47 research outputs found
Effect of angular opening on the dynamics of relativistic hydro jets
Context. Relativistic jets emerging from AGN cores transfer energy from the
core to their surrounding ISM/IGM. Because jets are observed to have finite
opening angles, one needs to quantify the role of conical versus cylindrical
jet propagation in this energy transfer. Aims. We use FR-II AGN jets parameter
with finite opening angles. We study the effect of the variation of the opening
angle on the dynamics and energy transfer of the jet. We also point out how the
characteristics of this external medium, such as its density profile, play a
role in the dynamics. Methods. This study exploits our parallel AMR code
MPI-AMRVAC with its special relativistic hydrodynamic model, incorporating an
equation of state with varying effective polytropic index. We studied mildly
under-dense jets up to opening angles of 10 degrees, at Lorentz factors of
about 10, inspired by observations. Instantaneous quantification of the various
ISM volumes and their energy content allows one to quantify the role of mixing
versus shock-heated cocoon regions over the time intervals. Results. We show
that a wider opening angle jet results in a faster deceleration of the jet and
leads to a wider cocoon dominated by Kelvin-Helmholtz and Rayleigh-Taylor
instabilities. The energy transfer mainly occurs in the shocked ISM region by
both the frontal bow shock and cocoon-traversing shock waves, in a roughly 3 to
1 ratio to the energy transfer of the mixing zone, for a 5 degree opening angle
jet. A rarefaction wave induces a dynamically formed layered structure of the
jet beam. Conclusions. Finite opening angle jets can efficiently transfer
significant fractions (25 % up to 70 %) of their injected energy over a growing
region of shocked ISM matter. The role of the ISM stratification is prominent
for determining the overall volume that is affected by relativistic jet
injection
Shape and evolution of wind-blown bubbles of massive stars: on the effect of the interstellar magnetic field
The winds of massive stars create large (>10 pc) bubbles around their
progenitors. As these bubbles expand they encounter the interstellar coherent
magnetic field which, depending on its strength, can influence the shape of the
bubble. We wish to investigate if, and how much, the interstellar magnetic
field can contribute to the shape of an expanding circumstellar bubble around a
massive star. We use the MPI-AMRVAC code to make magneto-hydrodynamical
simulations of bubbles, using a single star model, combined with several
different field strengths: B = 5, 10, and 20 muG for the interstellar magnetic
field. This covers the typical field strengths of the interstellar magnetic
fields found in the galactic disk and bulge. Furthermore, we present two
simulations that include both a 5 muG interstellar magnetic field and a 10,000
K interstellar medium and two different ISM densities to demonstrate how the
magnetic field can combine with other external factors to influence the
morphology of the circumstellar bubbles. Our results show that low magnetic
fields, as found in the galactic disk, inhibit the growth of the circumstellar
bubbles in the direction perpendicular to the field. As a result, the bubbles
become ovoid, rather than spherical. Strong interstellar fields, such as
observed for the galactic bulge, can completely stop the expansion of the
bubble in the direction perpendicular to the field, leading to the formation of
a tube-like bubble. When combined with a warm, high-density ISM the bubble is
greatly reduced in size, causing a dramatic change in the evolution of
temporary features inside the bubble. The magnetic field of the interstellar
medium can affect the shape of circumstellar bubbles. This effect may have
consequences for the shape and evolution of circumstellar nebulae and supernova
remnants, which are formed within the main wind-blown bubble.Comment: Proposed for acceptance for publication in Astronomy & Astrophysics.
The published version will contain animations of each simulatio
Synchrotron radiation of self-collimating relativistic MHD jets
The goal of this paper is to derive signatures of synchrotron radiation from
state-of-the-art simulation models of collimating relativistic
magnetohydrodynamic (MHD) jets featuring a large-scale helical magnetic field.
We perform axisymmetric special relativistic MHD simulations of the jet
acceleration region using the PLUTO code. The computational domain extends from
the slow magnetosonic launching surface of the disk up to 6000^2 Schwarzschild
radii allowing to reach highly relativistic Lorentz factors. The Poynting
dominated disk wind develops into a jet with Lorentz factors of 8 and is
collimated to 1 degree. In addition to the disk jet, we evolve a thermally
driven spine jet, emanating from a hypothetical black hole corona. Solving the
linearly polarized synchrotron radiation transport within the jet, we derive
VLBI radio and (sub-) mm diagnostics such as core shift, polarization
structure, intensity maps, spectra and Faraday rotation measure (RM), directly
from the Stokes parameters. We also investigate depolarization and the
detectability of a lambda^2-law RM depending on beam resolution and observing
frequency. We find non-monotonic intrinsic RM profiles which could be detected
at a resolution of 100 Schwarzschild radii. In our collimating jet geometry,
the strict bi-modality in polarization direction (as predicted by Pariev et
al.) can be circumvented. Due to relativistic aberration, asymmetries in the
polarization vectors across the jet can hint to the spin direction of the
central engine.Comment: Submitted to Ap
Can the magnetic field in the Orion arm inhibit the growth of instabilities in the bow shock of Betelgeuse?
Many evolved stars travel through space at supersonic velocities, which leads
to the formation of bow shocks ahead of the star where the stellar wind
collides with the interstellar medium (ISM). Herschel observations of the bow
shock of -Orionis show that the shock is almost free of instabilities,
despite being, at least in theory, subject to both Kelvin-Helmholtz and
Rayleigh-Taylor instabilities. A possible explanation for the lack of
instabilities lies in the presence of an interstellar magnetic field. We wish
to investigate whether the magnetic field of the interstellar medium (ISM) in
the Orion arm can inhibit the growth of instabilities in the bow shock of
-Orionis. We used the code MPI-AMRVAC to make magneto-hydrodynamic
simulations of a circumstellar bow shock, using the wind parameters derived for
-Orionis and interstellar magnetic field strengths of , and G, which fall within the boundaries of the observed
magnetic field strength in the Orion arm of the Milky Way. Our results show
that even a relatively weak magnetic field in the interstellar medium can
suppress the growth of Rayleigh-Taylor and Kelvin-Helmholtz instabilities,
which occur along the contact discontinuity between the shocked wind and the
shocked ISM. The presence of even a weak magnetic field in the ISM effectively
inhibits the growth of instabilities in the bow shock. This may explain the
absence of such instabilities in the Herschel observations of -Orionis.Comment: 5 pages, including 7 figures. The published version will include 4
animations. Accepted for publication in A&
Computing the dust distribution in the bowshock of a fast moving, evolved star
We study the hydrodynamical behavior occurring in the turbulent interaction
zone of a fast moving red supergiant star, where the circumstellar and
interstellar material collide. In this wind-interstellar medium collision, the
familiar bow shock, contact discontinuity, and wind termination shock
morphology forms, with localized instability development. Our model includes a
detailed treatment of dust grains in the stellar wind, and takes into account
the drag forces between dust and gas. The dust is treated as pressureless gas
components binned per grainsize, for which we use ten representative grainsize
bins. Our simulations allow to deduce how dust grains of varying sizes become
distributed throughout the circumstellar medium. We show that smaller dust
grains (radius <0.045 micro-meters) tend to be strongly bound to the gas and
therefore follow the gas density distribution closely, with intricate
finestructure due to essentially hydrodynamical instabilities at the
wind-related contact discontinuity. Larger grains which are more resistant to
drag forces are shown to have their own unique dust distribution, with
progressive deviations from the gas morphology. Specifically, small dust grains
stay entirely within the zone bound by shocked wind material. The large grains
are capable of leaving the shocked wind layer, and can penetrate into the
shocked or even unshocked interstellar medium. Depending on how the number of
dust grains varies with grainsize, this should leave a clear imprint in
infrared observations of bowshocks of red supergiants and other evolved stars.Comment: Accepted for publication in ApJL, 4 figure
Using numerical models of bow shocks to investigate the circumstellar medium of massive stars
Many massive stars travel through the interstellar medium at supersonic
speeds. As a result they form bow shocks at the interface between the stellar
wind. We use numerical hydrodynamics to reproduce such bow shocks numerically,
creating models that can be compared to observations. In this paper we discuss
the influence of two physical phenomena, interstellar magnetic fields and the
presence of interstellar dust grains on the observable shape of the bow shocks
of massive stars.
We find that the interstellar magnetic field, though too weak to restrict the
general shape of the bow shock, reduces the size of the instabilities that
would otherwise be observed in the bow shock of a red supergiant. The
interstellar dust grains, due to their inertia can penetrate deep into the bow
shock structure of a main sequence O-supergiant, crossing over from the ISM
into the stellar wind. Therefore, the dust distribution may not always reflect
the morphology of the gas. This is an important consideration for infrared
observations, which are dominated by dust emission.
Our models clearly show, that the bow shocks of massive stars are useful
diagnostic tools that can used to investigate the properties of both the
stellar wind as well as the interstellar medium.Comment: 7 pages, 4 figures, to be published in the Journal of Physics:
Conference Series (JPCS) as part of the proceedings of the 13th Annual
International Astrophysics Conferenc
Simulations of recoiling black holes: adaptive mesh refinement and radiative transfer
(Abridged) We here continue our effort to model the behaviour of matter when
orbiting or accreting onto a generic black hole by developing a new numerical
code employing advanced techniques geared solve the equations of in
general-relativistic hydrodynamics. The new code employs a number of
high-resolution shock-capturing Riemann-solvers and reconstruction algorithms,
exploiting the enhanced accuracy and the reduced computational cost of AMR
techniques. In addition, the code makes use of sophisticated ray-tracing
libraries that, coupled with general-relativistic radiation-transfer
calculations, allow us to compute accurately the electromagnetic emissions from
such accretion flows. We validate the new code by presenting an extensive
series of stationary accretion flows either in spherical or axial symmetry and
performed either in 2D or 3D. In addition, we consider the highly nonlinear
scenario of a recoiling black hole produced in the merger of a supermassive
black hole binary interacting with the surrounding circumbinary disc. In this
way we can present, for the first time, ray-traced images of the shocked fluid
and the light-curve resulting from consistent general-relativistic
radiation-transport calculations from this process. The work presented here
lays the ground for the development of a generic computational infrastructure
employing AMR techniques to deal accurately and self-consistently with
accretion flows onto compact objects. In addition to the accurate handling of
the matter, we provide a self-consistent electromagnetic emission from these
scenarios by solving the associated radiative-transfer problem. While magnetic
fields are presently excluded from our analysis, the tools presented here can
have a number of applications to study accretion flows onto black holes or
neutron stars.Comment: 20 pages, 20 figures, accepted for publication in A&
Pinwheels in the sky, with dust: 3D modeling of the Wolf-Rayet 98a environment
The Wolf-Rayet 98a (WR 98a) system is a prime target for interferometric
surveys, since its identification as a "rotating pinwheel nebulae", where
infrared images display a spiral dust lane revolving with a 1.4 year
periodicity. WR 98a hosts a WC9+OB star, and the presence of dust is puzzling
given the extreme luminosities of Wolf-Rayet stars. We present 3D hydrodynamic
models for WR 98a, where dust creation and redistribution are self-consistently
incorporated. Our grid-adaptive simulations resolve details in the wind
collision region at scales below one percent of the orbital separation (~4 AU),
while simulating up to 1300 AU. We cover several orbital periods under
conditions where the gas component alone behaves adiabatic, or is subject to
effective radiative cooling. In the adiabatic case, mixing between stellar
winds is effective in a well-defined spiral pattern, where optimal conditions
for dust creation are met. When radiative cooling is incorporated, the
interaction gets dominated by thermal instabilities along the wind collision
region, and dust concentrates in clumps and filaments in a volume-filling
fashion, so WR 98a must obey close to adiabatic evolutions to demonstrate the
rotating pinwheel structure. We mimic Keck, ALMA or future E-ELT observations
and confront photometric long-term monitoring. We predict an asymmetry in the
dust distribution between leading and trailing edge of the spiral, show that
ALMA and E-ELT would be able to detect fine-structure in the spiral indicative
of Kelvin-Helmholtz development, and confirm the variation in photometry due to
the orientation. Historic Keck images are reproduced, but their resolution is
insufficient to detect the details we predict.Comment: Accepted for publication in mnra
3-D simulations of shells around massive stars
As massive stars evolve, their winds change. This causes a series of
hydrodynamical interactions in the surrounding medium. Whenever a fast wind
follows a slow wind phase, the fast wind sweeps up the slow wind in a shell,
which can be observed as a circumstellar nebula.
One of the most striking examples of such an interaction is when a massive
star changes from a red supergiant into a Wolf-Rayet star. Nebulae resulting
from such a transition have been observed around many Wolf-Rayet stars and show
detailed, complicated structures owing to local instabilities in the swept-up
shells.
Shells also form in the case of massive binary stars, where the winds of two
stars collide with one another. Along the collision front gas piles up, forming
a shell that rotates along with the orbital motion of the binary stars. In this
case the shell follows the surface along which the ram pressure of the two
colliding winds is in balance.
Using the MPI-AMRVAC hydrodynamics code we have made multi-dimensional
simulations of these interactions in order to model the formation and evolution
of these circumstellar nebulae and explore whether full 3D simulations are
necessary to obtain accurate models of such nebulae.Comment: 5 Pages, 4 figures, Proceedings of the 39th Liege Astrophysical
Colloquium, held in Liege 12-16 July 201