetic dipole, in the case in which there exists a sufficiently large charge density (whose origin we do not question) to satisfy the ideal MHD condition, ${\bf E\cdot B}=0$, everywhere. The unique distribution of electric current along the open magnetic field lines which is required for the solution to be continuous and smooth is obtained numerically. With the geometry of the field lines thus determined we compute the dynamics of the associated MHD wind. The main result is that the relativistic outflow contained in the magnetosphere is not accelerated to the extremely relativistic energies required for the flow to generate gamma rays. We expect that our solution will be useful as the starting point for detailed studies of pulsar magnetospheres under more general conditions, namely when either the force-free and/or the ideal MHD condition ${\bf E\cdot B}=0$ are not valid in the entire magnetosphere. Based on our solution, we consider that the most likely positions of such an occurrence are the polar cap, the crossings of the zero space charge surface by open field lines, and the return current boundary, but not the light cylinder.Comment: 15 pages AAS Latex, 5 postscript figure

Locking of the Rotation of Disk-Accreting Magnetized Stars

We investigate the rotational equilibrium state of a disk accreting magnetized stars using axisymmetric magnetohydrodynamic (MHD) simulations. In this ``locked'' state, the spin-up torque balances the spin-down torque so that the net average torque on the star is zero. We investigated two types of initial conditions, one with a relatively weak stellar magnetic field and a high coronal density, and the other with a stronger stellar field and a lower coronal density. We observed that for both initial conditions the rotation of the star is locked to the rotation of the disk. In the second case, the radial field lines carry significant angular momentum out of the star. However, this did not appreciably change the condition for locking of the rotation of the star. We find that in the equilibrium state the corotation radius $r_{co}$ is related to the magnetospheric radius $r_A$ as $r_{co}/r_A\approx 1.2-1.3$ for case (1) and $r_{co}/r_A\approx 1.4-1.5$ for case (2). We estimated periods of rotation in the equilibrium state for classical T Tauri stars, dwarf novae and X-ray millisecond pulsars.Comment: 10 pages, 9 figures. Accepted by ApJ, will appear in vol. 634, 2005 December

The role of damped Alfven waves on magnetospheric accretion models of young stars

We examine the role of Alfven wave damping in heating the plasma in the magnetic funnels of magnetospheric accretion models of young stars. We study four different damping mechanisms of the Alfven waves: nonlinear, turbulent, viscous-resistive and collisional. Two different possible origins for the Alfven waves are discussed: 1) Alfven waves generated at the surface of the star by the shock produced by the infalling matter; and 2) Alfven waves generated locally in the funnel by the Kelvin-Helmholtz instability. We find that, in general, the damping lengths are smaller than the tube length. Since thermal conduction in the tube is not efficient, Alfven waves generated only at the star's surface cannot heat the tube to the temperatures necessary to fit the observations. Only for very low frequency Alfven waves ~10^{-5} the ion cyclotron frequency, is the viscous-resistive damping length greater than the tube length. In this case, the Alfven waves produced at the surface of the star are able to heat the whole tube. Otherwise, local production of Alfven waves is required to explain the observations. The turbulence level is calculated for different frequencies for optically thin and thick media. We find that turbulent velocities varies greatly for different damping mechanisms, reaching \~100 km s^{-1} for the collisional damping of small frequency waves.Comment: 29 pages, 12 figures, to appear in The Astrophysical Journa

Ultra-Relativistic Magneto-Hydro-Dynamic Jets in the context of Gamma Ray Bursts

We present a detailed numerical study of the dynamics and evolution of ultrarelativistic magnetohydrodynamic jets in the black hole-disk system under extreme magnetization conditions. We find that Lorentz factors of up to 3000 are achieved and derived a modifiedMichel scaling (Gamma ~ sigma) which allows for a wide variation in the flow Lorentz factor. Pending contamination induced by mass-entrainment, the linear Michel scaling links modulations in the ultrarelativistic wind to variations in mass accretion in the disk for a given magnetization. The jet is asymptotically dominated by the toroidal magnetic field allowing for efficient collimation. We discuss our solutions (jets) in the context of Gamma ray bursts and describe the relevant features such as the high variability in the Lorentz factor and how high collimation angles (~ 0-5 degrees), or cylindrical jets, can be achieved. We isolate a jet instability mechanism we refer to as the "bottle-neck" instability which essentially relies on a high magnetization and a recollimation of the magnetic flux surfaces. The instability occurs at large radii where any dissipation of the magnetic energy into radiation would in principle result in an optically thin emission.Comment: 31 pages, 6 figures. Submitted to ApJ. Higher Quality figures at http://www.capca.ucalgary.ca/paper