Are Magnetic Wind-Driving Disks Inherently Unstable?

There have been claims in the literature that accretion disks in which a centrifugally driven wind is the dominant mode of angular momentum transport are inherently unstable. This issue is considered here by applying an equilibrium-curve analysis to the wind-driving, ambipolar diffusion-dominated, magnetic disk model of Wardle & Konigl (1993). The equilibrium solution curves for this class of models typically exhibit two distinct branches. It is argued that only one of these branches represents unstable equilibria and that a real disk/wind system likely corresponds to a stable solution.Comment: 5 pages, 2 figures, to be published in ApJ, vol. 617 (2004 Dec 20). Uses emulateapj.cl

Crystallization, data collection and data processing of maltose-binding protein (MalE) from the phytopathogen Xanthomonas axonopodis pv. citri

Maltose-binding protein is the periplasmic component of the ABC transporter responsible for the uptake of maltose/maltodextrins. The Xanthomonas axonopodis pv. citri maltose-binding protein MalE has been crystallized at 293 Kusing the hanging-drop vapour-diffusion method. The crystal belonged to the primitive hexagonal space group P6_122, with unit-cell parameters a = 123.59, b = 123.59, c = 304.20 Å, and contained two molecules in the asymetric unit. It diffracted to 2.24 Å resolution

The Genesis of Cosmological Tracker Fields

The role of the quintessence field as a probable candidate for the repulsive dark energy, the conditions for tracking and the requisites for tracker fields are examined. The concept of `integrated tracking' is introduced and a new criterion for the existence of tracker potentials is derived assuming monotonic increase in the scalar energy density parameter \Omega_\phi with the evolution of the universe as suggested by the astrophysical constraints. It provides a technique to investigate generic potentials of the tracker fields. The general properties of the tracker fields are discussed and their behaviour with respect to tracking parameter \epsilon is analyzed. It is shown that the tracker fields around the limiting value \epsilon \simeq \frac 23 give the best fit with the observational constraints.Comment: 8 pages, Latex file, 1 figure, comments adde

Is it really possible to grow isotropic on-lattice diffusion-limited aggregates?

In a recent paper (Bogoyavlenskiy V A 2002 \JPA \textbf{35} 2533), an algorithm aiming to generate isotropic clusters of the on-lattice diffusion-limited aggregation (DLA) model was proposed. The procedure consists of aggregation probabilities proportional to the squared number of occupied sites ($k^2$). In the present work, we analyzed this algorithm using the noise reduced version of the DLA model and large scale simulations. In the noiseless limit, instead of isotropic patterns, a $45^\circ$ ($30^\circ$) rotation in the anisotropy directions of the clusters grown on square (triangular) lattices was observed. A generalized algorithm, in which the aggregation probability is proportional to $k^\nu$, was proposed. The exponent $\nu$ has a nonuniversal critical value $\nu_c$, for which the patterns generated in the noiseless limit exhibit the original (axial) anisotropy for $\nu<\nu_c$ and the rotated one (diagonal) for $\nu>\nu_c$. The values $\nu_c = 1.395\pm0.005$ and $\nu_c = 0.82\pm 0.01$ were found for square and triangular lattices, respectively. Moreover, large scale simulations show that there are a nontrivial relation between noise reduction and anisotropy direction. The case $\nu=2$ (\bogo's rule) is an example where the patterns exhibit the axial anisotropy for small and the diagonal one for large noise reduction.Comment: 12 pages, 8 figure

Non-collinear coupling between magnetic adatoms in carbon nanotubes

The long range character of the exchange coupling between localized magnetic moments indirectly mediated by the conduction electrons of metallic hosts often plays a significant role in determining the magnetic order of low-dimensional structures. In addition to this indirect coupling, here we show that the direct exchange interaction that arises when the moments are not too far apart may induce a non-collinear magnetic order that cannot be characterized by a Heisenberg-like interaction between the magnetic moments. We argue that this effect can be manipulated to control the magnetization alignment of magnetic dimers adsorbed to the walls of carbon nanotubes.Comment: 13 pages, 5 figures, submitted to PR

Absorption lines from magnetically-driven winds in X-ray binaries

High resolution X-ray spectra of black hole X-ray binaries (BHBs) show blueshifted absorption lines from disk winds which seem to be equatorial. Winds occur in the Softer (disk-dominated) states of the outburst and are less prominent or absent in the Harder (power-law dominated) states. We use self-similar magneto-hydrodynamic (MHD) accretion-ejection models to explain the disk winds in BHBs. In our models, the density at the base of the outflow from the accretion disk is not a free parameter, but is determined by solving the full set of dynamical MHD equations. Thus the physical properties of the outflow are controlled by the global structure of the disk. We studied different MHD solutions characterized by different values of (a) the disk aspect ratio ($\varepsilon$) and (b) the ejection efficiency ($p$). We use two kinds of MHD solutions depending on the absence (cold solution) or presence (warm solution) of heating at the disk surface. Such heating could be from e.g. dissipation of energy due to MHD turbulence in the disk or from illumination. We use each of these MHD solutions to predict the physical parameters of an outflow; put limits on the ionization parameter ($\xi$), column density and timescales, motivated by observational results; and thus select regions within the outflow which are consistent with the observed winds. The cold MHD solutions cannot account for winds due to their low ejection efficiency. But warm solutions can explain the observed physical quantities in the wind because they can have sufficiently high values of $p$ ($\gtrsim 0.1$, implying larger mass loading at the base of the outflow). Further from our thermodynamic equilibrium curve analysis for the outflowing gas, we found that in the Hard state a range of $\xi$ is thermodynamically unstable, and had to be excluded. This constrain made it impossible to have any wind at all, in the Hard state.Comment: 16 Pages, 10 figures in the main body and 4 figures in the appendix. Accepted for publication in A&

AN ATTEMPT TO LINK THE BRAZILIAN HEIGHT SYSTEM TO A WORLD HEIGHT SYSTEM

This paper deals with the geopotential approach to investigate the present BrazilianHeight System (BHS). Geopotential numbers are derived from Global PositioningSystem (GPS) satellite surveying and disturbing potential on selected benchmarks.A model for the disturbing potential can be obtained by an existing set of sphericalharmonic coefficients such as the Earth Gravity Model 2008 (EGM08). Theapproach provides absolute evaluation of local normal geopotential numbers (akaspheropotential numbers) related to a so-called World Height System (WHS). Totest the validity of the proposed methodology, a numerical experiment was carriedout related to a test region in Southern Brazil. The accuracy of the derivedgeopotential numbers was tested versus local normal geopotential numbers based on262 GPS/leveling points. The root mean square error (RMSE) value for metricoffset of BHS derived from geopotential numbers and the disturbing potentialmodeling in the test area was estimated to be near 0.224 meters in the absolute view.Therefore, since these spheropotential numbers are referred to a local datum, theseresults of comparisons may be an indicator of the mean bias of local network due tothe effect of local Sea Surface Topography (SSTop) and possible offset between theunknown reference for the BHS and the quasigeoid model in the region