463 research outputs found
Synthetic synchrotron emission maps from MHD models for the jet of M87
We present self-consistent global, steady-state MHD models and synthetic
optically thin synchrotron emission maps for the jet of M87. The model consist
of two distinct zones: an inner relativistic outflow, which we identify with
the observed jet, and an outer cold disk-wind. While the former does not
self-collimate efficiently due to its high effective inertia, the latter
fulfills all the conditions for efficient collimation by the
magneto-centrifugal mechanism. Given the right balance between the effective
inertia of the inner flow and the collimation efficiency of the outer disk
wind, the relativistic flow is magnetically confined into a well collimated
beam and matches the measurements of the opening angle of M87 over several
orders of magnitude in spatial extent. The synthetic synchrotron maps reproduce
the morphological structure of the jet of M87, i.e. center-bright profiles near
the core and limb-bright profiles away from the core. At the same time, they
also show a local increase of brightness at some distance along the axis
associated to a recollimation shock in the MHD model. Its location coincides
with the position of the optical knot HST-1. In addition our best fitting model
is consistent with a number of observational constraints such as the magnetic
field in the knot HST-1, and the jet-to-counterjet brightness ratio.Comment: 9 pages, 9 figures, accepted by Ap
Magnetic acceleration of ultra-relativistic jets in gamma-ray burst sources
We present a relativistic-MHD numerical study of axisymmetric, magnetically
driven jets with parameters applicable to gamma-ray burst (GRB) flows. We also
present analytic expressions for the asymptotic jet shape and other flow
parameters that agree very well with the numerical results. All
current-carrying outflows exhibit self-collimation and consequent acceleration
near the rotation axis, but unconfined outflows lose causal connectivity across
the jet and therefore do not collimate or accelerate efficiently in their outer
regions. Magnetically accelerated jets confined by an external pressure that
varies with distance with a power-law index < 2 assume a paraboloidal shape and
have an acceleration efficiency > 50%. They attain Lorentz factors > 30 on
scales 10^9-3x10^10 cm, consistent with the possibility that short/hard GRB
jets are accelerated on scales where they can be confined by moderately
relativistic winds from accretion discs, and > 100 on scales 10^10-10^12 cm,
consistent with the possibility that long/soft GRB jets are accelerated within
the envelopes of collapsing massive stars. We also find that the Lorentz factor
of a magnetically accelerated jet is approximately inversely proportional to
the opening half-angle of the poloidal streamlines. This implies that the
gamma-ray emitting components of GRB outflows are very narrow, with a
half-angle < 1 degree in regions where the Lorentz factor exceeds 100, and that
the afterglow light curves of these components would either exhibit a very
early jet break or show no jet break at all.Comment: submitted to MNRAS, 32 pages, 23 figure
Magnetic collimation of the relativistic jet in M87
We apply a two-zone MHD model to the jet of M87. The model consists of an
inner relativistic outflow, which is surrounded by a non-relativistic outer
disk-wind. The outer disk-wind collimates very well through magnetic
self-collimation and confines the inner relativistic jet into a narrow region
around the rotation axis. Further, we show by example, that such models
reproduce very accurately the observed opening angle of the M87 jet over a
large range from the kiloparsec scale down to the sub-parsec scale.Comment: 4 pages, 2 figures, accepted by A&A Letter
Rarefaction acceleration of ultrarelativistic magnetized jets in gamma-ray burst sources
When a magnetically-dominated super-fast magnetosonic GRB jet leaves the
progenitor star the external pressure support may drop and the jet may enter
the regime of ballistic expansion during which its magnetic acceleration
becomes highly ineffective. However, recent numerical simulations suggested
that the transition to this regime is accompanied by a sudden "burst" of
acceleration. We confirm this finding and attribute the acceleration to the
sideways expansion of the jet - the magnetic energy is converted into the
kinetic one in the strong magnetosonic rarefaction wave, which is launched when
the jet loses its external support. This type of acceleration, the rarefaction
acceleration, is specific to relativistic jets because their energy budget can
still be dominated by magnetic energy even in highly super-fast magnetosonic
regime. Just like the collimation acceleration of externally confined
magnetized jets, it is connected with the geometry of magnetic flux sufaces. In
both cases, in the acceleration zone the poloidal field lines diverge faster
than in the monopolar configuration. On the other hand, whereas the collimation
acceleration keeps the product of jet opening angle and Lorentz factor somewhat
below unity, the rarefaction acceleration allows to make it significantly
larger, in agreement with the standard model of jet breaks in afterglow light
curves.Comment: Submitted to MNRA
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&
Two-component jet simulations: I. Topological stability of analytical MHD outflow solutions
Observations of collimated outflows in young stellar objects indicate that
several features of the jets can be understood by adopting the picture of a
two-component outflow, wherein a central stellar component around the jet axis
is surrounded by an extended disk-wind. The precise contribution of each
component may depend on the intrinsic physical properties of the YSO-disk
system as well as its evolutionary stage. In this context, the present article
starts a systematic investigation of two-component jet models via
time-dependent simulations of two prototypical and complementary analytical
solutions, each closely related to the properties of stellar-outflows and
disk-winds. These models describe a meridionally and a radially self-similar
exact solution of the steady-state, ideal hydromagnetic equations,
respectively. By using the PLUTO code to carry out the simulations, the study
focuses on the topological stability of each of the two analytical solutions,
which are successfully extended to all space by removing their singularities.
In addition, their behavior and robustness over several physical and numerical
modifications is extensively examined. It is found that radially self-similar
solutions (disk-winds) always reach a final steady-state while maintaining all
their well-defined properties. The different ways to replace the singular part
of the solution around the symmetry axis, being a first approximation towards a
two-component outflow, lead to the appearance of a shock at the super-fast
domain corresponding to the fast magnetosonic separatrix surface. Conversely,
the asymptotic configuration and the stability of meridionally self-similar
models (stellar-winds) is related to the heating processes at the base of the
wind.Comment: Accepted for publication in A&
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&
A new method to measure evolution of the galaxy luminosity function
We present a new efficient technique for measuring evolution of the galaxy
luminosity function. The method reconstructs the evolution over the
luminosity-redshift plane using any combination of three input dataset types:
1) number counts, 2) galaxy redshifts, 3) integrated background flux
measurements. The evolution is reconstructed in adaptively sized regions of the
plane according to the input data as determined by a Bayesian formalism. We
demonstrate the performance of the method using a range of different synthetic
input datasets. We also make predictions of the accuracy with which forthcoming
surveys conducted with SCUBA2 and the Herschel Space Satellite will be able to
measure evolution of the sub-millimetre luminosity function using the method.Comment: MNRAS in press. 14 pages, 7 figures
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
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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