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Register a new userMagnetic fields in star forming systems (I): Idealized synthetic signatures of dust polarization and Zeeman splitting in filaments
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We use the POLARIS radiative transport code to generate predictions of the two main observables directly sensitive to the magnetic field morphology and strength in filaments: dust polarization and gas Zeeman line splitting. We simulate generic gas filaments with power-law density profiles assuming two density-field strength dependencies, six different filament inclinations, and nine distinct magnetic field morphologies, including helical, toroidal, and warped magnetic field geometries. We present idealized spatially resolved dust polarization and Zeeman-derived field strengths and directions maps. Under the assumption that dust grains are aligned by radiative torques (RATs), dust polarization traces the projected plane-of-the-sky magnetic field morphology. Zeeman line splitting delivers simultaneously the intensity-weighted line-of-sight field strength and direction. We show that linear dust polarization alone is unable to uniquely constrain the 3D field morphology. We demonstrate that these ambiguities are ameliorated or resolved with the addition of the Zeeman directional information. Thus, observations of both the dust polarization and Zeeman splitting together provide the most promising means for obtaining constraints of the 3D magnetic field configuration. We find that the Zeeman-derived field strengths are at least a factor of a few below the input field strengths due to line-of-sight averaging through the filament density gradient. Future observations of both dust polarization and Zeeman splitting are essential for gaining insights into the role of magnetic fields in star and cluster forming filaments.
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Cold, dense filaments, some appearing as infrared dark clouds, are the nurseries of stars. Tremendous progress in terms of temperature, density distribution and gas kinematics has been made in understanding the nature of these filaments. However, very little is known about the role played by magnetic fields in the evolution of these filaments. Here, I summarize the recent observational efforts and ongoing projects (POLSTAR survey) in this direction.
We report 1.2 mm polarized continuum emission observations carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star formation region G5.89-0.39. The observations show a prominent 0.2 pc north-south filamentary structure. The UCHII in G5.89-0.39 breaks the filament in two pieces. Its millimeter emission shows a dusty belt with a mass of 55-115 M$_{odot}$ and 4,500 au in radius, surrounding an inner part comprising mostly ionized gas with a dust emission only accounting about 30% of the total millimeter emission. We also found a lattice of convex arches which may be produced by dragged dust and gas from the explosive dispersal event involving the O5 Feldts star. The north-south filament has a mass between 300-600 M$_{odot}$ and harbours a cluster of about 20 millimeter envelopes with a median size and mass of 1700 au and 1.5 M$_{odot}$, respectively, some of which are already forming protostars. We interpret the polarized emission in the filament as mainly coming from magnetically aligned dust grains. The polarization fraction is ~4.4% in the filaments and 2.1% at the shell. The magnetic fields are along the North Filament and perpendicular to the South Filament. In the Central Shell, the magnetic fields are roughly radial in a ring surrounding the dusty belt between 4,500 and 7,500 au, similar to the pattern recently found in the surroundings of Orion BN/KL. This may be an independent observational signpost of explosive dispersal outflows and should be further investigated in other regions.
Star formation is primarily controlled by the interplay between gravity, turbulence, and magnetic fields. However, the turbulence and magnetic fields in molecular clouds near the Galactic Center may differ substantially from spiral-arm clouds. Here we determine the physical parameters of the central molecular zone (CMZ) cloud G0.253+0.016, its turbulence, magnetic field and filamentary structure. Using column-density maps based on dust-continuum emission observations with ALMA+Herschel, we identify filaments and show that at least one dense core is located along them. We measure the filament width W_fil=0.17$pm$0.08pc and the sonic scale {lambda}_sonic=0.15$pm$0.11pc of the turbulence, and find W_fil~{lambda}_sonic. A strong velocity gradient is seen in the HNCO intensity-weighted velocity maps obtained with ALMA+Mopra, which is likely caused by large-scale shearing of G0.253+0.016, producing a wide double-peaked velocity PDF. After subtracting the gradient to isolate the turbulent motions, we find a nearly Gaussian velocity PDF typical for turbulence. We measure the total and turbulent velocity dispersion, 8.8$pm$0.2km/s and 3.9$pm$0.1km/s, respectively. Using magnetohydrodynamical simulations, we find that G0.253+0.016s turbulent magnetic field B_turb=130$pm$50$mu$G is only ~1/10 of the ordered field component. Combining these measurements, we reconstruct the dominant turbulence driving mode in G0.253+0.016 and find a driving parameter b=0.22$pm$0.12, indicating solenoidal (divergence-free) driving. We compare this to spiral-arm clouds, which typically have a significant compressive (curl-free) driving component (b>0.4). Motivated by previous reports of strong shearing motions in the CMZ, we speculate that shear causes the solenoidal driving in G0.253+0.016 and show that this reduces the star formation rate (SFR) by a factor of 6.9 compared to typical nearby clouds.
The magnetic field plays an important role in every stage of the star-formation process from the collapse of the initial protostellar core to the stars arrival on the main sequence. Consequently, the goal of this science case is to explore a wide range of magnetic phenomena that can be investigated using the polarization capabilities of the Next Generation Very Large Array (ngVLA). These include (1) magnetic fields in protostellar cores via polarized emission from aligned dust grains, including in regions optically thick at wavelengths observable by the Atacama Large Millimeter/submillimeter Array (ALMA); (2) magnetic fields in both protostellar cores and molecular outflows via spectral-line polarization from the Zeeman and Goldreich-Kylafis effects; (3) magnetic fields in protostellar jets via polarized synchrotron emission; and (4) gyrosynchrotron emission from magnetospheres around low-mass stars.
We present new measurements of the dust emissivity index, beta, for the high-mass, star-forming OMC 2/3 filament. We combine 160-500 um data from Herschel with long-wavelength observations at 2 mm and fit the spectral energy distributions across a ~ 2 pc long, continuous section of OMC 2/3 at 15000 AU (0.08 pc) resolution. With these data, we measure beta and reconstruct simultaneously the filtered-out large-scale emission at 2 mm. We implement both variable and fixed values of beta, finding that beta = 1.7 - 1.8 provides the best fit across most of OMC 2/3. These beta values are consistent with a similar analysis carried out with filtered Herschel data. Thus, we show that beta values derived from spatial filtered emission maps agree well with those values from unfiltered data at the same resolution. Our results contradict the very low beta values (~ 0.9) previously measured in OMC 2/3 between 1.2 mm and 3.3 mm data, which we attribute to elevated fluxes in the 3.3 mm observations. Therefore, we find no evidence or rapid, extensive dust grain growth in OMC 2/3. Future studies with Herschel data and complementary ground-based long-wavelength data can apply our technique to obtain robust determinations of beta in nearby cold molecular clouds.
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