Magnetic Fields and the Structure of the Filamentary Interstellar Medium
Abstracts and Recorded Talks
How turbulence and magnetic fields regulate ISM structure and star formation
Christoph Federrath (ANU)
Abstract: How turbulence and magnetic fields regulate ISM structure and star formation Turbulence and magnetic fields are key players in the process of star formation. This is because they control the structure of the ISM on large scales, but also regulate processes such as angular momentum transport and jet launching inside protostellar discs where stars form. I will attempt to explain some of the theoretical models for how filaments may form in molecular clouds (in the Galactic disc and in the Galactic centre), and how the interplay of gravity, turbulence, magnetic fields and stellar feedback controls the star formation rate and the initial mass function of stars.
The Scale-dependent Importance of Magnetic Field in Star Formation
Che-Yu Chen (LLNL)
Abstract: The role played by magnetic fields during the star-forming process has been long debated and is still an open question in astrophysics. Motivated by the recently-advanced polarimetric capabilities, our recent theoretical efforts utilizing 3D MHD simulations have provided new ways to characterize the 3D structure and the relative importance of the magnetic field in star-forming clouds. At sub-pc scale, our previous investigations of magnetized core formation in turbulent clouds showed that magnetic effect is not critical during core growth because gas preferably flows along the field lines towards dense cores. Further examinations on the progression of magnetic field geometry from cloud to core scales also lend support to the anisotropic core formation model we proposed. Taken together, we suggest that the magnetic field is dynamically significant in regulating the star-forming process at intermediate scales, where the large-scale super-sonic/Alfvenic turbulence already cascaded but gravity has not yet taken over the control.
Multi-scale and Multi-wavelength Magnetic Fields in the Central Molecular Zone
Yue Hu (University of Wisconsin-Madison)
Abstrac: Magnetic fields in the central molecular zone have attracted a vast of attention in recent years. To get an insight into the magnetic fields, we employ the Gradient Technique (GT), which is rooted in the anisotropy of magnetohydrodynamic turbulence. Our analysis combines the data of multiple wavelengths, including molecular emission lines, radio 1.4 GHz continuum image, and Herschel 70µm image, as well as ionized [Ne2] and Paschen-alpha emissions, with the observations of Planck 353 GHz and HAWC+ 53µm polarized dust emissions. We will show wavelength-dependent magnetic fields in the overall central molecular zone, the radio arc, and the Sagittarius A* west region, accessing multi scales from the order of 10 pc to 0.1 pc. The magnetic fields towards the central molecular zone traced by GT are globally compatible with the polarization measurements, accounting for the contribution from the galactic foreground. This correspondence suggests that the magnetic field and turbulence are dynamically crucial in the galactic center. We will show that the magnetic fields associated with the arched filaments and the thermal components of the radio arc agree with the HAWC+ polarization. The measurement towards the non-thermal radio arc reveals the poloidal magnetic field components in the galactic center. For Sagittarius A* region, we will present the agreement between the GT measurement using [Ne2] emission and HAWC+ 53µm observation. We use GT to predict the magnetic fields associated with ionized Paschen-alpha Gas.
Magnetic Fields at Multiple Scales as Seen In FIR-MM Polarimetry
Terry Jones (U. Minnesota)
Abstract: Far Infrared and MM wave polarimetry of dust emission provides a unique tool for the study of magnetic fields in the interstellar medium over a wide range of physical scales. Observations of polarized dust emission are not affected by scattering as is the case at optical and NIR wavelengths, nor Faraday effects present in radio synchrotron observations. Current advances in both observations and analysis techniques are discussed for scales ranging from protostellar molecular cloud cores to entire galaxies. Current challenges for present and future application of FIR-MM wave polarimetry for the study of magnetic fields are discussed.
Note: due to technical difficulties, this talk was only partially recorded. The entire set of slides for the talk is available here.
Polarization from Cores to Disks: Connecting Single-Dish and ALMA Dust Polarization
Sarah Sadavoy (Queen’s University)
Abstract: The field of star formation is rapidly changing with the development of high resolution and multi-wavelength instrumentation that can observe dust polarization at far-infrared and (sub)millimeter wavelengths. With these facilities, we are now able to probe dust polarization in protostellar environments on scales of 10-5000 au, and we can use those data to infer magnetic field structures down to the scales of disks. In this presentation, I will present an overview of magnetic fields traced by dust polarization from the scales of cores and filaments as observed by HAWC+ and POL-2 to the scales of disks around young stars as observed by ALMA. I will also discuss the difficulties in probing magnetic fields down to disk scales and the need for multi-scale and multi-wavelength observations to uncover their role in the formation of both stars and planets.
Multi-Scale Picture of Magnetic Field and Gravity in High-Mass Star-Forming Region W51
Patrick Koch (ASIAA)
Abstract: We present a suite of dust polarization observations in the high-mass star-forming region W51. These observations image the magnetic (B-) field morphology with progressively higher-resolutions from the pc-scale envelope, to globally collapsing cores, to the fragments within cores, and down to a network of core-connection fibers with the currently highest resolution around 500au. These observations cover a range in resolution of about a factor 1,000 in area. Together with these scale-dependent B-field morphologies we analyze the gravitational vector field. We find recurring similarities in the magnetic field structures and their corresponding gravitational vector fields. These self-similar structures point at a multi-scale collapse-within-collapse scenario. At the highest resolution, we find B-field orientations that are prevailingly parallel to the core-connecting extensions and fibers. This key structural feature is analyzed together with the gravitational vector field. We derive a stability criterion that defines a maximum magnetic field strength that can be overcome by an observed magnetic field-gravity configuration. Equivalently, this defines a minimum field strength that can stabilize extensions and fibers against a radial collapse. We find that the detected fibers and extensions are stable, hence possibly making them a fundamental component in the accretion onto central cores.
IRDC Structure: Magnetically Dominated Envelopes and Collapsing Cores
Mordecai-Mark Mac Low (AMNH)
Abstract: Models of stratified, radiatively-cooing, magnetized, supernova-driven turbulent gas representative of sections of galactic disk have density enhancements that reach densities characteristic of giant molecular clouds (GMCs). However, in the absence of self-gravity, these objects have dynamics clearly distinct from observed GMCs, with velocity dispersions under a kilometer per second at all scales, and a diffuse morphology. Inclusion of self-gravity, however, produces dense filaments strongly reminiscent of IRDC morphology, with dynamics in broad agreement with observed dependences of velocity dispersion on column density and size of cloud. Detailed investigation of these clouds magnetic field structure reveals that, in agreement with observed clouds, they have strongly magnetized envelopes and gravitationally dominated cores that collapse at close to the free-fall timescale. Stellar feedback appears to terminate star formation and disperse gas quickly, leading to the low observed star formation rates.
Simulations of dust, gas, and magnetic fields
Mika Juvela (University of Helsinki)
Abstract: Star-formation studies require precise knowledge of the structure, kinematics, and magnetic fields of dense clouds. The magnetic fields are observationally the most challenging component but, since one only sees one projection of the medium, even density and velocity information is often hard to interpret. Simulations help to understand the relationships between the measurements and the three-dimensional reality in the clouds. I will describe past studies where simulations were used to interpret Planck polarisation measurements of clumps (using random MHD cloud realisations) or to seek explanation for SCUBA POL-2 measurements of an IRDC (using toy models built for the individual source). I will also describe ongoing study of an IRDC-like object selected from a larger MHD simulation. The study will be based on a combination of synthetic observations of line emission and dust continuum emission and polarisation.
Reconstructing the full 3D morphology of magnetic fields associated with filamentary molecular clouds
Mehrnoosh Tahani (DRAO - National Research Council Canada)
Abstract: We present a new approach to study the complete 3D morphology of magnetic fields, including their direction, associated with filamentary molecular clouds. We considered the Perseus, California, and Orion A filamentary molecular clouds, which previously showed a line-of-sight magnetic field reversal across them (Tahani et al. 2018). We analyzed velocity and Galactic magnetic field information of these regions and compared them with models and magneto-hydrodynamics simulations. We found that the observational data in the Perseus cloud matched the models that predict a bow-shaped magnetic field morphology, indicating a high likelihood for this magnetic morphology associated with the cloud. Using Galactic magnetic field and velocity information, we then reconstructed the 3D bow-shaped magnetic field morphology in this region, with its concave side pointing toward us and its plane-of-sky projection pointing in the decreasing longitude direction. For the Orion A and California clouds the limitations in the observations restricted us from comparing their consistency with the bow-predicting models. However, using previous studies and observations we constructed the 3D morphology of the magnetic fields in the Orion A cloud. The method suggested in this work enables us to construct the complete three-dimensional large-scale magnetic field morphologies and vectors. More specifically, the technique allows us to map the signed direction of magnetic fields in the plane of the sky (without the 180 degree ambiguity).
Disentangling Dust Properties, Grain Alignment And Magnetic Field Structure in NGC 2071 with SOFIA and JCMT
Lapo Fanciullo (Academia Sinica)
Abstract: The thermal emission of interstellar dust is one of the most important tracers of interstellar environments. In particular, interstellar magnetic fields can be studied using polarized dust emission from grains aligned with the field lines. However, interpreting the polarization fraction in emission is a non-trivial task, since this quantity is determined by several degenerate factors: the orientation and structure of the magnetic field, the grain alignment efficiency and the optical properties of dust itself. Comparing polarized emission at multiple wavelengths is one possible way of breaking this degeneracy. I will show the preliminary results of a multi-wavelength polarimetric analysis in the star-forming region NGC 2071, conducted at wavelengths of 850 um (POL-2 observations from the BISTRO project at JCMT), 154 um and 214 um (HAWC+/SOFIA observations). The data show the potential for tracing variations of magnetic Field orientation with optical depth, and/or grain growth in the higher-density region of the cloud, although more detailed modelling is needed to conclusively distinguish the two scenarios. I will further discuss which analysis techniques do not transfer well across wavelength, and how including both short-wavelength (< 250 um) and Long-wavelength data (~ 1 mm) was key to obtaining our results.
NIR BSP, GPIPS, Gaia, and IRDCs
Dan Clemens (Boston University)
Abstract: Background starlight polarimetry (BSP) at optical wavelengths first revealed Galactic magnetic fields 70 years ago. Near-infrared (NIR) BSP enables probing magnetic fields through extinctions ranging from under one Av magnitude to about 60 Av magnitudes. Hence NIR BSP provides key magnetic field context for diffuse to dense ISM interfaces, atomic to molecular cloud phase changes, comparing magnetic energies to thermal and turbulent energies, and how clouds form cores and cores form stars. The Galactic Plane Infrared Polarization Survey (GPIPS) probed much of the Galactic mid-plane with unprecedented NIR BSP sensitivity and sampling, yielding over one million stars with high quality polarizations. Coupled with Gaia distances and proper motions, we now have the ability to begin 3-D tomography of the magnetic field in the Milky Way. GPIPS survey data and follow-up deeper Mimir observations towards several IRDCs reveal a surprisingly complex range of magnetic field properties, from highly ordered to highly disordered. How do these properties correlate with turbulence, star formation, and the effects of external coherent forces?
Magnetic fields at different spatial scales of a filament G34.43+0.24
Archana Soam (SOFIA Science Center, USRA, NASA Ames Research Center)
Abstract: As most of the molecular clouds are filamentary and elongated, the magnetic fields (B-fields) in these clouds are found either parallel or perpendicular to the main axes. In the talk, I will present the B-fields mapped in IRDC G34.43+0.24 (G34 now onwards) obtained with 850-micron polarized dust emission observed with POL-2/JCMT. We examined the magnetic field geometries and strengths in the northern, central, and southern regions of the filament. The overall field orientations are uniform at large (POL-2 at 14” and SHARP at 10”) to small scales (TADPOL at 2.5” and SMA at 1.5”) in the MM1 and MM2 regions. The SHARP/CSO observations in MM3 at 350-micron from Tang et al. show a similar trend as seen in our POL-2 observations. TADPOL observations demonstrate a well-defined field geometry in central region of G34 consistent with MHD simulations of accreting filaments. We obtained a plane-of-sky magnetic field strength of 470+/-190 micG, 100+/-40 micG, and 60+/-34 micG in the central, northern and southern regions of G34, respectively, using the updated Davis-Chandrasekhar-Fermi relation. The estimated value of field strength, combined with column density and velocity dispersion values available in the literature, suggests G34 to be marginally critical with criticality parameter values 0.8+/-0.4, 1.1+/-0.8, and 0.9+/-0.5 in the central, northern, and southern regions, respectively. The turbulent motions in G34 are sub-Alfvénic in the three regions. The observed aligned B-fields in G34 are consistent with theoretical models suggesting that B-fields play an important role in guiding the contraction of the cloud driven by gravity. I will also highlight our attempts to map B-fields at different spatial scales of some other filamentary clouds.
Planck/Herschel analysis of correlations between filamentary structures and magnetic fields in star forming regions
Jean-Sébastien Carriere (IRAP - CNRS / UPS)
Abstract: Pre-stellar cores form in the dense interstellar medium, mostly within filaments. Magnetic fields are believed to play a key, albeit poorly understood, role in the whole sequence of structure formation, from interstellar filaments down to stars. It is, therefore, instructive to study the interplay between magnetic fields and filaments hosting cold cores in star forming regions under various conditions (density, evolutionary stage). This can be investigated by studying the relative orientation between filaments and magnetic fields using both Herschel and Planck maps towards various star forming regions. For this purpose, we have developed the new SupRHT method, based on an improvement of the Rolling Hough Transform (RHT), for the extraction of filamentary structures and the characterisation of their sizes and orientations. This new method is user friendly and is able to identify filaments automatically and robustly in any kind of environments. SupRHT thus allows us to analyse the relative orientation between magnetic fields and filaments over a broad range of size and density, from striations to dense filaments. We present the results obtained for a sample of Herschel fields from the ‘Galactic Cold Cores’ project, probing star formation in different Galactic environments. We find a whole range of relative orientations, with a tendency for filaments to be aligned either parallel or perpendicular to the magnetic field (generally depending on column density).
Multi-wavelength Magnetic Field Observations in Infrared Dark Clouds
Thushara Pillai (Boston University)
Abstract: Detailed measurements of magnetic fields in star-forming filaments in a state prior to the onset of star formation have just started to become available, and are even more scarce for high-mass star forming (HMSF) infrared dark clouds (IRDCs). Recent HMSF theories explore the effect of weak to strong magnetic fields on star formation and cloud evolution. I will present a review of the near and far-infrared polarimetric observations in IRDCs and how it informs us on the magnetic field topology at different stages of evolution. While the current data consistenly show evidence for strong magnetization in IRDCs, outstanding questions remain as to its implications for star-formation itself. I will briefly discuss these as well before opening up the forum for discussion.
What the orientation of the magnetic field tells us and which effects should we expect from the magnetic field?
Patrick Hennebelle (CEA)
Abstract: Which role magnetic fields play within the interstellar medium has been the matter of intense research. Recently simulations and observations have been revealing that there is a preferred orientation between magnetic fields and density gradients which tend to be respectivelly perpendicular or parallel at low or high column densities. What does it physically mean ? Can we conclude that magnetic fields have a strong influence on the ISM ? Using a reformulation of the MHD equations, I will argue that the preferred magnetic field and density gradient orientation is a natural consequence of the transport equations. I will then discuss three cases where magnetic fields really quantitativelly modify the picture, namely the formation of diffuse filaments, the fragmentation of a collapsing core and the formation of proto-planetary disks.
The JCMT BISTRO Survey: The Evolution of Magnetic Fields in Dense Filaments
Kate Pattle (NUI)
Abstract: In this talk I will present results from the JCMT BISTRO (B-Fields in Star-Forming Region Observations) Survey, which has been using the POL-2 polarimeter to map magnetic fields in the dense structures of Milky Way star-forming regions. I will discuss the insights which these observations give into the transition to magnetically subcritical gas dynamics which takes place in dense filamentary structures within molecular clouds, and the first hints that we are getting about the effects of stellar feedback on magnetic fields in dense star-forming gas. I will present recent BISTRO Survey observations of several nearby molecular clouds, discussing the search for a characteristic size or density scale at which magnetic fields lose their dynamic importance in the evolution of a dense filament to gravitational instability.
Filaments, clumps and massive star formation
Qizhou Zhang (Center for Astrophysics | Harvard & Smithsonian)
Abstract: Massive protostars and protostellar clusters are often found at intersections of filamentary molecular clouds. How cloud materials condense and fragment from filaments to form dense cores and a cluster of stellar objects remains an unsolved problem in astrophysics. We investigate the role of magnetic fields in this process using polarimetric observations of polarized dust emission from mm/submm interferometers such as SMA and ALMA, as well as from single dish telescopes such as JCMT and SOFIA. The angular scales probed by these telescopes enable investigations of the dynamic role of magnetic fields during the collapse and fragmentation of parsec-scale clumps and the formation of <0.1 pc dense cores. In this talk, I will present polarimetric surveys of protocluster forming molecular clumps obtained from the SMA and ALMA, and a multi-scale analysis of magnetic fields in conjunction with data from JCMT and SOFIA. We found compelling evidence of a strong magnetic field influence on the gas dynamics during the formation of cores in protoclusters.
Revealing the diverse magnetic field morphologies in Taurus dense cores with sensitive sub-millimeter polarimetry
Eswaraiah Chakali (National Astronomical Observatories of China (NAOC)
Abstract: We have obtained sensitive dust continuum polarization observations at 850 μm in the B213 region of Taurus using POL-2 on SCUBA-2 at the James Clerk Maxwell Telescope (JCMT), as part of the BISTRO (B-fields in STar-forming Region Observations) survey. These observations allow us to probe magnetic field (B-field) at high spatial resolution ( ∼2000 au or ∼0.01 pc at 140 pc) in two protostellar cores (K04166 and K04169) and one prestellar core (Miz-8b) that lie within the B213 filament. Using the Davis-Chandrasekhar-Fermi method, we estimate the B-field strengths in K04166, K04169, and Miz-8b to be 38±14 μG, 44±16 μG, and 12±5 μG, respectively. These cores show distinct mean B-field orientations. B-field in K04166 is well ordered and aligned parallel to the orientations of the core minor axis, outflows, core rotation axis, and large-scale uniform B-field, in accordance with magnetically regulated star formation via ambipolar diffusion taking place in K04166. B-field in K04169 is found to be ordered but oriented nearly perpendicular to the core minor axis and large-scale B-field, and not well-correlated with other axes. In contrast, Miz-8b exhibits disordered B-field which show no preferred alignment with the core minor axis or large-scale field. We found that only one core, K04166, retains a memory of the large-scale uniform B-field. The other two cores, K04169 and Miz-8b, are decoupled from the large-scale field. Such a complex B-field configuration could be caused by gas inflow onto the filament, even in the presence of a substantial magnetic flux.
First Results from the CN-Bright Magnetic Fields Project
Peter Barnes (Space Science Institute)
Abstract: The role of B fields in star formation is still mysterious due to observational challenges in measuring their strength and geometry. Eg, only 14 Zeeman field strength measurements exist using the dense gas tracer CN, limiting our tests of theory in the high-density regime; the important transition from magnetic to gravitational domination near densities ~300 cm–3 has not been clearly defined in a large cloud sample; and we lack systematic information connecting the GMC-scale (10s of pc) field geometry from Planck to the sub-pc scale of protostellar core fields, where cluster-forming clumps dominate star formation. SOFIA+ALMA provide an exciting opportunity to fill these gaps by enabling high quality field mapping in the cold, dense, star-forming gas. We have identified a complete sample of 45 massive molecular clumps which are sufficiently bright in CN to feasibly measure Zeeman field strengths with ALMA, while at the same time affording high-sensitivity field morphology mapping with the Goldreich-Kylafis effect in CO, plus in the continuum with both facilities. In this talk I present first results from this project from recent SOFIA+ALMA data. We are using various polarisation analysis techniques to construct systematic high-SDR maps of dust polarization, detailed mapping of field geometries and strengths, and leveraged by existing continuum & spectroscopic data on clumps’ non-magnetic properties, statistical studies of all parameters across contiguous scales 0.1–10 pc.
Magnetic Properties of Star-Forming Dense Cores
Phil Myers (Center for Astrophysics | Harvard and Smithsonian)
Abstract: We present and interpret magnetic and energetic properties of star-forming dense cores, based on observations of submm and near-infrared polarization of 17 low-mass dense cores. The observations are analyzed with the DCF model of Alfvénic fluctuations of polarization angle and gas velocity. They indicate (1) mass-to-flux ratios are slightly supercritical, with M⁄M_B= 1-3; (2) correlation between plane-of-sky field strength and column density B_pos~N^p, with p=1.05±0.08, and (3) correlation between plane-of-sky field strength and density B_pos~ n^q, with q=0.66±0.05. These properties agree with earlier Zeeman studies (Crutcher et al. 2010), but have finer precision. They are interpreted by (1) the small range of M⁄M_B is due to the relation between virial and magnetic masses, and to selection of centrally concentrated, gravitationally bound cores with modest Alfvén amplitudes. (2) B~N because B~(M⁄M_B)N and because the range of M⁄M_B is less than the range of N, and (3) B~n^(2⁄3) because B~M^(1⁄3) n^(2⁄3) and because the range of M^(1⁄3) is less than the range of n^(2⁄3), for concentrated, bound, spheroidal cores. These results call for better models and simulations of core ensembles. They do not support spherical core evolution at constant mass (Mestel 1966), which predicts B~n^(2⁄3) but requires weak fields to maintain spherical shape. Instead these results describe cores close to equipartition, with gravitational binding that is centrally concentrated, and with fields that are nearly as strong as possible, among fields that allow gravitational contraction.
The Davis-Chandrasekhar-Fermi method, its caveats and areas of application
Martin Houde (Western University)
Abstract: In this presentation I will be focusing on the well-known and widely used DCF equation due to Davis (1951) and Chandrasekhar & Fermi (1953) for estimating magnetic field strengths in the interstellar medium. I will summarize the assumptions underlying the derivation of the DCF equation and discuss its applicability to different environments. I will further emphasize the ensuing shortcomings of this method and discuss some approaches that have been devised to alleviate them. Finally, I will emphasize the need for alternatives to the DCF equation, some of which are discussed in this workshop.
A Practical Guide to Grain Alignment Theory
Brandon Hensley (Princeton University)
Abstract: The physics of grain alignment is a challenging problem, as evidenced by the decades between first observations of dust polarization in the 1940s and a theory capable of explaining its wavelength dependence in detail. In this talk, I will present the key ingredients of modern grain alignment theory, including radiative torques (RATs), paramagnetic dissipation, and thermal flipping, with emphasis on predictions for changes in grain alignment properties with interstellar environment. I will conclude with a discussion of some outstanding questions in grain alignment theory.
Measuring B-strength: turbulence theory based alternative to the Davis-Chandrasekhar-Fermi approach
Alex Lazarian (University of Wisconsin-Madison)
Abstract: I shall present a new way of measuring magnetic field strength distribution using polarization. The technique’s foundation is the modern MHD turbulence theory. I will describe the advantages of the new approach compared to the Davis-Chandrasekhar-Fermi technique and provide numerical simulation that support these advantages.
Calibrating the Davis-Chandrasekhar-Fermi method with numerical simulations
Junhao Liu (Nanjing University)
Abstract: The Davis-Chandrasekhar-Fermi (DCF) method is widely used to indirectly estimate the magnetic field strength from the plane-of-sky field orientation. In this work, we present a set of 3D MHD simulations and synthetic polarization images using radiative transfer of clustered massive star-forming regions. We apply the DCF method on the synthetic polarization maps to investigate its reliability in high-density molecular clumps and dense cores where self-gravity is significant. We investigate the validity of the assumptions of the DCF method step by step and compare the model and estimated field strength to derive the correction factors for the estimated uniform and total (rms) magnetic field strength at clump and core scales. We find the DCF method works well in strong field cases. However, the magnetic field strength in weak field cases could be significantly overestimated by the DCF method when the turbulent magnetic energy is smaller than the turbulent kinetic energy. We investigate the accuracy of the angular dispersion function (ADF, a modified DCF method) method on the effects that may affect the measured angular dispersion and find that the ADF method correctly accounts for the ordered field structure, the beam-smoothing, and the interferometric filtering, but may not be applicable to account for the signal integration along the line of sight in most cases. Our results suggest that the DCF methods should be avoided to be applied below ~0.1 pc scales if the effect of line-of-sight signal integration is not properly Addressed.
Testing the grain alignment and rotational disruption by radiative torques using dust polarization in filaments
Nguyen B. Ngoc (Vietnam National Space Center)
Abstract: Filaments are ubiquitous in the interstellar medium and play an important role in the star formation process. Dust polarization induced by alignment of dust grains with the magnetic field is widely used to study the magnetic field and its role in star formation. In this study, we use dust polarization data observed toward several filaments (Musca, Vela C, and Orion A observed by Planck, BLASTPol, JCMT/POL2, and SOFIA/HAWC+) to test the grain alignment and rotational disruption mechanisms by radiative torques. We found a general correlation of polarization fraction with dust temperature at low temperature and anticorrelation at high temperature. Our numerical modeling of the dust polarization degree simultaneously taking into account the dust alignment and disruption by RAdiative Torque mechanisms (RATs) successfully reproduces the observed variation of the dust polarization with dust temperature.
Magnetic Field Measurements via the Zeeman Effect - Strengths and Limitations
Tom Troland (University of Kentucky)
Abstract: Nature is exceedingly devious in hiding her secrets of the interstellar magnetic field. Each method we have to reveal these secrets obscures some of them, and the radio frequency Zeeman effect is no exception. The Zeeman effect offers the only method to measure interstellar magnetic field strengths directly. It can even reveal field strengths independently in multiple velocity components along the line-of-sight. However, the Zeeman effect only reveals the line-of-sight field strength. The effect is very weak, requiring long integration times per measurement. It can be susceptible to maddening instrumental effects. And the Zeeman effect can only be applied to the radiation from a very few interstellar species. In particular, (non-maser) Zeeman effect measurements to date have been made with spectral lines of HI (21 cm), OH (18 cm), and CN (1 and 3 mm). Fortunately, each species samples a different density regime, providing field strength information in a wide range of interstellar environments. High spatial resolution Zeeman effect measurements are especially needed in dense, star forming environments. ALMA observations of the CN Zeeman effect, discussed in this conference, likely offer the best opportunity to make these measurements.
The slides for this talk are available here.
The Imprint of Magnetic Fields on Polarization Observations
Stefan Reissl (University of Heidelberg)
Abstract: Magnetic fields are the key ingredient in many astrophysical processes within the interstellar medium (ISM). Observations of aligned dust grains, line emission with Zeeman Effect, and the Faraday rotation measure (RM) are common tracer techniques to infer magnetic field properties. Driven by large sub-mm polarization capabilities, e.g. SOFIA/HAWC+, Planck, and ALMA, in this talk I focus on specific astrophysical problems that currently beg re-evaluation: Does grain alignment physics cause a detectable imprint on the polarization signal? What regions of the ISM are actually probed by the different tracers? What is the actual impact of the magnetic field morphology on the observed polarization signal?
In detail, I present selected results of past projects from my radiative transfer (RT) code POLARIS, which covers multiple facets of dust polarization, Zeeman Effect, and RM. Based on synthetic observations of post-processed large-scale numerical ISM simulations I statistically analyze the imprint of the magnetic field on polarized light. Especially, I discuss methods to distinguish between distinct magnetic field morphologies. Finally, I outline the caveats and numerical limitations of such RT post-processing techniques.
Beyond Davis-Chandrasekhar-Fermi: understanding the magnetized interstellar medium using statistical tools
Juan Diego Soler (MPIA)
Abstract: Seventy years ago, Leverett Davis Jr. made the first estimations of the interstellar magnetic field strength in the Milky Way using starlight polarization observations and what is now known as the Davis-Chandrasekhar-Fermi method. Today, we have an unprecedented amount of observations of the interstellar magnetic fields in the form of synchrotron polarization, Faraday rotation, Zeeman splitting, Goldreich-Kylafis (GK) effect, starlight polarization, and dust polarized emission. This plethora of observations calls for new statistical tools that integrate large volumes of data, compare them to the physical phenomena in numerical simulations, and provide insight into the cycles of matter and energy in the interstellar medium. I will review some of these techniques, mainly focusing on those applied to the analysis of dust polarized emission observations in and around star-forming clouds by SOFIA, Planck, BLASTPol, and other millimeter- and submillimeter-wavelength observatories.