54 research outputs found
Magnetic fields in molecular clouds: Limitations of the analysis of Zeeman observations
Context. Observations of Zeeman split spectral lines represent an important
approach to derive the structure and strength of magnetic fields in molecular
clouds. In contrast to the uncertainty of the spectral line observation itself,
the uncertainty of the analysis method to derive the magnetic field strength
from these observations is not been well characterized so far.
Aims. We investigate the impact of several physical quantities on the
uncertainty of the analysis method, which is used to derive the line-of-sight
(LOS) magnetic field strength from Zeeman split spectral lines.
Methods. We simulate the Zeeman splitting of the 1665 MHz OH line with the 3D
radiative transfer (RT) extension ZRAD. This extension is based on the line RT
code Mol3D (Ober et al. 2015) and has been developed for the POLArized
RadIation Simulator POLARIS (Reissl et al. 2016).
Results. Observations of the OH Zeeman effect in typical molecular clouds are
not significantly affected by the uncertainty of the analysis method. We
derived an approximation to quantify the range of parameters in which the
analysis method works sufficiently accurate and provide factors to convert our
results to other spectral lines and species as well. We applied these
conversion factors to CN and found that observations of the CN Zeeman effect in
typical molecular clouds are neither significantly affected by the uncertainty
of the analysis method. In addition, we found that the density has almost no
impact on the uncertainty of the analysis method, unless it reaches values
higher than those typically found in molecular clouds. Furthermore, the
uncertainty of the analysis method increases, if both the gas velocity and the
magnetic field show significant variations along the line-of-sight. However,
this increase should be small in Zeeman observations of most molecular clouds
considering typical velocities of ~1 km/s.Comment: 9 pages, 6 figure
Tracing the ISM magnetic field morphology: The potential of multi-wavelength polarization measurements
We present a case study to demonstrate the potential of
multi-wavelength polarization measurements. The aim is to investigate the
effects that dichroic polarization and thermal re-emission have on tracing the
magnetic field in the interstellar medium (ISM). Furthermore, we analyze the
crucial influence of imperfectly aligned compact dust grains on the resulting
synthetic continuum polarization maps. We developed an
extended version of the well-known 3D Monte-Carlo radiation transport code MC3D
for multi-wavelength polarization simulations running on an adaptive grid.We
investigated the interplay between radiation, magnetic fields and dust grains.
Our results were produced by post-processing both ideal density distributions
and sophisticated magnetohydrodynamic (MHD) collapse simulations with radiative
transfer simulations. We derived spatially resolved maps of intensity, optical
depth, and linear and circular polarization at various inclination angles and
scales in a wavelength range from 7 to 1 . We
predict unique patterns in linear and circular polarization maps for different
types of density distributions and magnetic field morphologies for test setups
and sophisticated MHD collapse simulations. We show that alignment processes of
interstellar dust grains can significantly influence the resulting synthetic
polarization maps. Multi-wavelength polarization measurements allow one to
predict the morphology of the magnetic field inside the ISM. The interpretation
of polarization measurements of complex structures still remains ambiguous
because of the large variety of the predominant parameters in the ISM.Comment: 14 pages, 12 figures, 1 table, Paper accepted 2014 by A&
Spectral shifting strongly constrains molecular cloud disruption by radiation pressure on dust
To test the hypothesis that radiation pressure from star
clusters acting on dust is the dominant feedback agent disrupting the largest
star-forming molecular clouds and thus regulating the star-formation process.
We perform multi-frequency, 3D, RT calculations including
scattering, absorption, and re-emission to longer wavelengths for clouds with
masses of -M, with embedded clusters and a star
formation efficiencies of 0.009%-91%, and varying maximum grain sizes up to
200m. We calculate the ratio between radiative force and gravity to
determine whether radiation pressure can disrupt clouds. We
find that radiation acting on dust almost never disrupts star-forming clouds.
UV and optical photons to which the cloud is optically thick do not scatter
much. Instead, they quickly get absorbed and re-emitted by at thermal
wavelengths. As the cloud is typically optically thin to far-IR radiation, it
promptly escapes, depositing little momentum. The resulting spectrum is more
narrowly peaked than the corresponding Planck function with an extended tail at
longer wavelengths. As the opacity drops significantly across the sub-mm and
mm, the resulting radiative force is even smaller than for the corresponding
single-temperature black body. The force from radiation pressure falls below
the strength of gravitational attraction by an order of magnitude or more for
either Milky Way or starbust conditions. For unrealistically large maximum
grain sizes, and star formation efficiencies far exceeding 50% do we find that
the strength of radiation pressure can exceed gravity. We
conclude that radiation pressure acting on dust does not disrupt star-forming
molecular clouds in any Local Group galaxies. Radiation pressure thus appears
unlikely to regulate the star-formation process on either local or global
scales.Comment: 20 pages, 17 figure
Linear dust polarization during the embedded phase of protostar formation
Measuring polarization from thermal dust emission can provide constraints on
the magnetic field structure around embedded protostars. However, interpreting
the observations is challenging without models that consistently account for
both the complexity of the protostellar birth environment and polarization
mechanisms. We aim to provide a better understanding with a focus on
bridge-like structures such as that observed towards the protostellar multiple
IRAS 16293--2422 by comparing synthetic polarization maps of thermal reemission
with observations. We analyze the magnetic field properties associated with the
formation of a protostellar multiple based on ideal MHD 3D zoom-in simulations
carried out with the RAMSES code. To compare with observations, we post-process
a snapshot of a bridge-like structure that is associated with a forming triple
star system with the radiative transfer code POLARIS and produce
multi-wavelength dust polarization maps. In the most prominent bridge of our
sample, the typical density is about 10^(-16) g cm^(-3), and the magnetic field
strength is about 1 to 2 mG. The magnetic field structure has an elongated
toroidal morphology and the dust polarization maps trace the complex
morphology. In contrast, the magnetic field strength associated with the
launching of asymmetric bipolar outflows is significantly more magnetized (~100
mG). At {\lambda}=1.3 mm, the orientation of grains in the bridge is similar
for the case accounting for radiative alignment torques (RATs) compared to
perfect alignment with magnetic field lines. However, the polarization fraction
in the bridge is three times smaller for the RAT scenario compared to assuming
perfect alignment. At shorter wavelengths ({\lambda} < 200 {\mu}m), dust
polarization does not trace the magnetic field because other effects such as
self-scattering and dichroic extinction dominate the orientation of the
polarization.Comment: 18 pages, 12 figures plus 3 figures in the appendix, accepted for
publication in A&
Dust attenuation in galaxies at cosmic dawn from the FirstLight simulations
We study the behavior of dust in galaxies at cosmic dawn, z=6-8, by coupling
the FirstLight simulations with the radiative transfer code POLARIS. The
starburst nature of these galaxies and their complex distribution of dust lead
to a large diversity of attenuation curves. These follow the Calzetti model
only for relatively massive galaxies, Mstars=10^9Msun. Galaxies with lower
masses have steeper curves, consistent with the model for the Small Magellanic
Cloud (SMC). The ultraviolet and optical slopes of the attenuation curves are
closer to the modified Calzetti model, with a slight preference for the
power-law model for galaxies with the highest values of attenuation. We have
also examined the relation between the slope in the far-ultraviolet, beta_UV ,
and the infrared excess, IRX. At z=6, it follows the Calzetti model with a
shift to slightly lower beta_UV values due to lower metallicities at lower
attenuation. The same relation at z=8 shows a shift to higher IRX values due to
a stronger CMB radiation at high-z.Comment: 9 pages, 6 figures, accepted at MNRA
Magnetic fields in star-forming systems (II): examining dust polarization, the Zeeman effect, and the Faraday rotation measure as magnetic field tracers
The degree to which the formation and evolution of clouds and filaments in
the interstellar medium is regulated by magnetic fields remains an open
question. Yet the fundamental properties of the fields (strength and 3D
morphology) are not readily observable. We investigate the potential for
recovering magnetic field information from dust polarization, the Zeeman
effect, and the Faraday rotation measure () in a SILCC-Zoom
magnetohydrodynamic (MHD) filament simulation. The object is analyzed at the
onset of star formation, and it is characterized by a line-mass of about M/L
out to a radius of pc and a kinked 3D
magnetic field morphology. We generate synthetic observations via POLARIS
radiative transfer (RT) post-processing, and compare with an analytical model
of helical or kinked field morphology to help interpreting the inferred
observational signatures. We show that the tracer signals originate close to
the filament spine. We find regions along the filament where the
angular-dependency with the line-of-sight (LOS) is the dominant factor and dust
polarization may trace the underlying kinked magnetic field morphology. We also
find that reversals in the recovered magnetic field direction are not
unambiguously associated to any particular morphology. Other physical
parameters, such as density or temperature, are relevant and sometimes dominant
compared to the magnetic field structure in modulating the observed signal. We
demonstrate that the Zeeman effect and the recover the line-of-sight
magnetic field strength to within a factor 2.1 - 3.4. We conclude that the
magnetic field morphology may not be unambiguously determined in low-mass
systems by observations of dust polarization, Zeeman effect, or , whereas
the field strengths can be reliably recovered.Comment: 22 pages, 17 figures, 3 table
From parallel to perpendicular -- On the orientation of magnetic fields in molecular clouds
We present synthetic dust polarization maps of simulated molecular clouds
(MCs) with the goal to systematically explore the origin of the relative
orientation of the magnetic field () with respect to the MC
sub-structures identified in density (; 3D) and column density (; 2D).
The polarization maps are generated with the radiative transfer code POLARIS,
including self-consistently calculated efficiencies for radiative torque
alignment. The MCs are formed in two sets of 3D MHD simulations: in (i)
colliding flows (CF), and (ii) the SILCC-Zoom simulations. In 3D, for the CF
simulations with an initial field strength below 5 G, is
oriented parallel or randomly with respect to the -structures. For CF runs
with stronger initial fields and all SILCC-Zoom simulations, which have an
initial field strength of 3 G, a flip from parallel to perpendicular
orientation occurs at high densities of 10 -
10 cm. We suggest that this flip happens if the MC's mass-to-flux
ratio, , is close to or below the critical value of 1. This corresponds to
a field strength around 3 - 5 G. In 2D, we use the Projected Rayleigh
Statistics (PRS) to study the orientation of . If present, the flip in
orientation occurs at 10 cm,
similar to the observed transition value from sub- to supercritical magnetic
fields in the ISM. However, projection effects can reduce the power of the PRS
method: Depending on the MC or LOS, the projected maps of the SILCC-Zoom
simulations do not always show the flip, although expected from the 3D
morphology. Such projection effects can explain the variety of recently
observed field configurations, in particular within a single MC. Finally, we do
not find a correlation between the observed orientation of and the
-PDF.Comment: 20 pages, 12 figures, accepted for publication in MNRA
The complex multiscale structure in simulated and observed emission maps of the proto-cluster cloud G0.253+0.016 (\u27the Brick\u27)
The Central Molecular Zone (the central ∼500 pc of the Milky Way) hosts molecular clouds in an extreme environment of strong shear, high gas pressure and density, and complex chemistry. G0.253+0.016, also known as \u27the Brick\u27, is the densest, most compact, and quiescent of these clouds. High-resolution observations with the Atacama Large Millimetre/submillimetre Array (ALMA) have revealed its complex, hierarchical structure. In this paper we compare the properties of recent hydrodynamical simulations of the Brick to those of the ALMA observations. To facilitate the comparison, we post-process the simulations and create synthetic ALMA maps of molecular line emission from eight molecules. We correlate the line emission maps to each other and to the mass column density and find that HNCO is the best mass tracer of the eight emission lines within the simulations. Additionally, we characterize the spatial structure of the observed and simulated cloud using the density probability distribution function (PDF), spatial power spectrum, fractal dimension, and moments of inertia. While we find good agreement between the observed and simulated data in terms of power spectra and fractal dimensions, there are key differences in the density PDFs and moments of inertia, which we attribute to the omission of magnetic fields in the simulations. This demonstrates that the presence of the Galactic potential can reproduce many cloud properties, but additional physical processes are needed to fully explain the gas structure
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