630 research outputs found
Optimising Boltzmann codes for the Planck era
High precision measurements of the Cosmic Microwave Background (CMB)
anisotropies, as can be expected from the Planck satellite, will require
high-accuracy theoretical predictions as well. One possible source of
theoretical uncertainty is the numerical error in the output of the Boltzmann
codes used to calculate angular power spectra. In this work, we carry out an
extensive study of the numerical accuracy of the public Boltzmann code CAMB,
and identify a set of parameters which determine the error of its output. We
show that at the current default settings, the cosmological parameters
extracted from data of future experiments like Planck can be biased by several
tenths of a standard deviation for the six parameters of the standard
Lambda-CDM model, and potentially more seriously for extended models. We
perform an optimisation procedure that leads the code to achieve sufficient
precision while at the same time keeping the computation time within reasonable
limits. Our conclusion is that the contribution of numerical errors to the
theoretical uncertainty of model predictions is well under control -- the main
challenges for more accurate calculations of CMB spectra will be of an
astrophysical nature instead.Comment: 13 pages, 4 figure
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
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
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
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
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
Rico: An Accurate Cosmological Recombination Code
We present Rico, a code designed to compute the ionization fraction of the
Universe during the epoch of hydrogen and helium recombination with an
unprecedented combination of speed and accuracy. This is accomplished by
training the machine learning code Pico on the calculations of a multi-level
cosmological recombination code which self-consistently includes several
physical processes that were neglected previously. After training, Rico is used
to fit the free electron fraction as a function of the cosmological parameters.
While, for example at low redshifts (z<~900), much of the net change in the
ionization fraction can be captured by lowering the hydrogen fudge factor in
Recfast by about 3%, Rico provides a means of effectively using the accurate
ionization history of the full recombination code in the standard cosmological
parameter estimation framework without the need to add new or refined fudge
factors or functions to a simple recombination model. Within the new approach
presented here it is easy to update Rico whenever a more accurate full
recombination code becomes available. Once trained, Rico computes the
cosmological ionization history with negligible fitting error in ~10
milliseconds, a speed-up of at least 10^6 over the full recombination code that
was used here. Also Rico is able to reproduce the ionization history of the
full code to a level well below 0.1%, thereby ensuring that the theoretical
power spectra of CMB fluctuations can be computed to sufficient accuracy and
speed for analysis from upcoming CMB experiments like Planck. Furthermore it
will enable cross-checking different recombination codes across cosmological
parameter space, a comparison that will be very important in order to assure
the accurate interpretation of future cosmic microwave background data.Comment: 14 pages, 11 figures, submitted to PR
Global axisymmetric simulations of photoevaporation and magnetically driven protoplanetary disk winds
Photoevaporation and magnetically driven winds are two independent mechanisms
to remove mass from protoplanetary disks. In addition to accretion, the effect
of these two principles acting concurrently could be significant and the
transition between those two has not been extensively studied and quantified in
the literature yet. In order to contribute to the understanding of disk winds,
we present the phenomena emerging in the framework of two-dimensional
axisymmetric, non-ideal magnetohydrodynamic simulations including EUV-/ X-ray
driven photoevaporation. Of particular interest are the examination of the
transition region between photoevaporation and magnetically driven wind, the
possibility of emerging magneto-centrifugal wind effects, as well as the
morphology of the wind itself depending on the strength of the magnetic field.
We use the PLUTO code in a 2.5D axisymmetric configuration with additional
treatment of EUV-/ X-ray heating and dynamic ohmic diffusion based on a
semi-analytical chemical model. We identify the transition between both outflow
types to occur for values of the initial plasma beta , while
magnetically driven winds generally outperform photoevaporation for stronger
fields. In our simulations we observe irregular and asymmetric outflows for
stronger magnetic fields. In the weak field regime the photoevaporation rates
are slightly lowered by perturbations of the gas density in the inner regions
of the disk. Overall, our results predict a wind with a lever arm smaller than
1.5, consistent with a hot magneto-thermal wind. Stronger accretion flows are
present for values of .Comment: Published in A&A 633, A21 (2020
Molecular outflows in the young open cluster IC348
We present a wide-field survey of the young open cluster IC348 for molecular
H2 outflows. Outflow activity is only found at its south-western limit, where a
new subcluster of embedded sources is in an early phase of its formation. If
the IC348 cluster had been built up by such subclusters forming at different
times, this could explain the large age-spread that Herbig (1998) found for the
IC348 member stars. In addition to several compact groups of H2 knots, our
survey reveals a large north-south oriented outflow, and we identify the newly
discovered far-infrared and mm-object IC348MMS as its source. New deep images
in the 1-0 S(1) line of molecular hydrogen trace the HH211 jet and counterjet
as highly-collimated chains of knots, resembling the interferometric CO and SiO
jets. This jet system appears rotated counter-clockwise by about 3 degrees with
respect to the prominent H2 bow shocks. Furthermore, we resolve HH211-mm as a
double point-like source in the mm-continuum.Comment: 10 pages, 9 figures, accepted for publication in Ap
Ultra-Relativistic Magneto-Hydro-Dynamic Jets in the context of Gamma Ray Bursts
We present a detailed numerical study of the dynamics and evolution of
ultrarelativistic magnetohydrodynamic jets in the black hole-disk system under
extreme magnetization conditions. We find that Lorentz factors of up to 3000
are achieved and derived a modifiedMichel scaling (Gamma ~ sigma) which allows
for a wide variation in the flow Lorentz factor. Pending contamination induced
by mass-entrainment, the linear Michel scaling links modulations in the
ultrarelativistic wind to variations in mass accretion in the disk for a given
magnetization. The jet is asymptotically dominated by the toroidal magnetic
field allowing for efficient collimation. We discuss our solutions (jets) in
the context of Gamma ray bursts and describe the relevant features such as the
high variability in the Lorentz factor and how high collimation angles (~ 0-5
degrees), or cylindrical jets, can be achieved. We isolate a jet instability
mechanism we refer to as the "bottle-neck" instability which essentially relies
on a high magnetization and a recollimation of the magnetic flux surfaces. The
instability occurs at large radii where any dissipation of the magnetic energy
into radiation would in principle result in an optically thin emission.Comment: 31 pages, 6 figures. Submitted to ApJ. Higher Quality figures at
http://www.capca.ucalgary.ca/paper
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