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
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.
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
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.
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+.
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.