480 research outputs found
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
A disk-wind model with correct crossing of all MHD critical surfaces
The classical Blandford & Payne (1982) model for the magnetocentrifugal
acceleration and collimation of a disk-wind is revisited and refined. In the
original model, the gas is cold and the solution is everywhere subfast
magnetosonic. In the present model the plasma has a finite temperature and the
self-consistent solution of the MHD equations starts with a subslow
magnetosonic speed which subsequently crosses all critical points, at the slow
magnetosonic, Alfven and fast magnetosonic separatrix surfaces. The superfast
magnetosonic solution thus satisfies MHD causality. Downstream of the fast
magnetosonic critical point the poloidal streamlines overfocus towards the axis
and the solution is terminated. The validity of the model to disk winds
associated with young stellar objects is briefly discussed. ~Comment: 13 pages, MNRAS accepted for publicatio
A radio jet drives a molecular and atomic gas outflow in multiple regions within one square kiloparsec of the nucleus of the nearby galaxy IC5063
We analyzed near-infrared data of the nearby galaxy IC5063 taken with the
Very Large Telescope SINFONI instrument. IC5063 is an elliptical galaxy that
has a radio jet nearly aligned with the major axis of a gas disk in its center.
The data reveal multiple signatures of molecular and atomic gas that has been
kinematically distorted by the passage of the jet plasma or cocoon within an
area of ~1 kpc^2. Concrete evidence that the interaction of the jet with the
gas causes the gas to accelerate comes from the detection of outflows in four
different regions along the jet trail: near the two radio lobes, between the
radio emission tip and the optical narrow-line-region cone, and at a region
with diffuse 17.8 GHz emission midway between the nucleus and the north radio
lobe. The outflow in the latter region is biconical, centered 240 pc away from
the nucleus, and oriented perpendicularly to the jet trail. The diffuse
emission that is observed as a result of the gas entrainment or scattering
unfolds around the trail and away from the nucleus with increasing velocity. It
overall extends for >700 pc parallel and perpendicular to the trail. Near the
outflow starting points, the gas has a velocity excess of 600 km/s to 1200 km/s
with respect to ordered motions, as seen in [FeII], Pa alpha, or H2 lines. High
H2 (1-0) S(3)/S(1) flux ratios indicate non-thermal excitation of gas in the
diffuse outflow.Comment: Accepted for publication in Ap
Counter-rotation in relativistic magnetohydrodynamic jets
Young stellar object observations suggest that some jets rotate in the
opposite direction with respect to their disk. In a recent study, Sauty et al.
(2012) have shown that this does not contradict the magnetocentrifugal
mechanism that is believed to launch such outflows. Signatures of motions
transverse to the jet axis and in opposite directions have recently been
measured in M87 (Meyer et al. 2013). One possible interpretation of this motion
is the one of counter rotating knots. Here, we extend our previous analytical
derivation of counter-rotation to relativistic jets, demonstrating that
counter-rotation can indeed take place under rather general conditions. We show
that both the magnetic field and a non-negligible enthalpy are necessary at the
origin of counter-rotating outflows, and that the effect is associated with a
transfer of energy flux from the matter to the electromagnetic field. This can
be realized in three cases : if a decreasing enthalpy causes an increase of the
Poynting flux, if the flow decelerates, or, if strong gradients of the magnetic
field are present. An illustration of the involved mechanism is given by an
example of relativistic MHD jet simulation.Comment: Accepted for publication in ApJ
Two-component jet simulations: II. Combining analytical disk and stellar MHD outflow solutions
Theoretical arguments along with observational data of YSO jets suggest the
presence of two steady components: a disk wind type outflow needed to explain
the observed high mass loss rates and a stellar wind type outflow probably
accounting for the observed stellar spin down.
Each component's contribution depends on the intrinsic physical properties of
the YSO-disk system and its evolutionary stage. The main goal of this paper is
to understand some of the basic features of the evolution, interaction and
co-existence of the two jet components over a parameter space and when time
variability is enforced. Having studied separately the numerical evolution of
each type of the complementary disk and stellar analytical wind solutions in
Paper I of this series, we proceed here to mix together the two models inside
the computational box. The evolution in time is performed with the PLUTO code,
investigating the dynamics of the two-component jets, the modifications each
solution undergoes and the potential steady state reached.Comment: accepted for publication in A&
Young stellar object jet models: From theory to synthetic observations
Astronomical observations, analytical solutions and numerical simulations
have provided the building blocks to formulate the current theory of young
stellar object jets. Although each approach has made great progress
independently, it is only during the last decade that significant efforts are
being made to bring the separate pieces together. Building on previous work
that combined analytical solutions and numerical simulations, we apply a
sophisticated cooling function to incorporate optically thin energy losses in
the dynamics. On the one hand, this allows a self-consistent treatment of the
jet evolution and on the other, it provides the necessary data to generate
synthetic emission maps. Firstly, analytical disk and stellar outflow solutions
are properly combined to initialize numerical two-component jet models inside
the computational box. Secondly, magneto-hydrodynamical simulations are
performed in 2.5D, following properly the ionization and recombination of a
maximum of ions. Finally, the outputs are post-processed to produce
artificial observational data. The first two-component jet simulations, based
on analytical models, that include ionization and optically thin radiation
losses demonstrate promising results for modeling specific young stellar object
outflows. The generation of synthetic emission maps provides the link to
observations, as well as the necessary feedback for the further improvement of
the available models.Comment: accepted for publication A&A, 20 pages, 11 figure
Velocity asymmetries in YSO jets: Intrinsic and extrinsic mechanisms
It is a well established fact that some YSO jets (e.g. RW Aur) display
different propagation speeds between their blue and red shifted parts, a
feature possibly associated with the central engine or the environment in which
the jet propagates. In order to understand the origin of asymmetric YSO jet
velocities, we investigate the efficiency of two candidate mechanisms, one
based on the intrinsic properties of the system and one based on the role of
the external medium. In particular, a parallel or anti-parallel configuration
between the protostellar magnetosphere and the disk magnetic field is
considered and the resulting dynamics are examined both in an ideal and a
resistive magneto-hydrodynamical (MHD) regime. Moreover, we explore the effects
of a potential difference in the pressure of the environment, as a consequence
of the non-uniform density distribution of molecular clouds. Ideal and
resistive axisymmetric numerical simulations are carried out for a variety of
models, all of which are based on a combination of two analytical solutions, a
disk wind and a stellar outflow. We find that jet velocity asymmetries can
indeed occur both when multipolar magnetic moments are present in the star-disk
system as well as when non-uniform environments are considered. The latter case
is an external mechanism that can easily explain the large time scale of the
phenomenon, whereas the former one naturally relates it to the YSO intrinsic
properties. [abridged]Comment: accepted for publication in A&
Nonradial and nonpolytropic astrophysical outflows VIII. A GRMHD generalization for relativistic jets
Steady axisymmetric outflows originating at the hot coronal magnetosphere of
a Schwarzschild black hole and surrounding accretion disk are studied in the
framework of general relativistic magnetohydrodynamics (GRMHD). The assumption
of meridional self-similarity is adopted for the construction of
semi-analytical solutions of the GRMHD equations describing outflows close to
the polar axis. In addition, it is assumed that relativistic effects related to
the rotation of the black hole and the plasma are negligible compared to the
gravitational and other energetic terms. The constructed model allows us to
extend previous MHD studies for coronal winds from young stars to spine jets
from Active Galactic Nuclei surrounded by disk-driven outflows. The outflows
are thermally driven and magnetically or thermally collimated. The collimation
depends critically on an energetic integral measuring the efficiency of the
magnetic rotator, similarly to the non relativistic case. It is also shown that
relativistic effects affect quantitatively the depth of the gravitational well
and the coronal temperature distribution in the launching region of the
outflow. Similarly to previous analytical and numerical studies, relativistic
effects tend to increase the efficiency of the thermal driving but reduce the
effect of magnetic self-collimation.Comment: 20 page, Accepted in A&A 10/10/200
Monitoring the Bi-Directional Relativistic Jets of the Radio Galaxy 1946+708
We report on a multi-frequency, multi-epoch campaign of Very Long Baseline
Interferometry observations of the radio galaxy 1946+708 using the VLBA and a
Global VLBI array. From these high-resolution observations we deduce the
kinematic age of the radio source to be 4000 years, comparable with the
ages of other Compact Symmetric Objects (CSOs). Ejections of pairs of jet
components appears to take place on time scales of 10 years and these
components in the jet travel outward at intrinsic velocities between 0.6 and
0.9 c. From the constraint that jet components cannot have intrinsic velocities
faster than light, we derive H_0 > 57 km s^-1 Mpc^-1 from the fastest pair of
components launched from the core. We provide strong evidence for the ejection
of a new pair of components in ~1997. From the trajectories of the jet
components we deduce that the jet is most likely to be helically confined,
rather than purely ballistic in nature.Comment: 20 pages, 8 figures, accepted to Ap
Overdensity of SMGs in fields containing z ∼ 0.3 galaxies: magnification bias and the implications for studies of galaxy evolution
We report a remarkable overdensity of high-redshift submillimetre galaxies (SMG), 4–7 times the background, around a statistically complete sample of twelve 250 μm selected galaxies at z = 0.35, which were targeted by ALMA in a study of gas tracers. This overdensity is consistent with the effect of lensing by the haloes hosting the target z = 0.35 galaxies. The angular cross-correlation in this sample is consistent with statistical measures of this effect made using larger sub-mm samples. The magnitude of the overdensity as a function of radial separation is consistent with intermediate scale lensing by haloes of the order of 7×1013 M⊙ , which should host one or possibly two bright galaxies and several smaller satellites. This is supported by observational evidence of interaction with satellites in four out of the six fields with SMG, and membership of a spectroscopically defined group for a fifth. We also investigate the impact of these SMG on the reported Herschel fluxes of the z = 0.35 galaxies, as they produce significant contamination in the 350 and 500 μm Herschel bands. The higher than random incidence of these boosting events implies a significantly larger bias in the sub-mm colours of Herschel sources associated with z < 0.7 galaxies than has previously been assumed, with fboost = 1.13, 1.26, 1.44 at 250, 350, and 500 μm . This could have implications for studies of spectral energy distributions, source counts, and luminosity functions based on Herschel samples at z = 0.2–0.7
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