Magnetic Fields and the Structure of the Filamentary Interstellar Medium
Abstracts for Pre-Recorded Talks
Magnetic Fields in Massive Star-Forming Regions (MagMaR) II. Tomography Through Dust and Molecular Line Polarization in NGC 6334
Paulo Cortes (ALMA Observatory)
Abstract: We will report ALMA detections of polarized emission from dust, CS (J = 5 →4),
and C33S (J = 5 → 4) toward the high-mass star-forming region NGC6334I(N).A clear
“hourglass” magnetic field morphology was inferred from the polarized dust emission
which is also directly seen from the polarized CS emission across velocity, where the
polarization appears to be parallel to the field. By considering previous findings, the
field retains a pinched shape which can be traced to larger length-scales (2 pc) from
the scales traced by ALMA (3 mpc), suggesting that the field is dynamically
Important across multiple length-scales in this region. The CS total intensity emission
is found to be optically thick (τCS = 32 ± 12) while the C33S emission appears to be
Optically thin (τC33S = 0.1 ± 0.01). This suggests that sources of anisotropy other
than large velocity gradients, i.e. anisotropies in the radiation field, along with interferometric
filtering, are required to explain the polarized emission from CS seen by ALMA. By
using four variants of the Davis-Chandrasekhar-Fermi technique and the polarization
position angle dispersion function methods, we estimate a magnetic field strength onto
the plane of the sky of Bpos i = 16 ± 8 mG from the dust and Bpos i ∼ 2 mG±1 mG
from the CS emission, where each emission traces different molecular hydrogen
Number densities. This effectively enables a tomographic view of the magnetic field
within a single ALMA observation.
The dynamical impact and orientation of magnetic fields in the interstellar medium
Philipp Girichidis (Institute for Theoretical Astrophysics, University of
Heidelberg, Germany)
Abstract: Magnetic fields are ubiquitously observed in the interstellar medium (ISM)
with dynamically relevant energy densities. Using 3D-MHD simulations of the
supernova-driven ISM we investigate the dynamical impact of magnetic fields
on the formation of molecular gas out of the atomic gas, GMCs and filaments.
We show how magnetic fields delay the formation of H2 and help preventing the
gas from forming stars too efficiently. The simulation covers a box of 500 pc
and resolves scales down to sub-parsec, which allows to investigate the
magnetic field strength and orientation from scales of the galactic disk down
to the densest and self-gravitating parts of the molecular clouds. We also
correlate the orientation of the magnetic field with respect to gas
structures and gas velocities with the density and the degree of importance
of contracting gravitational forces. We also illustrate why the orientation
of the field is well identified locally in 3D simulations but might be hard
to observe because of the short correlation lengths of the density structures
and the magnetic fields and the resulting line of sight integration effects.
A Comparison between Magnetic Field Directions Inferred from Planck and Starlight Polarimetry toward Gould Belt Clouds
Qilao Gu (Shanghai Astronomical Observatory)
Abstract: We compare the magnetic field (B-field) orientations inferred from
Planck 353 GHz thermal dust polarization and starlight polarimetry data and
study the cloud-field alignment based on these two tracers within Gould Belt
clouds, which show good agreement with each other. Furthermore, we analyze
two fundamentally different alignment studies—global (cloud scale,
~10–100 pc) cloud-field alignment, which compares mean fields and global
cloud orientations, and local (pixel size scale, ~0.1–1 pc) structure-field
alignment, which compares this relation pixel by pixel—and find the
connection between them.
Studying magnetic fields toward M17 cloud using dust polarization taken with SOFIA/HAWC+
Thuong Duc Hoang (University of Science and Technology of Hanoi)
Abstract: Understanding the role of magnetic fields in the evolution of dense
clouds and star formation process is a challenge in modern astrophysics. This
study reports the observation of magnetic fields in the plane-of-sky of the
M17 cloud using SOFIA/HAWC+ polarimetric data at 154 μm wavelength. Using the
Davis-Chandrasekhar-Fermi technique, we found the presence of strong magnetic
fields (326 and 839 μG in lower-density and higher-density regions,
respectively). The mean values of the field strengths are used to determine
the sub-Alfv ́enic Mach numbers (i.e. 0.02-0.004) which are the results of
The well-aligned magnetic fields morphology. Sub-critical values of the
mass-to-flux ratios (i.e. 0.21-0.55) are in agreement with the lack of massive
stars formed in M17. We also study the relationship between the dust
polarization degree and dust temperature to explain the effect of grain
alignment and rotation disruption by radiative torques.
The Explosion in Orion-KL as Seen by Mosaicking the Magnetic Field with ALMA
Chat Hull (National Astronomical Observatory of Japan)
Abstract: We present the first linear-polarization mosaicked observations
performed by the Atacama Large Millimeter/submillimeter Array (ALMA). We
mapped the Orion-Kleinmann-Low (Orion-KL) nebula using super-sampled mosaics
at 3.1 and 1.3 mm as part of the ALMA Extension and Optimization of
Capabilities program. We derive the magnetic field morphology in the plane of
the sky by assuming that dust grains are aligned with respect to the ambient
magnetic field. At the center of the nebula, we find a quasi-radial magnetic
field pattern that is aligned with the explosive CO outflow up to a radius of
approximately 12" (∼5000 au), beyond which the pattern smoothly transitions
into a quasi-hourglass shape resembling the morphology seen in larger-scale
observations by the James-Clerk-Maxwell Telescope (JCMT). We estimate an
average magnetic field strength 9.4 mG and a total magnetic energy of
2×10^45 erg, which is three orders of magnitude less than the energy in the
explosive CO outflow. We conclude that the field has been overwhelmed by the
outflow and that a shock is propagating from the center of the nebula, where
the shock front is seen in the magnetic field lines at a distance of ∼5000
au from the explosion center.
The magnetic field geometry in the halo of NGC891
Jin-Ah Kim (University of Minnesota)
Abstract: Measurements of the magnetic field geometry in galactic halos using
interstellar polarization are sparse compared to observations within galactic
disks due to observational constraints. The magnetic field well away from the
disk has essential information on how the magnetic field in halos evolves and
how the mixing of disk and halo materials affects the magnetic field
configuration. While an X-shaped magnetic field structure or more complex
vertical magnetic field geometries in halos have been studied using radio
observations, far-IR polarimetry observations of interstellar polarization in
halos have only recently become feasible. Far-IR observations of polarized
dust emission, which is dependent on the presence of warm dust and is not
influenced by Faraday effects, provides a unique and complementary tool for
the study of the magnetic field geometry in galactic halos. Using SOFIA HAWC+
in polarimetry mode at 154um, we found possible evidence for vertical
magnetic fields in the well-known edge-on galaxy NGC891. The vertical fields
seen in NGC891 could be extensions of the mean field formed by dynamo action
in the disk and moderate galactic winds. Or, the fields could be correlated
with super bubbles or loop structures, perhaps triggered by local enhanced
star formation. To investigate vertical magnetic field in the halo of NGC891,
we have been awarded further observations with SOFIA HAWC+ and we will use
these observations to determine whether the vertical magnetic fields are
clearly present in the dust halo and whether the vertical fields are
correlated with local features such as super bubbles.
A GASKAP View of HI Filaments in the SMC: The Relationship with Magnetic Fields
Yik Ki (Jackie) Ma (Australian National University)
Abstract: Neutral hydrogen (HI) in the interstellar medium often exhibits as
a vast network of filamentary structures. The anisotropy of these HI
filaments can be induced by supernova explosions, tidal forces, or magnetic
fields. In the Milky Way, nearby HI filaments have been found to be oriented
along the ambient magnetic fields traced by polarised starlight, suggesting
an important role of magnetic fields in the formation of these neutral
structures. We investigate if the same relation holds in the Small Magellanic
Cloud (SMC), which harbours vastly different astrophysical conditions
compared to the Milky Way. This study has been made possible by the new data
from the Galactic ASKAP (GASKAP) survey, which provides an unprecedented
combination of angular resolution, velocity resolution, and sensitivity of HI
in the SMC. We identified ~100 pc scale HI filaments automatically using the
Rolling Hough Transform machine vision algorithm, and carefully compared
their orientations with the polarisation angles from a recent starlight
polarisation catalogue of the SMC. In this talk, I will present the
preliminary results from this work.
Modeling of CO Emission in Shocks of W 28 and IC 443
Fuda Nguyen (Vietnam National University)
Abstract: Modeling of CO Emission in Shocks of W 28 and IC 443
Abstract: Shocks into the interstellar medium is an incredibly effective
laboratory for tracing the interplay of energies exchange in the interstellar
medium. Carbon monoxide (CO) is amongst the most abundant gas in the ISM,
second only to diatomic hydrogen (H_2). In the cooling phase of the
shock, pure-rotational transitions of the ^16C^12O (∆ J=1) molecule, covering ranges of frequencies, are very
accessible to observations. In this work, we made use of the Large Velocity
Gradient (LVG) approximation to make the first radiative transfer model of CO
emissions directly from the MHD Paris-Durham 1D shock code. Our model
predicts well the CO emission observed with SOFIA from C-type,
magnetic-dominated, multi-components shocks, which are strongly present in
the bright molecular regions of W 28 and IC 443. The model provided
constraints for pre-shock density of n < 1E4 cm^-3 and ISM magnetic field
of B < 350 uG, and also served as an effective guideline for identifying
portion in the evolution of the shock which likely contributed to the final
molecular emission.
The North Polar Spur puzzle: feedback near versus feedback far
Gina Panopoulou (Caltech)
Abstract: Filamentary structure is prevalent in the magnetized ISM. The
relation between filament morphology and magnetic field geometry can shed
light on the different formation mechanisms for such structures. I will
discuss one of the prototypical examples of the link between the magnetic
field and filament morphology in the diffuse ISM: the North Polar Spur.
Existing determinations of the distance to the NPS are contradictory, fueling
the ongoing debate on the origin of this object. Observations of stellar
extinction and polarization strongly suggest that the NPS is a reheated SNR,
with a diameter of ~200 pc. However, recent X-ray observations find that the
NPS is a kpc-size structure created by activity near the Galactic center. I
will review the evidence for/against each model and present new constraints
on the distance to the structure from a combination of three magnetic field
tracers: polarized dust emission, radio synchrotron and stellar
Polarizations.
Characterizing Mechanisms Producing the Unusual Magnetic Field of the Galactic Center Radio Arc
Dylan Michelson Paré (University of Iowa)
Abstract: The Galactic Center (GC) is an unusual region of the Galaxy that
contains a unique population of structures known as the non-thermal filaments
(NTFs). The NTFs are highly polarized with polarized intensity distributions
which trace the total intensity of the NTFs. In addition, their intrinsic
magnetic fields are found to be parallel to the extent of the NTFs. By
contrast, the most prominent NTF, the Radio Arc, exhibits polarized intensity
structures which extend into regions of low total intensity. In addition, the
intrinsic magnetic field of the Radio Arc alternates between being parallel
and rotated with respect to the total intensity. The origin of these unusual
features for the Radio Arc remains unclear. We obtained an Australia
Telescope Compact Array (ATCA) data set of the Radio Arc covering a 4 – 11
GHz frequency range to probe the mechanisms responsible for the unique
features seen for the Radio Arc. Using the 4 MHz spectral resolution of our
ATCA data, we fit models of several Faraday rotation mechanisms to the
spectral polarization data. This model fitting of the data allows us to
analyze the number and nature of rotating media located along the line of
sight to determine whether an additional intervening medium is responsible
for the unusual polarized intensity and magnetic field properties observed
for the Radio Arc. This work expands our understanding of the environment
local to the Radio Arc, shedding light on the complex region it is embedded
within.
Magnetic filaments in the ISM due to the small-scale dynamo
Amit Seta (Australian National University)
Abstract: Magnetic fields in the interstellar medium (ISM) of galaxies is
amplified and maintained by a dynamo action, whereby a part of kinetic energy
gets converted to magnetic energy. A dynamo that produces magnetic structures
at scales smaller than the driving scale of turbulence is known as the
small-scale dynamo. We explore magnetic structures in the amplifying and
statistically steady state of the small-scale dynamo in driven turbulence
simulations. Using the Minkowski functionals, we quantify the shape of the
magnetic structures produced by the dynamo as magnetic filaments and derive
the scalings of the typical length, width, and thickness of these filaments
with the magnetic dissipation. We show that all three of these magnetic
length scales increases as the magnetic field amplifies. The study would help
compare the theory of small-scale dynamo and magnetic filaments seen in
various ISM simulations with direct and indirect observations of magnetic
filaments in the ISM.
Cloud-scale Radio Survey of Star Formation and Feedback in Triangulum Galaxy (CRASSFIT)
Fatemeh Tabatabaei (Institute for Research in Fundamental Sciences (IPM))
Abstract: CRASSFIT is a full polarization survey of M33 at C band (5-7 GHz)
and L band (1-2 GHz) with the Karl G. Jansky Very Large Array (JVLA) to study
star formation and feedback at spatial scales down to 40 pc. These scales are
important to investigate the thermal and non-thermal processes controlling
giant molecular clouds and regulating the formation of star clusters. The
sensitive JVLA polarization observations show tangled magnetic field in star
forming regions traced in synchrotron. These regions had been
beam-depolarized in previous studies. We also find alignments of the
B-vectors with gaseous filaments in the some parts of the main spiral arm in
this galaxy. These regions are found to have a lower star formation activity
compared to other regions. Similar to the clouds in the center of NGC1097, an
anti-correlation is found between the magnetic field strength and the star
formation efficiency in these clouds in M33. The aligned magnetic field, that
is likely generated by shear gas motions, can support the gas against gravity
decelerating the formation of cores of star clusters.
Magnetic fields in the ISM and their effect on filaments, stars & discs
James Wurster (University of St Andrews)
Abstract:
Magnetic fields affect the evolution of the ISM in star forming regions on a
variety of scales. They modify its large scale structure (e.g. its
filamentary structure), which affects the magnetic field structure in star
forming cores, which ultimately affects the magnetic field structure of any
protostellar discs that form. Since the ISM is only weakly ionised,
non-ideal MHD is a more realistic description of magnetic fields.
In this talk, I will present the results of numerical simulations that model
star forming clusters. I will discuss the large- and small-scale effects of
the magnetic field, the importance of ideal and non-ideal MHD and of the
initial magnetic field strength. I will discuss the resulting gas structure
of the ISM, the stellar populations, and the magnetic field in the discs that
Form.
Anchoring Magnetic Fields in Turbulent Molecular Clouds. II. From 0.1 to 0.01 pc
Yapeng Zhang (Department of Astronomy, Beijing Normal University)
Abstract: We compared the magnetic field directions inferred from polarimetry data
obtained from 100 pc scale inter-cloud media (ICM) and from sub-parsec scale
molecular cloud cores. The highly correlated result led us to conclude that
cloud turbulence must be sub-Alfvénic. Here we extend the study with 0.01 pc
cores observed by interferometers. The inferred field directions at this
scale significantly deviate from that of the surrounding ICM. An obvious
question to ask is whether this high-resolution result contradicts the
sub-Alfvénic picture concluded earlier. We performed MHD simulations of a
slightly super-critical clouds with Alfvénic Mach number MA=0.63 which can
reproduce the Paper I result, and observed the development toward smaller
scales. Interestingly, all subregions hosting cores with n(H2)>10^5/cc (the
typical density observed by interferometers) possess MA=2~3. Not too
surprisingly, these slightly super-Alfvénic cores result in B-field
orientation offsets comparable to the interferometer observations. The result
suggests that gravity can concentrate (and maybe also contribute to, which
requires further study to confirm) turbulent energy and create slightly
super-Alfvénic cores out of sub-Alfvénic clouds. The results of our
simulations also agree with the observed velocity-scale, mass-scale, and
field-density relations.
Wednesday
Flash Contributed Talks, Session 1
Flash Contributed Talks, Session 2
Note: an incorrect slide was shown for Pak Shing Li's talk; the correct slide can be viewed here.
Classification of Filament Formation Mechanisms in Magnetized Molecular Clouds
Daisei Abe (Nagoya University)
Abstract: Recent observations of molecular clouds show that dense filaments
are the sites of present-day star formation. Thus, it is necessary to
understand the filament formation process because these filaments provide the
initial condition for star formation. Theoretical research suggests that
shock waves in molecular clouds trigger filament formation. Since several
different mechanisms have been proposed for filament formation, the formation
mechanism of the observed star-forming filaments requires clarification. In
the present study, we perform a series of isothermal magnetohydrodynamics
simulations of filament formation. We focus on the influences of shock
velocity and turbulence on the formation mechanism and identified three
different mechanisms for filament formation. The results indicate that when
the shock is fast, at shock velocity v_sh = 7 km/s, the gas flows driven by
the curved shock wave create filaments irrespective of the presence of
turbulence and self-gravity. However, at a slow shock velocity v_sh = 2.5
km/s, the compressive flow component involved in the initial turbulence
induces filament formation. When both the shock velocities and turbulence are
low, the self-gravity in the shock-compressed sheet becomes important for
filament formation. Moreover, we analyzed the line-mass distribution of the
filaments and showed that strong shock waves can naturally create
high-line-mass filaments such as those observed in the massive star-forming
regions in a short time. We conclude that the dominant filament formation
mode changes with the velocity of the shock wave triggering the filament
Formation.
The Monoceros OB1 East filamentary cloud and its large-scale magnetic field
Dana Alina (Nazarbayev University)
Abstract: Monoceros OB1 East molecular cloud is a large complex that has a filamentary
shape in its Northern part and that contains a stellar cluster in its
Southern part. It has a rich star formation history and an on-going an active
star formation process.
We study the large-scale magnetic field structure of the cloud using the
Planck satellite data and the velocity gradients technique applied to the
TRAO 14m telescope.
This allows us to reveal the following dynamical characteristics of the
cloud.
First, we confirm a shock region between the Northern filaments, in which the
magnetic field is elongated along the filament’s long axes. Second,
possible inflows of gas along magnetic field lines towards the densest part
of the cloud, that contain the NGC2264 cluster, are detected.
We conclude that the large-scale magnetic field has a dynamically important
role in Monoceros OB1 East: in the Northern filaments it might be providing
support against gravity while in the Southern part it might be guiding gas
Inflow.
Magnetic Field Morphology in High-Mass Star Forming Filaments
Akanksha Bij (Queen's University)
Abstract: We implement a multi-scale and multi-tracer analysis of the
orientation between magnetic field morphology and elongated gas structures in
the densest filament of the young giant molecular cloud Vela C. This region
is of particular interest as it is within 1 pc of a young ~1Myr old cluster
that is powering the bipolar HII region RCW 36, allowing us to probe the
impact of feedback on magnetic field geometry. To trace the magnetic field on
filament scales (~0.03-0.1 pc), we use public observations from SOFIA/HAWC+
79 microns (Band C) and 214 microns (Band E). To trace the dense filament and
PDR, we use column density and temperature maps derived from Herschel and
integrated line intensity maps such as 12CO(J=3-2) and 13CO(J=3-2) from
APEX/LAsMA and CII from SOFIA/FEEDBACK. We also trace the core and
sub-filament scales with ~4500 AU resolution using ALMA band 6 (1.1-1.4 mm)
continuum maps. Previous studies using BLASTpol toward this region have found
that the high column density filamentary structure has a strong statistical
preference to align perpendicular to the cloud-scale magnetic field. Using
the higher resolution magnetic field data, we also find that the column
density structures preferentially align perpendicular to the magnetic field.
However, for several of the line tracers (e.g 12CO, 13CO) the lower intensity
structures are more likely to align perpendicular to the magnetic field
whereas the higher intensity structures show non-preferential or parallel
alignment. This could be caused by feedback from the nearby massive star
cluster or gravitational collapse altering the magnetic field geometry.
The role of the magnetic field in the formation of the Musca filament
Lars Bonne (SOFIA/USRA)
Abstract: The Musca filament in the Southern Hemisphere is a low-mass
filament at the stage just before fragmentation into prestellar cores, and
thus an ideal target to study the initial conditions of low-mass star
formation. Analyzing several CO transitions, continuous mass accretion on the
filament was established. Further analyzing large CO(1-0) and HI maps
indicates that the bending of the magnetic field, as described in e.g. Inoue
et al. 2018, probably drives the mass accretion on the filament. This bending
of the magnetic field appears to be the result of a 50 pc scale HI colliding
flow in the Chameleon-Musca complex (Bonne et al. 2020a,b). Cycle 6 HAWC+
observations show that inside the Musca filament, the magnetic field remains
roughly perpendicular to the orientation of the filament, as was found on
large scales around the filament with Planck and optical polarization data.
Comparing the HAWC+ magnetic field orientation with the C18O(2-1) velocity
field in the filament shows that they are closely aligned which confirms that
the magnetic field and mass inflow to the filament are closely related.
Combining the HAWC+ data with the optical polarization observations (Pereyra
& Magalhaes 2004) confirms some reorientation of the magnetic field directly
around the filament that was found with Planck at lower spatial resolution.
This reorientation can have several potential interpretations such as bending
of the magnetic field, an increasing role of gravity or shocks.
SILCC-Zoom: the effect of magnetic fields on the morphology, dynamics and fragmentation of molecular clouds
Shashwata Ganguly Affiliation (University of Cologne)
Abstract: SILCC-Zoom: the effect of magnetic fields on the morphology,
dynamics and fragmentation of molecular clouds
Abstract: To what extent magnetic fields affect how molecular clouds (MC)
fragment and create dense structures is an open question. We present a
numerical study of cloud fragmentation using the SILCC-Zoom simulations
(Seifried+2017). These simulations follow the self-consistent formation of
MCs in a few hundred pc sized region of a stratified galactic disc; and
include magnetic fields, self-gravity, supernova driven turbulence, as well
as a non-equilibrium chemical network. To discern the role of magnetic fields
in the time evolution of MCs, we study seven simulated clouds, five with
magnetic fields and two without, for 1.5 Myr with a maximum resolution of 0.1
pc. Using a dendrogram we identify hierarchical structures which form within
the clouds. We find that five out of seven clouds are sheet-like on the
largest scales with filamentary structures embedded within, consistent with
the bubble driven MC formation mechanism proposed by Inutsuka+2015.
Hydrodynamic simulations tend to produce more sheet-like structures, while
MHD simulations seem equally likely to produce filaments or sheets. Moreover,
magnetic fields lead to shallower density gradients such that dense
structures are embedded in a more extended diffuse medium compared to
hydrodynamic clouds. Analysing cloud energetics, we find that magnetic fields
are dynamically important for less dense atomic structures, while the denser,
potentially star forming structures, are energetically dominated by
self-gravity and turbulence.
The distribution of magnetic field strengths in the OMC-1 region
Jihye Hwang (Korea Astronomy and Space Science Institute/University of
Science and Technology)
Abstract: Estimate of magnetic field strengths in a molecular cloud is
necessary to determine whether magnetic field can support the molecular cloud
against gravitational collapse. We suggest a new application of the
Davis-Chandrasekhar-Fermi (DCF) method to estimate magnetic field strengths
in the OMC-1 region. We use dust polarization emission at 450 and 850 μm and
C18O (3-2) spectral line data obtained with the JCMT as a part of the JCMT
BISTRO survey. We estimate the volume density, the velocity dispersion and
the polarization angle dispersion in a box, 40′′×40′′ (5×5 pixels),
which moves over the OMC-1 region. We estimate the distribution of magnetic
field strengths in each box using the DCF method. The magnetic field
strengths range from 0.8 to 26.4 mG and their mean value is about 6 mG. We
also get maps of the mass-to-flux ratio in units of a critical value and the
Alfve ́n Mach number. The central parts of two clumps (BN-KL and S) in the
OMC-1 region are magnetically supercritical, but the outer parts of the
region are magnetically subcritical. The Alfve ́n Mach numbers are smaller
than 0.6 over the OMC-1 region, which implies that the magnetic field
pressure exceeds the turbulent pressure in the OMC-1 region.
Multi-wavelength analysis of the magnetic field in Ophiuchus A using SOFIA/HAWC+ and JCMT/POL-2
Janik Karoly (University of Central Lancashire)
Abstract: We use previously published linear polarization observations of the
Ophiuchus-A core carried out by SOFIA/HAWC+ at 89 and 154μm and from the
BISTRO survey using JCMT/POL-2 at 850μm to determine the magnetic field in,
and surrounding, the main core. Using N(H2) column density maps at each
wavelength, we suggest that each wavelength traces a different component of
the magnetic field, both spatially and depth wise. The 89μm magnetic field
lies spatially to the east of the main Oph-A core and lies in the more
diffuse layer, at column densities on the order of 10^19 cm^-2. The 154μm
and 850μm are spatially coincident but trace the magnetic field at different
depths in the main core, with the column density at 154μm on the order of
10^20-10^21 cm^-2 and at 850μm on the order of 10^22 cm^-2. The magnetic
fields observed at 154 and 850μm are largely in agreement but do differ in
certain regions. We suggest this supports previous findings that there is
good grain alignment in the Oph-A core.
HAWC+/SOFIA Polarimetry in L1688: Relative Orientation of Magnetic Field and Elongated Cloud Structure
Dennis Lee (Northwestern University)
Abstract: We present a study of the relative orientation between the magnetic
field and elongated cloud structures for the Rho Oph A and Rho Oph E regions
in L1688. Combining inferred magnetic field orientation from HAWC+ 154 um
observations of polarized thermal emission with column density maps created
using Herschel submillimeter observations, we find consistent perpendicular
relative alignment at scales of 0.02 pc (33.6” at d ~ 137 pc) using the
histogram of relative orientations (HRO) technique. This supports the
conclusions of Planck Intermediate Results XXXV for nearby clouds and extends
the results to higher column densities. Combining this HAWC+ HRO analysis
with a Planck HRO analysis of L1688, the transition from parallel to
perpendicular alignment in L1688 is observed to occur at a molecular hydrogen
column density of approximately 10^(21.7) cm^-2. This value for the alignment
transition column density agrees well with values from Planck Intermediate
Results XXXV. Using existing turbulent, MHD simulations of molecular clouds
formed by colliding flows (from Chen et al. 2016; Vol 829, pg 84) as a model
for L1688, we conclude that the molecular hydrogen volume density associated
with this transition is approximately ~10^4 cm^-3. We discuss the limitations
of our analysis, including incomplete sampling of the dense regions in L1688
By HAWC+
Mapping the magnetic field in the Taurus/B211 filamentary cloud with SOFIA HAWC+ and comparing with simulation
Pak Shing Li (Astronomy Depart, University of California at Berkeley)
Abstract: Molecular cloud L1495 in the Taurus region is a typical long, filamentary
molecular cloud. Early optical and infrared polarization mapping and recent
Planck observations of the cloud show that the large-scale field is
approximately perpendicular to the long axis of the cloud. Numerical
simulation of filamentary cloud formation shows that filamentary
substructures inside filamentary clouds can strongly perturb the magnetic
field inside the cloud. We use the HAWC+ polarimeter on SOFIA to probe the
complex magnetic field in the B211 region of the cloud and reveal a
dispersion of polarization angles of 36.1 degree, about five times that
measured on a larger scale by Planck. It is extraordinary that in the small
0.82 pc mapped region, two distinctly different sub-regions are found with
magnetic field strengths estimated by the Davis-Chandrasekhar-Fermi (DCF)
method that differ by more than a factor 3, with velocity information
obtained from IRAM 30m C18O(1-0) observation. The more quiet sub-region is
subcritical and sub-Alfvenic; the field is comparable to the average field
measured in molecular clumps based on Zeeman observations. The more chaotic
super-Alfvenic sub-region shows at least 3 velocity components, indicating
interaction among multiple substructures. Its field is much less than the
average Zeeman field, suggesting that the DCF value of the field there may be
an underestimate. We also show that the standard DCF method is valid even if
the turbulence is not due to Alfven waves, and we present an alternate
derivation of the structure function version of the DCF method.
The characteristic widths of magnetized filaments
Felix Priestley (Cardiff University)
Abstract: Observations of the filamentary structures in molecular clouds find
that they have a characteristic width of ~0.1 pc, and that they tend to be
oriented perpendicularly to the local magnetic field, at least at high column
densities. I show that the first property can be explained if filaments are
formed via converging supersonic flows; for the gas properties and turbulent
velocities typical of molecular clouds, an accretion shock forms at ~0.1 pc
from the point of convergence, which effectively sets the boundary of the
filament and thus its full-width half-maximum. The observed distribution of
filament widths can be reproduced with a realistic distribution of the inflow
Mach number. A perpendicularly-aligned magnetic field results in the observed
width depending on viewing angle, and a broader filament width distribution
compared to the non-magnetised case. I investigate whether this scenario is
consistent with the observations, and to what extent filament widths can
constrain the dynamical importance of magnetic fields.
Magnetic field structure of OMC-3 in the far-infrared unveiled by SOFIA/HAWC+
Niko Zielinski (CAU Kiel, Institut für Theoretische Physik und
Astrophysik)
Abstract: We report SOFIA/HAWC+ band D (154 μm) and E (214 μm) polarimetric
observations of the filamentary structure OMC-3, part of the Orion molecular
cloud. The polarization pattern is uniform for both bands and parallel to the
filament structure. The polarization degree decreases towards regions with
high intensity for both bands, revealing a so called "polarization hole". We
identify an optical depth effect in which polarized emission and extinction
act as counteracting mechanisms as a potential contributor to this
phenomenon. Assuming that the detected polarization is caused by the emission
of magnetically aligned non-spherical dust grains, the inferred magnetic
field is uniform and oriented perpendicular to the filament. The magnetic
field strength derived from the polarization patterns at 154 μm and 214 μm
amounts to 201 μG and 261 μG, respectively. The derived magnetic field
direction is consistent with that derived from previous polarimetric
observations in the far-infrared and sub-mm wavelength range. Investigating
the far-infrared polarization spectrum derived from the SOFIA/HAWC+
observations we do not find a clear correlation between the polarization
spectrum and cloud properties, i.e., column density and temperature.
Thursday
Relative Orientations between Velocity Gradients and Magnetic Fields in Perseus NGC 1333 Filaments
Mike Chen (Queen's University)
Abstract:
Motions of star-forming gas are guided by magnetic field lines when magnetic
field energy dominates over its gas kinematic counterpart, driven either by
turbulence, gravity, or both. The relative orientations between a magnetic
field and the gas velocity gradients can thus place important constraints on
the role of magnetic fields in assembling star-forming gas. Here we present
the relative orientation analysis of Perseus NGC 1333 filaments between
magnetic fields and velocity gradient fields measured by JCMT POL-2 and VLA
ammonia observations, respectively, both on a 0.02 pc scale. The ammonia
data, in particular, are fitted with two spectral components to decouple
velocity structures along the lines of sight and ensure accurate measurements
of velocity centroid and dispersion. While we find no strong correlation
between column densities and relative orientations in NGC 1333, localized
filaments and cores often display preferential parallel or perpendicular
alignments over regions that are about 0.05pc - 0.15pc in sizes. This result
suggests that while magnetic fields alone do not solely regulate the dense
gas flow on the 0.02 pc scale, magnetic fields can still strongly influence
the gas kinematics in filaments under certain conditions.
The role of the magnetic field in the evolution of the hub-filament structures in IC5146
Eun Jung Chung (Chungnam National University)
Abstract: Hub-filament structures (HFSs) are usually associated with stellar
clusters and thus provide good laboratories for the star formation study. We
have carried out polarimetric observations with the SCUBA2/POL-2 instrument
on the James Clerk Maxwell Telescope (JCMT) toward the western hub (W-Hub)
of IC5146 and investigated the roles of the magnetic field, turbulence, and
gravity in forming cores. Using the molecular lines data obtained from TRAO
14m antenna, we estimated the magnetic field strengths, mass-to-flux ratios,
and the Alfvenic Mach numbers of the cores in the W-Hub. In the presentation,
we will show the result of the JCMT/Pol2 observation and make a comparison to
the eastern hub of IC5146. We will discuss the roles of gravity, turbulence,
and magnetic field in the formation of dense cores in the W-Hub of IC5146.
BISTRO: Magnetic Fields in Serpens Main
Woojin Kwon (Seoul National University)
Abstract: We have studied the magnetic fields in the Serpens Main molecular
cloud on about 6000 au scales, as a part of the B-fields In STar-forming
Region Observations (BISTRO) survey using the JCMT and POL-2. Serpens Main
consists of two subclusters and has six filamentary structures with various
physical properties including star formation activities. Utilizing the
histogram of relative orientations technique, we found that magnetic fields
are parallel to the less dense filaments while being perpendicular to dense
ones. In addition, the magnetic field orientations with respect to the
structures change again in denser regions. We also estimated magnetic field
strengths by the Davis-Chandrasekhar-Fermi method and found that magnetic
field pressure is comparable to or larger than that of the turbulence in all
filaments.
BISTRO Survey: Dust Grain Properties from the Observation of Magnetic Fields Surrounding LkHa 101
Diep Pham Ngoc (Vietnam National Space Center)
Abstract: Using the polarimeter POL2 of the James Clerk Maxwell Telescope,
the BISTRO Collaboration conducted the highest spatial resolution
measurements of the magnetic fields surrounding the densest region of the
Auriga-California molecular cloud (LkHa 101). The magnetic field morphology,
the possible causes of the observed polarization hole, and the implications
on the properties of the dust grains in the region will be discussed.
How do magnetic fields regulate star formation? The case of 30 Doradus
Cornelia Pabst (Leiden Observatory)
Abstract: The interaction of massive stars with their environment is crucial
to the lifecycle of the interstellar medium as this feedback regulates the
star formation potential of galaxies. Previously, we have studied the
interaction of the Trapezium star cluster with the Orion Molecular Cloud.
Here we report a similar study of the 30 Doradus region in the Large
Magellanic Cloud. As the nearest starburst region 30 Doradus serves as a
template to study star formation and stellar feedback in extreme
environments. With SOFIA/upGREAT we have obtained velocity-resolved
observations of the [CII] 158um line of the entire nebula. The [CII] line is
an excellent tracer of large-scale dynamics and expansion of the neutral gas.
Together with archival SOFIA/HAWC+ observations of the dust polarization
these data allow to investigate the role magnetic fields play in stellar
feedback processes. I will show some preliminary results.
Filaments segmentation: Mask RCNN approach
Adai Shomanov (Nazarbayev University)
Abstract:The interstellar filaments are believed to play a major role in star
formation.
Filament detection algorithms were developed to find a location and extent of
the filaments in the sky. Two of the commonly used methods for this task are
the RHT (Rolling Hough Transform) and the DisPerse algorithms. Different by
their nature, these algorithms depend on a large number of parameters and
hence finding filaments in the images usually require a time-consuming
process of finding the optimal set of these parameters. To facilitate this process
and make it easier and more efficient we propose a deep neural network solution
for the process of the filament segmentation. Our work depends on a particular
architecture called Mask R-CNN, which is a variant of convolutional neural
network that provides as an output the masked images, in which corresponding
filaments can be found. In our work we have demonstrated a high mean average
precision (mAP) of 90 percent for the filament segmentation task. In our
approach we used datasets with different levels of complexity, both simulated
and observational. We applied transfer learning for each consecutive step.
The neural network was trained on a GeForce 2080 RTX GPU with a keras library
and tensorflow backend used as software.
Some insights from the JCMT BISTRO Surveys
Derek Ward-Thompson (UCLAN)
Abstract: In this talk I will present some of the highlights of the JCMT
BISTRO Surveys so far. I will not give a comprehensive review of the surveys
but rather will share some insights into things we have learnt in general
about magnetic fields in filamentary environments in the ISM. These will
include the relationship between fields and outflows, a possible 'critical
scale length' for magnetic fields, and interactions between fields and
'bubbles'. I will compare the BISTRO data with data at other wavelengths and
different resolutions from other instruments and telescopes, such as Planck,
ALMA and SOFIA HAWC+.
Friday
Constructing Maps of Plane-of-Sky Magnetic Field Strength using FIR Polarimetric Data
Jordan Guerra (Villanova University)
Abstract: Far Infrared (FIR) dust polarimetry enables the study of the
magnetic field in astrophysical objects such as molecular clouds. On one
hand, polarization orientation provide information about the plane-of-sky
(POS) direction of the magnetic field. On the other hand, the patterns
observed in the polarization orientation can be used to estimate the POS
magnetic field strength via the Davis-Chandrasekhar-Fermi (DCF) method. The
advent of high angular-resolution polarimetric data from facilities such as
HAWC+/SOFIA provide an unprecedented opportunity to study the magnetic field
in further detail. We present here the application of the DCF method in
combination with a moving kernel approach. This combination allows the
construction of maps of the magnetic field strength. We will present such
maps for the Orion Molecular Cloud region 1 (OMC-1) and their implications in
the magnetic field – gravity balance that is fundamental for star formation
process. In addition, we will discuss the shortcomings of the DCF method and
what can be done to mitigate them by using simulated data.
Rotational Disruption and Alignment of Astrophysical Dust
Thiem Hoang (KASI & UST)
Abstract: The alignment of dust grains causes the polarization of starlight
as well as the polarization of thermal radiation from themselves. Efficient
grain alignment, as required to reproduce observed dust polarization, is only
achieved when grains can rotate suprathermally. The grain suprathermal
rotation (e.g., by radiative torques) induces centrifugal stress within the
grain that can exceed the binding energy of the grain material, resulting in
the disruption of the grain into small fragments. The rotational disruption
changes the grain size distribution, which affects all observable properties
of dust (extinction, emission, polarization) and surface chemistry. In this
talk, I will discuss the fundamental link between grain alignment, rotational
disruption, and present our recent results in numerical modeling and
observational evidence for the grain rotational disruption by radiative torques.
Precession Galactic magnetometry through dust polarization
Raphael Skalidis (University of Crete and Institute of Astrophysics, FORTH)
Abstract:
Dust polarization is used to probe the magnetic field properties in the interstellar medium
(ISM), but it does not provide a direct measurement of its strength. Various methods have
been developed employing dust polarization and spectroscopic data in order to infer the
magnetic field strength. All of these methods rely on the assumption that the observed
linewidths of the emission spectra and the spread in the distribution of the
polarization angle is due to the propagation of Alfvén waves. Observations, however, indicate
that non-Alfvénic (compressible) waves may be important in the ISM kinematics. With simple
Energetics arguments, we developed a new method which takes into account the
compressible modes. We created synthetic observations from 3D MHD turbulent simulations in order to assess the accuracy of our method. For comparison, we applied two of the most widely
accepted past methods developed by Davis 1951, Chandrasekhar & Fermi 1953 and Hildebrand et al. 2009 and Houde et al. 2009, which are solely based on Alfvénic modes. The omission of compressible modes highly affects these methods, which in some cases produced estimates
which deviated more than a factor of two from the true magnetic field strength. In contrast to
them, the proposed method produced estimates with a mean relative deviation from the true value equal to 17%. Our method achieved a uniformly low error in the estimation of the magnetic
field strength independently of the turbulent properties of the medium.
Magnetic helicity inverse transfer in supersonic isothermal MHD turbulence
Jean-Mathieu Teissier (Technische Universität Berlin)
Abstract:
Magnetic helicity is an ideal invariant of the magnetohydrodynamic (MHD)
equations which exhibits an inverse transfer in spectral space. Up to the
present day, its transport has been studied in direct numerical simulations
only in incompressible or in subsonic or transonic flows. Inspired by typical
values of the turbulent root mean square (RMS) Mach number in the
interstellar medium, this work presents some aspects of the magnetic helicity
inverse transfer in high Mach number isothermal compressible turbulence, with
RMS Mach number up to the order of ten:
1) a clear Mach-number dependence of the spectral magnetic helicity scaling
but an invariant scaling exponent of the co-spectrum of the Alfvén velocity
and its curl,
2) the approximate validity of a dynamical balance relation found by
incompressible turbulence closure theory,
3) a characteristic structuring of helically-decomposed nonlinear
shell-to-shell fluxes that can be disentangled into different spectrally
local and non-local transfer processes.
Understanding polarized dust emission from Rho Ophiuchi A in light of grain alignment and disruption by radiative torques
Le Ngoc Tram (Max-Planck Institute for RadioAstronomy)
Abstract: The alignment of dust grains with the ambient magnetic field
produces polarization of starlight and thermal dust emission. Using the
SOFIA/HAWC+ polarimetric data observed toward the Rho Ophiuchus (Oph) A
cloud hosted by a B star at 89 and 154 µm, we find that the fractional
polarization of thermal dust emission first increases with the grain
temperature and then decreases once the grain temperature exceeds ≃
25-32 K. The latter trend differs from the popular RAdiative Torques (RATs)
alignment theory prediction, which implies a monotonic increase of the
polarization fraction with the grain temperature. We perform numerical
modeling of polarized dust emission for the Rho Oph-A cloud and calculate
the fraction of dust polarization by simultaneously considering the dust
grain alignment and rotational disruption by RATs. Our modeling results could
successfully reproduce both the rising and declining trends of the
observational data, which suggest grains in the Rho Oph-A cloud have a
composite structure, and the grain size distribution has a steeper slope than
the standard size distribution for the interstellar medium. This study
revealed the importance of the rotational disruption mechanism that needs to
be considered together with RAT alignment to interpret SOFIA/HAWC+
observations of the polarized thermal dust emission toward an intense
radiation source.
Turbulence anisotropy: a new method for measuring magnetic fields in cold interstellar phases
Siyao Xu (Institute for Advanced Study)
Abstract: The interstellar turbulence is magnetized and thus anisotropic. The
anisotropy of turbulent magnetic fields and velocities is imprinted in the
related observables, including filamentary density structures. As
theoretically expected and numerically tested, density filaments aligned with
the local magnetic fields are naturally generated in sub-Alfvénic MHD
turbulence, and their morphology is shaped by the turbulence anisotropy.
Based on this knowledge, we found the difference between the turbulent
fluctuations measured in different directions is closely correlated with the
magnetic field strength. Their relation has been quantitatively confirmed by
our synthetic observations of 12CO emission. The turbulence anisotropy
provides a new method for measuring the plane-of-sky magnetic fields in cold
interstellar phases.
Velocity Caustics as the tracer of magnetic field and turbulence statistics in interstellar media
Ka Ho Yuen (University of Wisconsin Madison)
Abstract: The success of the velocity gradient technique allows us to
retrieve the magnetic field structures in both interstellar media and
molecular clouds with relatively high accuracy. Yet, the fundamental reason
why the gradient technique works rely on the fact that the velocity channels
in the position-position-velocity cube are filamentary in nature and dominant
by velocity fluctuations. The latter is hard to attain when there are other
ongoing physical processes like shocks or outflows, strong absorptions, or
underlying thermal broadening. The recent development of the Velocity
Decomposition Algorithm allows observers to obtain velocity caustics from
observations, which is suggested by the MHD turbulence theory to be the best
magnetic field trader available in observations. In the talk, I will
illustrate how to obtain velocity caustics from different molecular clouds,
show the respective magnetic field structures outlined by the filamentary
features from caustics, and reveal the underlying MHD turbulence statistics
stored in the statistics of velocity caustics. The availability of velocity
caustics for every PPV cube in molecular clouds will open a new avenue in
studying magnetic fields or, more generally, studying turbulence in
Observations.