438 research outputs found
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
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&
The Dynamics of Radiative Shock Waves: Linear and Nonlinear Evolution
The stability properties of one-dimensional radiative shocks with a power-law
cooling function of the form are the main
subject of this work. The linear analysis originally presented by Chevalier &
Imamura, is thoroughfully reviewed for several values of the cooling index
and higher overtone modes. Consistently with previous results, it is
shown that the spectrum of the linear operator consists in a series of modes
with increasing oscillation frequency. For each mode a critical value of the
cooling index, , can be defined so that modes with are unstable, while modes with
are stable. The perturbative analysis is complemented by several numerical
simulations to follow the time-dependent evolution of the system for different
values of . Particular attention is given to the comparison between
numerical and analytical results (during the early phases of the evolution) and
to the role played by different boundary conditions. It is shown that an
appropriate treatment of the lower boundary yields results that closely follow
the predicted linear behavior. During the nonlinear regime, the shock
oscillations saturate at a finite amplitude and tend to a quasi-periodic cycle.
The modes of oscillations during this phase do not necessarily coincide with
those predicted by linear theory, but may be accounted for by mode-mode
coupling.Comment: 33 pages, 12 figures, accepted for publication on the Astrophysical
Journa
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
Aerial scene classification through fine-tuning with adaptive learning rates and label smoothing
Remote Sensing (RS) image classification has recently attracted great attention for its application in different tasks, including environmental monitoring, battlefield surveillance, and geospatial object detection. The best practices for these tasks often involve transfer learning from pre-trained Convolutional Neural Networks (CNNs). A common approach in the literature is employing CNNs for feature extraction, and subsequently train classifiers exploiting such features. In this paper, we propose the adoption of transfer learning by fine-tuning pre-trained CNNs for end-to-end aerial image classification. Our approach performs feature extraction from the fine-tuned neural networks and remote sensing image classification with a Support Vector Machine (SVM) model with linear and Radial Basis Function (RBF) kernels. To tune the learning rate hyperparameter, we employ a linear decay learning rate scheduler as well as cyclical learning rates. Moreover, in order to mitigate the overfitting problem of pre-trained models, we apply label smoothing regularization. For the fine-tuning and feature extraction process, we adopt the Inception-v3 and Xception inception-based CNNs, as well the residual-based networks ResNet50 and DenseNet121. We present extensive experiments on two real-world remote sensing image datasets: AID and NWPU-RESISC45. The results show that the proposed method exhibits classification accuracy of up to 98%, outperforming other state-of-the-art methods
Low-mass planets in nearly inviscid disks: Numerical treatment
Embedded planets disturb the density structure of the ambient disk and
gravitational back-reaction will induce possibly a change in the planet's
orbital elements. The accurate determination of the forces acting on the planet
requires careful numerical analysis. Recently, the validity of the often used
fast orbital advection algorithm (FARGO) has been put into question, and
special numerical resolution and stability requirements have been suggested. In
this paper we study the process of planet-disk interaction for small mass
planets of a few Earth masses, and reanalyze the numerical requirements to
obtain converged and stable results. One focus lies on the applicability of the
FARGO-algorithm. Additionally, we study the difference of two and
three-dimensional simulations, compare global with local setups, as well as
isothermal and adiabatic conditions. We study the influence of the planet on
the disk through two- and three-dimensional hydrodynamical simulations. To
strengthen our conclusions we perform a detailed numerical comparison where
several upwind and Riemann-solver based codes are used with and without the
FARGO-algorithm.
With respect to the wake structure and the torque density acting on the
planet we demonstrate that the FARGO-algorithm yields correct results, and that
at a fraction of the regular cpu-time. We find that the resolution requirements
for achieving convergent results in unshocked regions are rather modest and
depend on the pressure scale height of the disk. By comparing the torque
densities of 2D and 3D simulations we show that a suitable vertical averaging
procedure for the force gives an excellent agreement between the two. We show
that isothermal and adiabatic runs can differ considerably, even for adiabatic
indices very close to unity.Comment: accepted by Astronomy & Astrophysic
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&
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