108 research outputs found
Three-dimensional magnetohydrodynamical simulations of the morphology of head-tail radio galaxies based on magnetic tower jet model
The distinctive morphology of head-tail radio galaxies reveals strong
interactions between the radio jets and their intra-cluster environment, the
general consensus on the morphology origin of head-tail sources is that radio
jets are bent by violent intra-cluster weather. We demonstrate in this paper
that such strong interactions provide a great opportunity to study the jet
properties and also the dynamics of intra-cluster medium (ICM). By
three-dimensional magnetohydrodynamical simulations, we analyse the detailed
bending process of a magnetically dominated jet, based on the magnetic tower
jet model. We use stratified atmospheres modulated by wind/shock to mimic the
violent intra-cluster weather. Core sloshing is found to be inevitable during
the wind-cluster core interaction, which induces significant shear motion and
could finally drive ICM turbulence around the jet, making it difficult for jet
to survive. We perform detailed comparison between the behaviour of pure
hydrodynamical jets and magnetic tower jet, and find that the jet-lobe
morphology could not survive against the violent disruption in all of our pure
hydrodynamical jet models. On the other hand, the head-tail morphology is well
reproduced by using a magnetic tower jet model bent by wind, in which
hydrodynamical instabilities are naturally suppressed and the jet could always
keep its integrity under the protection of its internal magnetic fields.
Finally, we also check the possibility for jet bending by shock only. We find
that shock could not bend jet significantly, so could not be expected to
explain the observed long tails in head-tail radio galaxies.Comment: submitted to ApJ on December 9, 2016, and accepted on March 1st, 201
Multiple Disk Gaps and Rings Generated by a Single Super-Earth: II. Spacings, Depths, and Number of Gaps, with Application to Real Systems
ALMA has found multiple dust gaps and rings in a number of protoplanetary
disks in continuum emission at millimeter wavelengths. The origin of such
structures is in debate. Recently, we documented how one super-Earth planet can
open multiple (up to five) dust gaps in a disk with low viscosity
(). In this paper, we examine how the positions, depths,
and total number of gaps opened by one planet depend on input parameters, and
apply our results to real systems. Gap locations (equivalently, spacings) are
the easiest metric to use when making comparisons between theory and
observations, as positions can be robustly measured. We fit the locations of
gaps empirically as functions of planet mass and disk aspect ratio. We find
that the locations of the double gaps in HL Tau and TW Hya, and of all three
gaps in HD 163296, are consistent with being opened by a sub-Saturn mass
planet. This scenario predicts the locations of other gaps in HL Tau and TW
Hya, some of which appear consistent with current observations. We also show
how the Rossby wave instability may develop at the edges of several gaps and
result in multiple dusty vortices, all caused by one planet. A planet as low in
mass as Mars may produce multiple dust gaps in the terrestrial planet forming
region.Comment: 16 pages; ApJ accepte
Multiple Disk Gaps and Rings Generated by a Single Super-Earth
We investigate the observational signatures of super-Earths (i.e.,
Earth-to-Neptune mass planets) in their natal disks of gas and dust. Combining
two-fluid global hydrodynamics simulations with a radiative transfer code, we
calculate the distributions of gas and of sub-mm-sized dust in a disk perturbed
by a super-Earth, synthesizing images in near-infrared scattered light and the
mm-wave thermal continuum for direct comparison with observations. In low
viscosity gas (), a super-Earth opens two annular gaps
to either side of its orbit by the action of Lindblad torques. This double gap
and its associated gas pressure gradients cause dust particles to be dragged by
gas into three rings: one ring sandwiched between the two gaps, and two rings
located at the gap edges farthest from the planet. Depending on system
parameters, additional rings may manifest for a single planet. A double gap
located at tens of AUs from a host star in Taurus can be detected in the dust
continuum by the Atacama Large Millimeter Array (ALMA) at an angular resolution
of ~0".03 after two hours of integration. Ring and gap features persist in a
variety of background disk profiles, last for thousands of orbits, and change
their relative positions and dimensions depending on the speed and direction of
planet migration. Candidate double gaps have been observed by ALMA in systems
like HL Tau (D5 and D6) and TW Hya (at 37 and 43 AU); we submit that each
double gap is carved by one super-Earth in nearly inviscid gas.Comment: 23 pages, 1 table, 14 figures, ApJ accepte
Identifying Anticyclonic Vortex Features Produced by the Rossby Wave Instability in Protoplanetary Disks
Several nearby protoplanetary disks have been observed to display large scale
crescents in the (sub)millimeter dust continuum emission. One interpretation is
that these structures correspond to anticyclonic vortices generated by the
Rossby wave instability within the gaseous disk. Such vortices have local gas
over-densities and are expected to concentrate dust particles with Stokes
number around unity. This process might catalyze the formation of
planetesimals. Whereas recent observations showed that dust crescent are indeed
regions where millimeter-size particles have abnormally high concentration
relative to the gas and smaller grains, no observations have yet shown that the
gas within the crescent region counter-rotates with respect to the
protoplanetary disk. Here we investigate the detectability of anticyclonic
features through measurement of the line-of-sight component of the gas velocity
obtained with ALMA. We carry out 2D hydrodynamic simulations and 3D radiative
transfer calculation of a protoplanetary disk characterized by a vortex created
by the tidal interaction with a massive planet. As a case study, the disk
parameters are chosen to mimic the IRS 48 system, which has the most prominent
crescent observed to date. We generate synthetic ALMA observations of both the
dust continuum and 12CO emission around the frequency of 345 GHz. We find that
the anticyclonic features of vortex are weak but can be detected if both the
source and the observational setup are properly chosen. We provide a recipe for
maximizing the probability to detect such vortex features and present an
analysis procedure to infer their kinematic properties.Comment: 14 pages, 8 figures, Accepted for publication in Astrophysical
Journa
Three-Dimensional MHD Simulation of Caltech Plasma Jet Experiment: First Results
Magnetic fields are believed to play an essential role in astrophysical jets
with observations suggesting the presence of helical magnetic fields. Here, we
present three-dimensional (3D) ideal MHD simulationsof the Caltech plasma jet
experiment using a magnetic tower scenario as the baseline model. Magnetic
fields consist of an initially localized dipole-like poloidal component and a
toroidal component that is continuously being injected into the domain. This
flux injection mimics the poloidal currents driven by the anode-cathode voltage
drop in the experiment. The injected toroidal field stretches the poloidal
fields to large distances, while forming a collimated jet along with several
other key features. Detailed comparisons between 3D MHD simulations and
experimental measurements provide a comprehensive description of the interplay
among magnetic force, pressure and flow effects. In particular, we delineate
both the jet structure and the transition process that converts the injected
magnetic energy to other forms. With suitably chosen parameters that are
derived from experiments, the jet in the simulation agrees quantitatively with
the experimental jet in terms of magnetic/kinetic/inertial energy, total
poloidal current, voltage, jet radius, and jet propagation velocity.
Specifically, the jet velocity in the simulation is proportional to the
poloidal current divided by the square root of the jet density, in agreement
with both the experiment and analytical theory. This work provides a new and
quantitative method for relating experiments, numerical simulations and
astrophysical observation, and demonstrates the possibility of using
terrestrial laboratory experiments to study astrophysical jets.Comment: accepted by ApJ 37 pages, 15 figures, 2 table
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