The role of magnetic fields in the fragmentation of the Taurus B213 filament into Sun-type star-forming cores

Fragmentation is a key step in the process of transforming clouds (and their substructures such as filaments, clumps, and cores) into protostars. The thermal gas pressure and gravitational collapse are believed to be the primary agents governing this process, referred to as the thermal Jeans fragmentation. However, the contributions of other factors (such as magnetic fields and turbulence) to the fragmentation process remain less explored. In this work, we have tested possible fragmentation mechanisms by estimating the mean core mass and mean inter-core separation of the B213 filament. We have used the $\sim$14" resolution James Clerk Maxwell Telescope (JCMT) Submillimetre Common-User Bolometer Array 2 (SCUBA-2)/POL-2 850 $\mu$m dust continuum map and combined it with a Planck 850 $\mu$m map and Herschel data. We find that in addition to the thermal contribution, the presence of ordered magnetic fields could be important in the fragmentation of the B213 filament.Comment: 10 pages, 3 figures, and 5 tables ; Accepted for publication in JOA

Multiwavelength polarimetry of the filamentary cloud IC 5146. II. Magnetic field structures

The IC 5146 cloud is a nearby star-forming region in Cygnus, consisting of molecular gas filaments in a variety of evolutionary stages. We used optical and near-infrared polarization data toward the IC 5146 cloud, reported in the first paper of this series, to reveal the magnetic fields in this cloud. Using the newly released Gaia data, we found that the IC 5146 cloud may contain two separate clouds: a first cloud, including the densest main filament at a distance of∼600 pc, and a second cloud, associated with the Cocoon Nebula at a distance of∼800 pc. The spatially averaged H-band polarization map revealed a well-ordered magnetic field morphology, with the polarization segments perpendicular to the main filament but parallel to the nearby sub filaments, consistent with models assuming that the magnetic field is regulating cloud evolution. We estimated the magnetic field strength using the Davis–Chandrasekhar–Fermi method and found that the magnetic field strength scales with volume density with a power-law index of∼0.5 in the density range from N_H2 ∼ 10 to 3000 cm_−3, which indicates an an isotropic cloud contraction with a preferred direction along the magnetic field. In addition, the mass-to-flux ratio of the cloud gradually changes from subcritical to supercritical from the cloud envelope to the deep regions. These features are consistent with strong magnetic field star formation models and suggest that the magnetic field is important in regulating the evolution of the IC 5146 cloud.Accepted manuscript and published version

Unveiling the Importance of Magnetic Fields in the Evolution of Dense Clumps Formed at the Waist of Bipolar H ii Regions: A Case Study of Sh 2-201 with JCMT SCUBA-2/POL-2

Abstract: We present the properties of magnetic fields (B fields) in two clumps (clump 1 and clump 2), located at the waist of the bipolar H ii region Sh 2-201, based on James Clerk Maxwell Telescope SCUBA-2/POL-2 observations of 850 μm polarized dust emission. We find that B fields in the direction of the clumps are bent and compressed, showing bow-like morphologies, which we attribute to the feedback effect of the H ii region on the surface of the clumps. Using the modified Davis–Chandrasekhar–Fermi method, we estimate B-field strengths of 266 and 65 μG for clump 1 and clump 2, respectively. From virial analyses and critical mass ratio estimates, we argue that clump 1 is gravitationally bound and could be undergoing collapse, whereas clump 2 is unbound and stable. We hypothesize that the interplay of the thermal pressure imparted by the H ii region, the B-field morphologies, and the various internal pressures of the clumps (such as magnetic, turbulent, and gas thermal pressures) has the following consequences: (a) formation of clumps at the waist of the H ii region; (b) progressive compression and enhancement of the B fields in the clumps; (c) stronger B fields that will shield the clumps from erosion by the H ii region and cause pressure equilibrium between the clumps and the H ii region, thereby allowing expanding ionization fronts to blow away from the filament ridge, forming bipolar H ii regions; and (d) stronger B fields and turbulence that will be able to stabilize the clumps. A study of a larger sample of bipolar H ii regions would help to determine whether our hypotheses are widely applicable

ATOMS : ALMA three-millimeter observations of massive star-forming regions - XII: Fragmentation and multiscale gas kinematics in protoclusters G12.42+0.50 and G19.88-0.53

We present new continuum and molecular line data from the ALMA Three-millimeter Observations of Massive Star-forming regions (ATOMS) survey for the two protoclusters, G12.42+0.50 and G19.88-0.53. The 3 mm continuum maps reveal seven cores in each of the two globally contracting protoclusters. These cores satisfy the radius-mass relation and the surface mass density criteria for high-mass star formation. Similar to their natal clumps, the virial analysis of the cores suggests that they are undergoing gravitational collapse (alpha(vir) < < 2). The clump to core scale fragmentation is investigated and the derived core masses and separations are found to be consistent with thermal Jeans fragmentation. We detect large-scale filamentary structures with velocity gradients and multiple outflows in both regions. Dendrogram analysis of the (HCO+)-C-13 map identities several branch and leaf structures with sizes similar to 0.1 and 0.03 pc, respectively. The supersonic gas motion displayed by the branch structures is in agreement with the Larson power law indicating that the gas kinematics at this spatial scale is driven by turbulence. The transition to transonic/subsonic gas motion is seen to occur at spatial scales of similar to 0.1 pc indicating the dissipation of turbulence. In agreement with this, the leaf structures reveal gas motions that deviate from the slope of Larson's law. From the largescale converging filaments to the collapsing cores, the gas dynamics in G12.42+0.50 and G19.88-0.53 show scale-dependent dominance of turbulence and gravity and the combination of these two driving mechanisms needs to be invoked to explain massive star formation in the protoclusters.Peer reviewe

Dust polarization in OMC-1

We present ALMA Band 7 polarization observations of the OMC-1 region of the Orion molecular cloud. We find that the polarization pattern observed in the region is likely to have been significantly altered by the radiation field of the >104 L⊙ high-mass protostar Orion Source I. In the protostar’s optically thick disc, polarization is likely to arise from dust self-scattering. In material to the south of Source I – previously identified as a region of ‘anomalous’ polarization emission – we observe a polarization geometry concentric around Source I. We demonstrate that Source I’s extreme luminosity may be sufficient to make the radiative precession time-scale shorter than the Larmor time-scale for moderately large grains (⁠>0.005−0.1μm), causing them to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This requires relatively unobscured emission from Source I, supporting the hypothesis that emission in this region arises from the cavity wall of the Source I outflow. This is one of the first times that evidence for k-RAT alignment has been found outside of a protostellar disc or AGB star envelope. Alternatively, the grains may remain aligned by B-RATs and trace gas infall on to the Main Ridge. Elsewhere, we largely find the magnetic field geometry to be radial around the BN/KL explosion centre, consistent with previous observations. However, in the Main Ridge, the magnetic field geometry appears to remain consistent with the larger-scale magnetic field, perhaps indicative of the ability of the dense Ridge to resist disruption by the BN/KL explosion

Direct observational evidence of the multi-scale, dynamical mass accretion toward a high-mass star forming hub-filament system

There is growing evidence that high-mass star formation and hub-filament systems (HFS) are intricately linked. The gas kinematics along the filaments and the forming high-mass star(s) in the central hub are in excellent agreement with the new generation of global hierarchical high-mass star formation models. In this paper, we present an observational investigation of a typical HFS cloud, G310.142+0.758 (G310 hereafter) which reveals unambiguous evidence of mass inflow from the cloud scale via the filaments onto the forming protostar(s) at the hub conforming with the model predictions. Continuum and molecular line data from the ATOMS and MALT90 surveys are used that cover different spatial scales. Three filaments (with total mass $5.7\pm1.1\times 10^3~M_{\odot}$) are identified converging toward the central hub region where several signposts of high-mass star formation have been observed. The hub region contains a massive clump ($1280\pm260~M_{\odot}$) harbouring a central massive core. Additionally, five outflow lobes are associated with the central massive core implying a forming cluster. The observed large-scale, smooth and coherent velocity gradients from the cloud down to the core scale, and the signatures of infall motion seen in the central massive clump and core, clearly unveil a nearly-continuous, multi-scale mass accretion/transfer process at a similar mass infall rate of $\sim 10^{-3}~M_{\odot}~yr^{-1}$ over all scales, feeding the central forming high-mass protostar(s) in the G310 HFS cloud.Comment: Accepted to publish in ApJ. 10 pages with 6 figures and 2 table

ATOMS : ALMA three-millimeter observations of massive star-forming regions - VII. A catalogue of SiO clumps from ACA observations

To understand the nature of SiO emission, we conducted ACA observations of the SiO (2-1) lines toward 146 massive star-forming regions, as part of the ALMA Three-millimeter Observations of Massive Star-forming regions (ATOMS) survey. We detected SiO emission in 128 (87.7 per cent) sources and identified 171 SiO clumps, 105 of which are spatially separated from 3 mm continuum emission. A large amount of the SiO line profiles (60 per cent) are non-Gaussian. The velocity dispersion of the SiO lines ranges from 0.3 to 5.43 km s(-1). In 63 sources the SiO clumps are associated with H II regions characterized by H40 alpha emission. We find that 68 per cent (116) of the SiO clumps are associated with strong outflows. The median velocity dispersion of the SiO line for outflow sources and non-outflow sources is 1.91 km s(-1) and 0.99 km s(-1), respectively. These results indicate that outflow activities could be connected to strongly shocked gas. The velocity dispersion and [SiO]/[(HCO+)-C-13] intensity ratio do not show any correlation with the dust temperature and particle number density of clumps. We find a positive correlation between the SiO line luminosity and the bolometric luminosity, implying stronger shock activities are associated with more luminous protoclusters. The SiO clumps in associations with HII regions were found to show a steeper feature in Lsio/Lbol. The SiO line luminosity and the fraction of shocked gas have no apparent evidence of correlation with the evolutionary stages traced by luminosity to mass ratio (L-bol/M).Peer reviewe

The JCMT BISTRO Survey: multiwavelength polarimetry of bright regions in NGC 2071 in the far-infrared/submillimetre range, with POL-2 and HAWC+

Polarized dust emission is a key tracer in the study of interstellar medium and of star formation. The observed polarization, however, is a product of magnetic field structure, dust grain properties, and grain alignment efficiency, as well as their variations in the line of sight, making it difficult to interpret polarization unambiguously. The comparison of polarimetry at multiple wavelengths is a possible way of mitigating this problem. We use data from HAWC+ /SOFIA and from SCUBA-2/POL-2 (from the BISTRO survey) to analyse the NGC 2071 molecular cloud at 154, 214, and 850 μm. The polarization angle changes significantly with wavelength over part of NGC 2071, suggesting a change in magnetic field morphology on the line of sight as each wavelength best traces different dust populations. Other possible explanations are the existence of more than one polarization mechanism in the cloud or scattering from very large grains. The observed change of polarization fraction with wavelength, and the 214-to-154 μm polarization ratio in particular, are difficult to reproduce with current dust models under the assumption of uniform alignment efficiency. We also show that the standard procedure of using monochromatic intensity as a proxy for column density may produce spurious results at HAWC+wavelengths. Using both long-wavelength (POL-2, 850 μm) and short-wavelength (HAWC+, ≲200μm) polarimetry is key in obtaining these results. This study clearly shows the importance of multi-wavelength polarimetry at submillimetre bands to understand the dust properties of molecular clouds and the relationship between magnetic field and star formation