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1. Magnetic Field Strength from Turbulence Theory. I. Using Differential Measure Approach

   https://doi.org/10.3847/1538-4357/ac6877  

   Lazarian, A. ; Yuen, Ka Ho ; Pogosyan, Dmitri ( August 2022 , The Astrophysical Journal)

   The mean plane-of-sky magnetic field strength is traditionally obtained from the combination of polarization and spectroscopic data using the Davis–Chandrasekhar–Fermi (DCF) technique. However, we identify the major problem of the DCF technique to be its disregard of the anisotropic character of MHD turbulence. On the basis of the modern MHD turbulence theory we introduce a new way of obtaining magnetic field strength from observations. Unlike the DCF technique, the new technique uses not the dispersion of the polarization angle and line-of-sight velocities, but increments of these quantities given by the structure functions. To address the variety of astrophysical conditions for which our technique can be applied, we consider turbulence in both media with magnetic pressure higher than the gas pressure, corresponding, e.g., to molecular clouds, and media with gas pressure higher than the magnetic pressure, corresponding to the warm neutral medium. We provide general expressions for arbitrary admixtures of Alfvén, slow, and fast modes in these media and consider in detail particular cases relevant to diffuse media and molecular clouds. We successfully test our results using synthetic observations obtained from MHD turbulence simulations. We demonstrate that our differential measure approach, unlike the DCF technique, can be used to measuremore »the distribution of magnetic field strengths, can provide magnetic field measurements with limited data, and is much more stable in the presence of induced large-scale variations of nonturbulent nature. Furthermore, our study uncovers the deficiencies of earlier DCF research.

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2. Large-scale magnetic field in the Monoceros OB 1 east molecular cloud

   https://doi.org/10.1051/0004-6361/202039065  

   Alina, D. ; Montillaud, J. ; Hu, Y. ; Lazarian, A. ; Ristorcelli, I. ; Abdikamalov, E. ; Sagynbayeva, S. ; Juvela, M. ; Liu, T. ; Carrière, J.-S. ( February 2022 , Astronomy & Astrophysics)

   Context. The role of large-scale magnetic fields in the evolution of star-forming regions remains elusive. Its investigation requires the observational characterization of well-constrained molecular clouds. The Monoceros OB 1 molecular cloud is a large complex containing several structures that have been shown to be engaged in an active interaction and to have a rich star formation history. However, the magnetic fields in this region have only been studied on small scales. Aims. We study the large-scale magnetic field structure and its interplay with the gas dynamics in the Monoceros OB 1 east molecular cloud. Methods. We combined observations of dust polarized emission from the Planck telescope and CO molecular line emission observations from the Taeduk Radio Astronomy Observatory 14-metre telescope. We calculated the strength of the plane-of-sky magnetic field using a modified Chandrasekhar-Fermi method and estimated the mass-over-flux ratios in different regions of the cloud. We used the comparison of the velocity and intensity gradients of the molecular line observations with the polarimetric observations to trace dynamically active regions. Results. The molecular complex shows an ordered large-scale plane-of-sky magnetic field structure. In the northern part, it is mostly orientated along the filamentary structures, while the southern part shows at leastmore »two regions with distinct magnetic field orientations. Our analysis reveals a shock region in the northern part right between two filamentary clouds that, in previous studies, were suggested to be involved in a collision. The magnetic properties of the north-main and north-eastern filaments suggest that these filaments once formed a single one, and that the magnetic field evolved together with the material and did not undergo major changes during the evolution of the cloud. In the southern part, we find that either the magnetic field guides the accretion of interstellar matter towards the cloud or it is dragged by the matter falling towards the main cloud. Conclusions. The large-scale magnetic field in the Monoceros OB 1 east molecular cloud is tightly connected to the global structure of the complex. In the northern part, it seems to serve a dynamically important role by possibly providing support against gravity in the direction perpendicular to the field and to the filament. In the southern part, it is probably the most influential factor governing the morphological structure by guiding possible gas inflow. A study of the whole Monoceros OB 1 molecular complex at large scales is necessary to form a global picture of the formation and evolution of the Monoceros OB 1 east cloud and the role of the magnetic field in this process.« less
3. The JCMT BISTRO Survey: Revealing the Diverse Magnetic Field Morphologies in Taurus Dense Cores with Sensitive Submillimeter Polarimetry

   https://doi.org/10.3847/2041-8213/abeb1c  

   Eswaraiah, Chakali ; Li, Di ; Furuya, Ray S. ; Hasegawa, Tetsuo ; Ward-Thompson, Derek ; Qiu, Keping ; Ohashi, Nagayoshi ; Pattle, Kate ; Sadavoy, Sarah ; Hull, Charles L. ; et al ( May 2021 , The Astrophysical Journal Letters)

   Abstract We have obtained sensitive dust continuum polarization observations at 850 μ m in the B213 region of Taurus using POL-2 on SCUBA-2 at the James Clerk Maxwell Telescope as part of the B -fields in STar-forming Region Observations (BISTRO) 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, 44 ± 16, and 12 ± 5 μ G, respectively. These cores show distinct mean B -field orientations. The 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. The 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 a disordered B -fieldmore »that shows 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.« less
4. Unveiling polarized emission from interstellar dust of the Large Magellanic Cloud with Planck

   https://doi.org/10.1093/mnras/stac3164  

   Alina, D. ; Bernard, J-Ph ; Yuen, K. H. ; Lazarian, A. ; Hughes, A. ; Iskakova, M. ; Akimkhan, A. ; Mukanova, A. ( November 2022 , Monthly Notices of the Royal Astronomical Society)

   Polarization of interstellar dust emission is a powerful probe of dust properties and magnetic field structure. Yet studies of external galaxies are hampered by foreground dust contribution. The study aims at separating the polarized signal from the Large Magellanic Cloud (LMC) from that of the Milky Way (MW) to construct a wide-field, spatially complete map of dust polarization using the Planck 353 GHz data. To estimate the foreground polarization direction, we used velocity gradients in H i spectral line data and assessed the performance of the output by comparing to starlight extinction polarization. We estimate the foreground intensity using the dust-to-gas correlation and the average intensity around the LMC and we assume the foreground polarization to be uniform and equal to the average of the MW around the galaxy to derive foreground I, Q, and U parameters. After foreground removal, the geometry of the plane-of-the-sky magnetic field tends to follow the structure of the atomic gas. This is notably the case along the molecular ridges extending south and south-east of the 30 Doradus star-forming complex and along the more diffuse southern arm extending towards the Small Magellanic Cloud. There is also an alignment between the magnetic field and the outer arm inmore »the western part. The median polarization fraction in the LMC is slightly lower than that observed for the MW as well as the anticorrelation between the polarization angle dispersion function and the polarization fraction. Overall, polarization fraction distribution is similar to that observed in the MW.

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5. Dust polarized emission observations of NGC 6334: BISTRO reveals the details of the complex but organized magnetic field structure of the high-mass star-forming hub-filament network

   https://doi.org/10.1051/0004-6361/202038624  

   Arzoumanian, D. ; Furuya, R. S. ; Hasegawa, T. ; Tahani, M. ; Sadavoy, S. ; Hull, C. L. ; Johnstone, D. ; Koch, P. M. ; Inutsuka, S. ; Doi, Y. ; et al ( March 2021 , Astronomy & Astrophysics)

   Context. Molecular filaments and hubs have received special attention recently thanks to new studies showing their key role in star formation. While the (column) density and velocity structures of both filaments and hubs have been carefully studied, their magnetic field (B-field) properties have yet to be characterized. Consequently, the role of B-fields in the formation and evolution of hub-filament systems is not well constrained. Aims. We aim to understand the role of the B-field and its interplay with turbulence and gravity in the dynamical evolution of the NGC 6334 filament network that harbours cluster-forming hubs and high-mass star formation. Methods. We present new observations of the dust polarized emission at 850 μ m toward the 2 pc × 10 pc map of NGC 6334 at a spatial resolution of 0.09 pc obtained with the James Clerk Maxwell Telescope (JCMT) as part of the B-field In STar-forming Region Observations (BISTRO) survey. We study the distribution and dispersion of the polarized intensity ( PI ), the polarization fraction ( PF ), and the plane-of-the-sky B-field angle ( χ B_POS ) toward the whole region, along the 10 pc-long ridge and along the sub-filaments connected to the ridge and the hubs. We derivedmore »the power spectra of the intensity and χ B POS along the ridge crest and compared them with the results obtained from simulated filaments. Results. The observations span ~3 orders of magnitude in Stokes I and PI and ~2 orders of magnitude in PF (from ~0.2 to ~ 20%). A large scatter in PI and PF is observed for a given value of I . Our analyses show a complex B-field structure when observed over the whole region (~ 10 pc); however, at smaller scales (~1 pc), χ B POS varies coherently along the crests of the filament network. The observed power spectrum of χ B POS can be well represented with a power law function with a slope of − 1.33 ± 0.23, which is ~20% shallower than that of I . We find that this result is compatible with the properties of simulated filaments and may indicate the physical processes at play in the formation and evolution of star-forming filaments. Along the sub-filaments, χ B POS rotates frombeing mostly perpendicular or randomly oriented with respect to the crests to mostly parallel as the sub-filaments merge with the ridge and hubs. This variation of the B-field structure along the sub-filaments may be tracing local velocity flows of infalling matter in the ridge and hubs. Our analysis also suggests a variation in the energy balance along the crests of these sub-filaments, from magnetically critical or supercritical at their far ends to magnetically subcritical near the ridge and hubs. We also detect an increase in PF toward the high-column density ( N H 2 ≳ 10 23  cm −2 ) star cluster-forming hubs. These latter large PF values may be explained by the increase in grain alignment efficiency due to stellar radiation from the newborn stars, combined with an ordered B-field structure. Conclusions. These observational results reveal for the first time the characteristics of the small-scale (down to ~ 0.1 pc) B-field structure of a 10 pc-long hub-filament system. Our analyses show variations in the polarization properties along the sub-filaments that may be tracing the evolution of their physical properties during their interaction with the ridge and hubs. We also detect an impact of feedback from young high-mass stars on the local B-field structure and the polarization properties, which could put constraints on possible models for dust grain alignment and provide important hints as to the interplay between the star formation activity and interstellar B-fields.« less