661 research outputs found
Numerical simulations of radiative magnetized Herbig-Haro jets: the influence of pre-ionization from X-rays on emission lines
We investigate supersonic, axisymmetric magnetohydrodynamic (MHD) jets with a
time-dependent injection velocity by numerical simulations with the PLUTO code.
Using a comprehensive set of parameters, we explore different jet
configurations in the attempt to construct models that can be directly compared
to observational data of microjets. In particular, we focus our attention on
the emitting properties of traveling knots and construct, at the same time,
accurate line intensity ratios and surface brightness maps. Direct comparison
of the resulting brightness and line intensity ratios distributions with
observational data of microjets shows that a closer match can be obtained only
when the jet material is pre-ionized to some degree. A very likely source for a
pre-ionized medium is photoionization by X-ray flux coming from the central
object.Comment: Accepted for publication in Ap
Mass Accretion Processes in Young Stellar Objects: Role of Intense Flaring Activity
According to the magnetospheric accretion scenario, young low-mass stars are
surrounded by circumstellar disks which they interact with through accretion of
mass. The accretion builds up the star to its final mass and is also believed
to power the mass outflows, which may in turn have a significant role in
removing the excess angular momentum from the star-disk system. Although the
process of mass accretion is a critical aspect of star formation, some of its
mechanisms are still to be fully understood. On the other hand, strong flaring
activity is a common feature of young stellar objects (YSOs). In the Sun, such
events give rise to perturbations of the interplanetary medium. Similar but
more energetic phenomena occur in YSOs and may influence the circumstellar
environment. In fact, a recent study has shown that an intense flaring activity
close to the disk may strongly perturb the stability of circumstellar disks,
thus inducing mass accretion episodes (Orlando et al. 2011). Here we review the
main results obtained in the field and the future perspectives.Comment: 4 pages, 2 Figures; accepted for publication on Acta Polytechnica
(Proceedings of the Frascati Workshop 2013
On the convergence of Magnetorotational turbulence in stratified isothermal shearing boxes
We consider the problem of convergence in stratified isothermal shearing
boxes with zero net magnetic flux. We present results with the highest
resolution to-date--up to 200 grid-point per pressure scale height--that show
no clear evidence of convergence. Rather, the Maxwell stresses continue to
decrease with increasing resolution. We propose some possible scenarios to
explain the lack of convergence based on multi-layer dynamo systems.Comment: 10 pages, 4 figures, accepted for publication in ApJ Letter
Fully Convective Magnetorotational Turbulence in Stratified Shearing Boxes
We present a numerical study of turbulence and dynamo action in stratified
shearing boxes with zero magnetic flux. We assume that the fluid obeys the
perfect gas law and has finite (constant) thermal diffusivity. We choose
radiative boundary conditions at the vertical boundaries in which the heat flux
is propor- tional to the fourth power of the temperature. We compare the
results with the corresponding cases in which fixed temperature boundary
conditions are applied. The most notable result is that the formation of a
fully convective state in which the density is nearly constant as a function of
height and the heat is transported to the upper and lower boundaries by
overturning motions is robust and persists even in cases with radiative
boundary conditions. Interestingly, in the convective regime, although the
diffusive transport is negligible the mean stratification does not relax to an
adiabatic state.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
Magnetic Helicities and Dynamo Action in Magneto-rotationally Driven Turbulence
We examine the relationship between magnetic flux generation, taken as an
indicator of large-scale dynamo action, and magnetic helicity, computed as an
integral over the dynamo volume, in a simple dynamo. We consider dynamo action
driven by Magneto-Rotational Turbulence (MRT) within the shearing-box
approximation. We consider magnetically open boundary conditions that allow a
flux of helicity in or out of the computational domain. We circumvent the
problem of the lack of gauge invariance in open domains by choosing a
particular gauge -- the winding gauge -- that provides a natural interpretation
in terms of average winding number of pairwise field lines. We use this gauge
precisely to define and measure the helicity and helicity flux for several
realizations of dynamo action. We find in these cases, that the system as a
whole does not break reflectional symmetry and the total helicity remains small
even in cases when substantial magnetic flux is generated. We find no
particular connection between the generation of magnetic flux and the helicity
or the helicity flux through the boundaries. We suggest that this result may be
due to the essentially nonlinear nature of the dynamo processes in MRT.Comment: 26 pages, 10 figures, ApJ accepte
Relativistic MHD Simulations of Jets with Toroidal Magnetic Fields
This paper presents an application of the recent relativistic HLLC
approximate Riemann solver by Mignone & Bodo to magnetized flows with vanishing
normal component of the magnetic field.
The numerical scheme is validated in two dimensions by investigating the
propagation of axisymmetric jets with toroidal magnetic fields.
The selected jet models show that the HLLC solver yields sharper resolution
of contact and shear waves and better convergence properties over the
traditional HLL approach.Comment: 12 pages, 5 figure
MAGNETOROTATIONAL TURBULENCE IN STRATIFIED SHEARING BOXES WITH PERFECT GAS EQUATION OF STATE AND FINITE THERMAL DIFFUSIVITY
We present a numerical study of turbulence and dynamo action in stratified
shearing boxes with zero mean magnetic flux. We assume that the fluid obeys the
perfect gas law and has finite (constant) thermal diffusivity. The calculations
begin from an isothermal state spanning three scale heights above and below the
mid-plane. After a long transient the layers settle to a stationary state in
which thermal losses out of the boundaries are balanced by dissipative heating.
We identify two regimes. A conductive regime in which the heat is transported
mostly by conduction and the density decreases with height. In the limit of
large thermal diffusivity this regime resembles the more familiar isothermal
case. Another, the convective regime, observed at smaller values of the thermal
diffusivity, in which the layer becomes unstable to overturning motions, the
heat is carried mostly by advection and the density becomes nearly constant
throughout the layer. In this latter constant-density regime we observe
evidence for large-scale dynamo action leading to a substantial increase in
transport efficiency relative to the conductive cases.Comment: 18 pages, 10 figures, ApJ accepte
Symmetries, scaling laws and convergence in shearing-box simulations of MRI driven turbulence
We consider the problem of convergence in homogeneous shearing box simula-
tions of magneto-rotationally driven turbulence. When there is no mean magnetic
flux, if the equations are non dimensionalized with respect to the diffusive
scale, the only free parameter in the problem is the size of the computational
domain. The problem of convergence then relates to the asymptotic form of the
solutions as the computational box size becomes large. By using a numerical
code with a high order of accuracy we show that the solutions become
asymptotically inde- pendent of domain size. We also show that cases with weak
magnetic flux join smoothly to the zero flux cases as the flux vanishes. These
results are consistent with the operation of a subcritical small-scale dynamo
driving the turbulence. We conclude that for this type of turbulence the
angular momentum transport is a proportional to the diffusive flux and
therefore has limited relevance in as- trophysical situations.Comment: 28 pages, 7 figures, submitted to Ap
Aspect ratio dependence in magnetorotational instability shearing box simulations
Aims: We study the changes in the properties of turbulence driven by the
magnetorotational instability in a shearing box, as the computational domain
size in the radial direction is varied relative to the height Methods: We
perform 3D simulations in the shearing box approximation, with a net magnetic
flux, and we consider computational domains with different aspect ratios
Results: We find that in boxes of aspect ratio unity the transport of angular
momentum is strongly intermittent and dominated by channel solutions in
agreement with previous work. In contrast, in boxes with larger aspect ratio,
the channel solutions and the associated intermittent behavior disappear.
Conclusions: There is strong evidence that, as the aspect ratio becomes larger,
the characteristics of the solution become aspect ratio independent. We
conclude that shearing box calculations with aspect ratio unity or near unity
may introduce spurious effects.Comment: 5 pages, 6 figures, Astronomy and Astrophysics accepte
Time-dependent MHD shocks and line intensity ratios in the HH 30 jet: a focus on cooling function and numerical resolution
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