880 research outputs found
Magnetic fields in circumstellar disks: The potential of Zeeman observations
Context. Recent high angular resolution polarimetric continuum observations
of circumstellar disks provide new insights into their magnetic field. However,
direct constraints are limited to the plane of sky component of the magnetic
field. Observations of Zeeman split spectral lines are a potential approach to
enhance these insights by providing complementary information.
Aims. We investigate which constraints for magnetic fields in circumstellar
disks can be obtained from Zeeman observations of the CN
lines. Furthermore, we analyze the requirements to perform these observations
and their dependence on selected quantities.
Methods. We simulate the Zeeman splitting with the radiative transfer (RT)
code POLARIS (Reissl et al. 2016) extended by our Zeeman splitting RT extension
ZRAD (Brauer et al. 2017), which is based on the line RT code Mol3D (Ober et
al. 2015).
Results. We find that Zeeman observations of the CN lines
provide significant insights into the magnetic field of circumstellar disks.
However, with the capabilities of recent and upcoming instrument/observatories,
even spatially unresolved observations would be challenging. Nevertheless,
these observations are feasible for the most massive disks with a strong
magnetic field and high abundance of CN/H. The most restrictive quantity is the
magnetic field strength, which should be at least in the order of
. In addition, the inclination of the disk should be around
to preserve the ability to derive the line-of-sight (LOS) magnetic
field strength and to obtain a sufficiently high circularly polarized flux.Comment: 15 pages, 14 figure
Radiation Magnetohydrodynamics In Global Simulations Of Protoplanetary Disks
Our aim is to study the thermal and dynamical evolution of protoplanetary
disks in global simulations, including the physics of radiation transfer and
magneto-hydrodynamic (MHD) turbulence caused by the magneto-rotational
instability. We develop a radiative transfer method based on the flux-limited
diffusion approximation that includes frequency dependent irradiation by the
central star. This hybrid scheme is implemented in the PLUTO code. The focus of
our implementation is on the performance of the radiative transfer method.
Using an optimized Jacobi preconditioned BiCGSTAB solver, the radiative module
is three times faster than the MHD step for the disk setup we consider. We
obtain weak scaling efficiencies of 70% up to 1024 cores. We present the first
global 3D radiation MHD simulations of a stratified protoplanetary disk. The
disk model parameters are chosen to approximate those of the system AS 209 in
the star-forming region Ophiuchus. Starting the simulation from a disk in
radiative and hydrostatic equilibrium, the magnetorotational instability
quickly causes MHD turbulence and heating in the disk. For the disk parameters
we use, turbulent dissipation heats the disk midplane and raises the
temperature by about 15% compared to passive disk models. A roughly flat
vertical temperature profile establishes in the disk optically thick region
close to the midplane. We reproduce the vertical temperature profile with a
viscous disk models for which the stress tensor vertical profile is flat in the
bulk of the disk and vanishes in the disk corona. The present paper
demonstrates for the first time that global radiation MHD simulations of
turbulent protoplanetary disks are feasible with current computational
facilities. This opens up the windows to a wide range of studies of the
dynamics of protoplanetary disks inner parts, for which there are significant
observational constraints.Comment: Accepted to A&
Frontier Army Life Revealed by Charles King, 1844-1933
The purport of this study is to picture army life of the old West as revealed by the works of Charles King, author and military man of the late nineteenth and early twentieth centuries. I have emphasized the period of frontier policing by the army during the settling of the West, more commonly referred to as the Indian Wars of the 1870\u27s.https://scholars.fhsu.edu/fort_hays_studies_series/1023/thumbnail.jp
Gaps, Rings, and Non-Axisymmetric Structures in Protoplanetary Disks - From Simulations to ALMA Observations
Recent observations by the Atacama Large Millimeter/submillimeter Array
(ALMA) of disks around young stars revealed distinct asymmetries in the dust
continuum emission. In this work we want to study axisymmetric and
non-axisymmetric structures, evocated by the magneto-rotational instability in
the outer regions of protoplanetary disks. We combine the results of
state-of-the-art numerical simulations with post-processing radiative transfer
(RT) to generate synthetic maps and predictions for ALMA. We performed
non-ideal global 3D MHD stratified simulations of the dead-zone outer edge
using the FARGO MHD code PLUTO. The stellar and disk parameters are taken from
a parameterized disk model applied for fitting high-angular resolution
multi-wavelength observations of circumstellar disks. The 2D temperature and
density profiles are calculated consistently from a given surface density
profile and Monte-Carlo radiative transfer. The 2D Ohmic resistivity profile is
calculated using a dust chemistry model. The magnetic field is a vertical net
flux field. The resulting dust reemission provides the basis for the simulation
of observations with ALMA. The fiducial model develops a large gap followed by
a jump in surface density located at the dead-zone outer edge. The jump in
density and pressure is strong enough to stop the radial drift of particles. In
addition, we observe the generation of vortices by the Rossby wave instability
(RWI) at the jumps location close to 60 AU. The vortices are steadily generated
and destroyed at a cycle of 40 local orbits. The RT results and simulated ALMA
observations predict the feasibility to observe such large scale structures
appearing in magnetized disks without having a planet.Comment: Language update, added comments, added citations, in press. (A&A
3D MHD Simulations of Planet Migration in Turbulent Stratified Disks
We performed 3D MHD simulations of planet migration in stratified disks using
the Godunov code PLUTO, where the disk is turbulent due to the
magnetorotational instability. We study the migration for planets with
different planet-star mass ratios . In agreement with previous
studies, for the low-mass planet cases ( and ),
migration is dominated by random fluctuations in the torque. For a Jupiter-mass
planet for , we find a reduction of
the magnetic stress inside the orbit of the planet and around the gap region.
After an initial stage where the torque on the planet is positive, it reverses
and we recover migration rates similar to those found in disks where the
turbulent viscosity is modelled by an viscosity. For the
intermediate-mass planets ( and ) we
find a new and so far unexpected behavior. In some cases they experience
sustained and systematic outwards migration for the entire duration of the
simulation. For this case, the horseshoe region is resolved and torques coming
from the corotation region can remain unsaturated due to the stresses in the
disk. These stresses are generated directly by the magnetic field. The
magnitude of the horseshoe drag can overcome the negative Lindblad contribution
when the local surface density profile is flat or increasing outwards, which we
see in certain locations in our simulations due to the presence of a zonal
flow. The intermediate-mass planet is migrating radially outwards in locations
where there is a positive gradient of a pressure bump (zonal flow).Comment: Accepted for publication in Ap
Intrinsic polarisation of elongated porous dust grains
ALMA observations revealed recently polarised radiation of several
protoplanetary disks in the (sub-)millimetre wavelength range. Besides
self-scattering of large particles, thermal emission by elongated grains is a
potential source for the detected polarisation signal. We calculate the
wavelength dependent absorption and intrinsic polarisation of spheroidally
shaped, micrometre and sub-millimetre sized dust grains using the discrete
dipole approximation. In particular, we analyse the impact of dust grain
porosity which appears to be present in disks when small grains coagulate to
form larger aggregates. For the first time our results show that (a) the
intrinsic polarisation decreases for increasing grain porosity and (b) the
polarisation orientation flips by 90 degree for certain ratios of wavelength to
grain size. We present a new method to constrain grain porosity and the grain
size in protoplanetary disks using multi-wavelength polarisation observations
in the far-infrared to millimetre wavelengths. Finally, we find that moderate
grain porosities () potentially explain the observed
polarisation fraction in the system HD 142527 while highly porous grains
() fail unless the grain's axis ratio is extraordinarily
large.Comment: 10 pages, 10 figure
A conservative orbital advection scheme for simulations of magnetized shear flows with the PLUTO code
Explicit numerical computations of super-fast differentially rotating disks
are subject to the time-step constraint imposed by the Courant condition. When
the bulk orbital velocity largely exceeds any other wave speed the time step is
considerably reduced and a large number of steps may be necessary to complete
the computation.
We present a robust numerical scheme to overcome the Courant limitation by
extending the algorithm previously known as FARGO (Fast Advection in Rotating
Gaseous Objects) to the equations of magnetohydrodynamics (MHD). The proposed
scheme conserves total angular momentum and energy to machine precision and
works in Cartesian, cylindrical, or spherical coordinates. The algorithm is
implemented in the PLUTO code for astrophysical gasdynamics and is suitable for
local or global simulations of accretion or proto-planetary disk models.
By decomposing the total velocity into an average azimuthal contribution and
a residual term, the algorithm solves the MHD equations through a linear
transport step in the orbital direction and a standard nonlinear solver applied
to the MHD equations written in terms of the residual velocity. Since the
former step is not subject to any stability restriction, the Courant condition
is computed only in terms of the residual velocity, leading to substantially
larger time steps. The magnetic field is advanced in time using the constrained
transport method in order to preserve the divergence-free condition.
Conservation of total energy and angular momentum is enforced at the discrete
level by properly expressing the source terms in terms of upwind fluxes
available during the standard solver.
Our results show that applications of the proposed orbital-advection scheme
to problems of astrophysical relevance provides, at reduced numerical cost,
equally accurate and less dissipative results than standard time-marching
schemes.Comment: 16 pages, 13 figures. Accepted for publication in A&
- …