3,332 research outputs found
Large scale magnetic fields in viscous resistive accretion disks. I. Ejection from weakly magnetized disks
Cold steady-state disk wind theory from near Keplerian accretion disks
requires a large scale magnetic field at near equipartition strength. However
the minimum magnetization has never been tested. We investigate the time
evolution of an accretion disk threaded by a weak vertical magnetic field. The
strength of the field is such that the disk magnetization falls off rapidly
with radius. Four 2.5D numerical simulations of viscous resistive accretion
disk are performed using the magnetohydrodynamic code PLUTO. In these
simulations, a mean field approach is used and turbulence is assumed to give
rise to anomalous transport coefficients (alpha prescription). The large scale
magnetic field introduces only a small perturbation to the disk structure, with
accretion driven by the dominant viscous torque. A super fast magnetosonic jet
is observed to be launched from the innermost regions and remains stationary
over more than 953 Keplerian orbits. The self-confined jet is launched from a
finite radial zone in the disk which remains constant over time. Ejection is
made possible because the magnetization reaches unity at the disk surface, due
to the steep density decrease. However, no ejection is reported when the
midplane magnetization becomes too small. The asymptotic jet velocity remains
nevertheless too low to explain observed jets due to the negligible power
carried away by the jet. Astrophysical disks with superheated surface layers
could drive analogous outflows even if their midplane magnetization is low.
Sufficient angular momentum would be extracted by the turbulent viscosity to
allow the accretion process to continue. The magnetized outflows would be no
more than byproducts, rather than a fundamental driver of accretion. However,
if the midplane magnetization increases towards the center, a natural
transition to an inner jet dominated disk could be achieved.Comment: Accepted by Astronomy and Astrophysic
Gamma-ray line emission from Al-26 produced by Wolf-Rayet stars
The recent satellite observations of the 1.8 MeV line from the decay of Al-26 has given a new impetus to the study of the nucleosynthesis of Al-26. The production and ejection of Al-26 by massive mass-losing stars (Of and WR stars) is discussed in the light of recent stellar models. The longitude distribution of the Al-26 gamma ray line emission produced by the galactic collection of WR stars is derived based on various estimates of their radial distribution. This longitude profile provides: (1) a specific signature of massive stars on the background of other potential Al-26 sources, as novae, supernovae, certain red giants and possibly AGB stars; and (2) a possible tool to improve the data analysis of the HEAO 3 and SMM experiments
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
MHD simulations of jet acceleration from Keplerian accretion disks: the effects of disk resistivity
Accretion disks and astrophysical jets are used to model many active
astrophysical objects, viz., young stars, relativistic stars, and active
galactic nuclei. In this paper we present self-consistent time-dependent
simulations of supersonic jets launched from magnetized accretion disks, using
high resolution numerical techniques. In particular we study the effects of the
disk magnetic resistivity, parametrized through an alpha-prescription, in
determining the properties of the inflow-outflow system. Moreover we analyze
under which conditions steady state solutions of the type proposed in the self
similar models of Blandford and Payne can be reached and maintained in a self
consistent nonlinear stage. We use the resistive MHD FLASH code with adaptive
mesh refinement, allowing us to follow the evolution of the structure for a
time scale long enough to reach steady state. A detailed analysis of the
initial configuration state is given. We obtain the expected solutions in the
axisymmetric (2.5D) limit. Assuming a magnetic field around equipartition with
the thermal pressure of the disk, we show how the characteristics of the disk
jet system, as the ejection efficiency and the energetics, are affected by the
anomalous resistivity acting inside the disk.Comment: 20 pages, 18 figures, accepted for publication in Astronomy and
Astrophysic
Redox and ion-exchange properties in surface-tethered DNA-conducting polymers
A poly(cyclopentadithiophene) matrix modified by DNA covalently fixed to the surface has been designed to study the redox and ion-exchange properties in surface-tethered DNA-conducting polymers. Voltammetric investigations show an improvement in conductivity, originating from DNA modification, probably due to changes in charged-density and size of dopant species. Cyclic voltammetry with concomitant QCM measurements indicate that the mass changes are consistent with an ejection of Na+ cations associated to the anionic phosphate groups, attesting a DNA contribution to the p-doping process. So, in contrast to the classic doping patterns, the p-doping process of surface-tethered DNA-copolymer exhibits a cation-controlled transport mechanism. Impedimetric investigations indicate that for long enough DNA target sequence, nucleic acid preserves certain flexibility and is involved in the p-doping process through a diffusion-like motion. These results give new opportunities for genesensors development and for a better understanding of bioactive conducting surfaces
Two-component magnetohydrodynamical outflows around young stellar objects Interplay between stellar magnetospheric winds and disc-driven jets
We present the first-ever simulations of non-ideal magnetohydrodynamical
(MHD) stellar magnetospheric winds coupled with disc-driven jets where the
resistive and viscous accretion disc is self-consistently described. These
innovative MHD simulations are devoted to the study of the interplay between a
stellar wind (having different ejection mass rates) and an MHD disc-driven jet
embedding the stellar wind. The transmagnetosonic, collimated MHD outflows are
investigated numerically using the VAC code. We first investigate the various
angular momentum transports occurring in the magneto-viscous accretion disc. We
then analyze the modifications induced by the interaction between the two
components of the outflow. Our simulations show that the inner outflow is
accelerated from the central object's hot corona thanks to both the thermal
pressure and the Lorentz force. In our framework, the thermal acceleration is
sustained by the heating produced by the dissipated magnetic energy due to the
turbulence. Conversely, the outflow launched from the resistive accretion disc
is mainly accelerated by the magneto-centrifugal force.}{The simulations show
that the MHD disc-driven outflow extracts angular momentum more efficiently
than do viscous effects in near-equipartition, thin-magnetized discs where
turbulence is fully developed. We also show that, when a dense inner stellar
wind occurs, the resulting disc-driven jet has a different structure, namely a
magnetic structure where poloidal magnetic field lines are more inclined
because of the pressure caused by the stellar wind. This modification leads to
both an enhanced mass-ejection rate in the disc-driven jet and a larger radial
extension that is in better agreement with the observations, besides being more
consistent.Comment: 16 pages, Accepted in A&A 04/08/200
Hybridization-induced interfacial changes detected by non-Faradaic impedimetric measurements compared to Faradaic approach
A biosensor for direct label-free DNA detection based on a polythiophene matrix is investigated by electrochemical impedance spectroscopy (EIS). Impedimetric experiments are performed with and without redox probe in solution. The non-Faradaic impedance measurements reveal two relaxation processes located at 50 Hz and 5 kHz, respectively. The first relaxation process, located at low frequencies, allows to detect biorecognition events by measuring the phase angle decrease, in accordance with a hindrance of the polaronic conduction. The second relaxation process, located at 5 kHz and originating from DNA modification, seems to increase with the length of the target sequence. These results suggest that this loaded support provides a platform for impedimetric detection of hybridization at high frequencies, leading to less time-consuming detection procedure. For a better understanding, results obtained in non-Faradaic mode are compared with Faradaic approach
Predicted rotation signatures in MHD disc winds and comparison to DG Tau observations
Motivated by the first detections of rotation signatures in the DG Tau jet
(Bacciotti et al. 2002), we examine possible biases affecting the relation
between detected rotation signatures and true azimuthal velocity for
self-similar MHD disc winds, taking into account projection, convolution as
well as excitation gradients effects. We find that computed velocity shifts are
systematically smaller than the true underlying rotation curve. When outer
slower streamlines dominate the emission, we predict observed shifts increasing
with transverse distance to the jet axis, opposite to the true rotation
profile. Determination of the full transverse rotation profile thus requires
high angular resolution observations (< 5 AU) on an object with dominant inner
faster streamlines. Comparison of our predictions with HST/STIS observations of
DG Tau clearly shows that self-similar, warm MHD disc wind models with lambda =
13 and an outer radius of the disc ~ 3 AU are able to reproduce detected
velocity shifts, while cold disc wind models (lambda > 50) are ruled out for
the medium-velocity component in the DG Tau jet.Comment: 4 Pages, 4 Figures, to be published in A&
MeV Dark Matter: Has It Been Detected?
We discuss the possibility that the recent detection of 511 keV gamma-rays
from the galactic bulge, as observed by INTEGRAL, is a consequence of low mass
(~MeV) particle dark matter annihilations. We discuss the type of halo profile
favored by the observations as well as the size of the annihilation cross
section needed to account for the signal. We find that such a scenario is
consistent with the observed dark matter relic density and other constraints
from astrophysics and particle physics.Comment: 4 pages, 1 figur
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