589 research outputs found
Changes of vibrational lifetimes with minor structural modification of small polyatomic molecules
Substantial changes of population lifetimes of CH-stretching modes are observed when two atoms are exchanged in CH2=CCl2 to form trans CHCl=CHCl and when three deuterons are substituted in C6H6 to form 1,3,5.-C6H3D3. The measured lifetimes are in good agreement with estimates based on Fermi resonance-mixing which is inferred from infrared and Raman spectra
Magnetohydrodynamic jets from different magnetic field configurations
Using axisymmetric MHD simulations we investigate how the overall jet
formation is affected by a variation in the disk magnetic flux profile and/or
the existence of a central stellar magnetosphere. Our simulations evolve from
an initial, hydrostatic equilibrium state in a force-free magnetic field
configuration. We find a unique relation between the collimation degree and the
disk wind magnetization power law exponent. The collimation degree decreases
for steeper disk magnetic field profiles. Highly collimated outflows resulting
from a flat profile tend to be unsteady. We further consider a magnetic field
superposed of a stellar dipole and a disk field in parallel or anti-parallel
alignment. Both stellar and disk wind may evolve in a pair of outflows,
however, a reasonably strong disk wind component is essential for jet
collimation. Strong flares may lead to a sudden change in mass flux by a factor
two. We hypothesize that such flares may eventually trigger jet knots.Comment: 5 pages, 4 figures; proceedings from conference: Protostellar Jets in
Context, held in Rhodes, July 7-12, 200
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, , 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 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
Collimation of astrophysical jets - the role of the accretion disk magnetic field distribution
We have applied axisymmetric MHD simulations to investigate the impact of the
accretion disk magnetic flux profile on the jet collimation. Using the ZEUS-3D
code modified for magnetic diffusivity, our simulations evolve from an initial
hydrostatic equilibrium state in a force-free magnetic field. Considering a
power law for the disk poloidal magnetic field profile Bp ~ r^{-mu} and for the
disk wind density profile rho ~ r^{-mu_rho} we performed a systematic study
over a wide parameter range mu and mu_rho. We find a degree of collimation
(ratio of mass flow rates in axial and lateral direction) decreasing for
steeper disk magnetic field profiles (increasing mu). Varying the total
magnetic flux doesn't change the degree of jet collimation substantially, it
only affects the time scale of outflow evolution and the terminal jet speed. As
our major result we find a general relation between the collimation degree with
the disk wind magnetization power law exponent. Outflows with high collimation
degree resulting from a flat disk magnetic field profile tend to be unsteady,
producing axially propagating knots as discussed earlier. Depending slightly on
the inflow density profile this unsteady behavior sets in for mu < 0.4. We also
performed simulations of jet formation with artificially enhanced decay of the
toroidal magnetic field in order to investigate the idea of a purely "poloidal
collimation" discussed in the literature. These outflows remain weakly
collimated and propagate with lower velocity. Thanks to our large numerical
grid size (7x14 AU for protostars), we may apply our results to recently
observed hints of jet rotation (DG Tau) indicating a relatively flat disk
magnetic field profile, mu ~ 0.5. In general, our results are applicable to
both stellar and extragalactic sources of MHD jets.Comment: accepted by ApJ, high resolution version under
www.mpia-hd.mpg.de/homes/fendt
Magnetic interaction of jets and molecular clouds in NGC 4258
NGC 4258 is a well known spiral galaxy with a peculiar large scale jet flow detected in the radio and in H-alpha. Due to the special geometry of the galaxy, the jets emerge from the nuclear region through the galactic disk. Also the distribution of molecular gas looks different from that in other spiral galaxies: [12]CO(1-0)emission has only been detected in the center and along the jets and only up to distances of about 50 arcsec (1.8 kpc) from the nucleus. The reason for the CO concentration along the inner jets in NGC 4258 was not understood and is the motivation for the observations presented here. Using the IRAM interferometer, we mapped the [12]CO(1-0) emission of the central part of NGC 4258 along the nuclear jet direction in the inner 3 kpc. We detected two parallel CO ridges along a position angle of -25 degr with a total length of about 80 arcsec (2.8 kpc), separated by a CO-depleted funnel with a width of about 5 arcsec (175 pc). The H-alpha emission is more extended and broader than the CO emission with its maximum just in between the two CO ridges. In CO we see a peculiar velocity distribution in the iso-velocity map and p-v diagrams. We discuss different scenarios for an interpretation and present a model which can explain the observational results consistently. We propose here that the concentration of CO along the ridges is due to interaction of the rotating gas clouds with the jet's magnetic field by ambipolar diffusion. This magnetic interaction is thought to increase the time the molecular clouds reside near the jet thus leading to the quasi-static CO ridge
Accretion-Powered Stellar Winds II: Numerical Solutions for Stellar Wind Torques
[Abridged] In order to explain the slow rotation observed in a large fraction
of accreting pre-main-sequence stars (CTTSs), we explore the role of stellar
winds in torquing down the stars. For this mechanism to be effective, the
stellar winds need to have relatively high outflow rates, and thus would likely
be powered by the accretion process itself. Here, we use numerical
magnetohydrodynamical simulations to compute detailed 2-dimensional
(axisymmetric) stellar wind solutions, in order to determine the spin down
torque on the star. We explore a range of parameters relevant for CTTSs,
including variations in the stellar mass, radius, spin rate, surface magnetic
field strength, the mass loss rate, and wind acceleration rate. We also
consider both dipole and quadrupole magnetic field geometries.
Our simulations indicate that the stellar wind torque is of sufficient
magnitude to be important for spinning down a ``typical'' CTTS, for a mass loss
rate of yr. The winds are wide-angle,
self-collimated flows, as expected of magnetic rotator winds with moderately
fast rotation. The cases with quadrupolar field produce a much weaker torque
than for a dipole with the same surface field strength, demonstrating that
magnetic geometry plays a fundamental role in determining the torque. Cases
with varying wind acceleration rate show much smaller variations in the torque
suggesting that the details of the wind driving are less important. We use our
computed results to fit a semi-analytic formula for the effective Alfv\'en
radius in the wind, as well as the torque. This allows for considerable
predictive power, and is an improvement over existing approximations.Comment: Accepted for publication in Ap
Collimating, relativistic, magnetic jets from rotating disks
The magnetic flux distribution is determined by the solution of the
Grad-Shafranov equation. With differential rotation, i.e. the variation of the
iso-rotation parameter, the shape of the light surface must be calculated in an
iterative way. For the first time, we have calculated the force-free magnetic
structure of truly two-dimensional, relativistic jets, anchored in a
differentially rotating disk. Such an approach allows for a direct connection
between parameters of the central source (mass, rotation) and the extension of
the radio jet. We present an analytical estimate for the jet opening angle
along the asymptotic branches of the light surface. In general, differential
rotation of the iso-rotation parameter leads to an increase of the jet opening
angle. Comparison to the M87 jet shows agreement in the collimation distance.
We derive a light cylinder radius of the M87 jet of 50 Schwarzschild radii.Comment: 11 pags, 10 figs, Latex, accepted for Astron.Astroph., [email protected],
[email protected]
Cannonballs in the context of Gamma Ray Bursts: Formation sites ?
We investigate possible formation sites of the cannonballs (in the gamma ray
bursts context) by calculating their physical parameters, such as density,
magnetic field and temperature close to the origin. Our results favor scenarios
where the cannonballs form as instabilities (knots) within magnetized jets from
hyperaccreting disks. These instabilities would most likely set in beyond the
light cylinder where flow velocity with Lorentz factors as high as 2000 can be
achieved. Our findings challenge the cannonball model of gamma ray bursts if
these indeed form inside core-collapse supernovae (SNe) as suggested in the
literature; unless hyperaccreting disks and the corresponding jets are common
occurrences in core-collapse SNe.Comment: 10 pages, 12 figure
Relativistic expansion of a magnetized fluid
We study semi-analytical time-dependent solutions of the relativistic
magnetohydrodynamic (MHD) equations for the fields and the fluid emerging from
a spherical source. We assume uniform expansion of the field and the fluid and
a polytropic relation between the density and the pressure of the fluid. The
expansion velocity is small near the base but approaches the speed of light at
the light sphere where the flux terminates. We find self-consistent solutions
for the density and the magnetic flux. The details of the solution depend on
the ratio of the toroidal and the poloidal magnetic field, the ratio of the
energy carried by the fluid and the electromagnetic field and the maximum
velocity it reaches.Comment: 17 pages, 6 figures, accepted by Geophysical and Astrophysical Fluid
Dynamic
Formation of protostellar jets - effects of magnetic diffusion
We investigate the evolution of a disk wind into a collimated jet under the
influence of magnetic diffusivity, assuming that the turbulent pattern in the
disk will also enter the disk corona and the jet. Using the ZEUS-3D code in the
axisymmetry option we solve the time-dependent resistive MHD equations for a
model setup of a central star surrounded by an accretion disk. We find that the
diffusive jets propagate slower into the ambient medium. Close to the star we
find that a quasi stationary state evolves after several hundred (weak
diffusion) or thousand (strong diffusion) disk rotations. Magnetic diffusivity
affects the protostellar jet structure as follows. The jet poloidal magnetic
field becomes de-collimated. The jet velocity increases with increasing
diffusivity, while the degree of collimation for the hydrodynamic flow remains
more or less the same. We suggest that the mass flux is a proper tracer for the
degree of jet collimation and find indications of a critical value for the
magnetic diffusivity above which the jet collimation is only weak.Comment: 16 pages, 12 figs, accepted by Astron. and Astrop
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