Tao-Chung Ching

Magnetic fields play a fundamental role in the formation of protostellar winds. In the magneto-centrifugal models, poloidal magnetic fields launch winds from accretion disks, and fast-rotating gas twists the fields into toroidal geometry that collimates and accelerates winds through magnetic hoop stress. However, toroidal fields in protostellar winds remain observationally unresolved. Here we report polarization observations of carbon monoxide emission toward the NGC1333 IRAS 4A protostellar outflow. The inferred magnetic fields are perpendicular to the outflow axis and aligned with the rotational structure of the outflow, indicating toroidal fields with strengths of a few milligauss, sufficient to collimate and accelerate the outflow at several hundred astronomical units from the protostar. A linear correlation is found between the curl of plane-of-the-sky magnetic field and the line-of-sight electric current density. Our analysis provides better constraints on ion-electron drift velocity in protostellar outflows and supports rotating outflows driven by the magneto-centrifugal mechanism.

For over a century, the origin of low-energy cosmic rays (LECRs), the dominant heaters and ionizers of dense interstellar gas, has remained elusive owing to solar modulation and uncertain transport processes. In this study, we introduce a new astrophysical approach based on H i narrow self-absorption (HINSA) to obtain spatially resolved measurements of LECR ionization rates using high-fidelity H i observations toward the Orion region from the FAST telescope. The LECR ionization rate is found to scale with local star formation rate as log 10 ζ = ( 1.4 ± 0.70 ) log 10 SFR + ( − 10.5 ± 2.9 ) . Moreover, it increases with visual extinction, and is found to exceed, toward active star-forming regions, the value predicted for diffuse regions based on Voyager measurements and an external propagation model. These findings demonstrate that LECRs are generated in situ by star-forming activities rather than penetrating from the broader Galactic cosmic-ray population. This is further supported by Fermi-LAT gamma-ray observations toward the Orion region. Together, these results resolve a key uncertainty in cosmic-ray origin and establish a new avenue for quantifying the energetic feedback that regulates the interstellar medium.

We present the Atacama Large Millimeter/submillimeter Array observations of linearly polarized 870 μ m continuum emission at ∼0 . ″ 2 (2000 au) resolutions toward the two massive clumps, Clump 1 and Clump 4, in the 20 km s ⁻¹ cloud. The derived magnetic field strength for both clumps ranges from ∼0.3 to 3.1 mG using the angular dispersion function method. The magnetic field orientations across multiple scales suggest that the magnetic field dominates at the cloud scale, whereas gravity appears to govern structures at the core (0.01−0.1 pc) and condensation (≤0.01 pc) scales. Furthermore, the study on the angular difference between the orientations of the local gravity gradient and the magnetic field suggests that the magnetic field predominantly governs the dynamics in the diffuse regions, while gravity and star formation feedback become increasingly significant within the dense regions. The ratio of the magnetic field tension force F _B to the gravitational force F _G suggests that the magnetic field may provide some support against gravity, but it is insufficient to prevent gas from infalling toward the dense cores.

We present 850 μ m polarized continuum observations carried out using SCUBA-2/POL-2 on the James Clerk Maxwell Telescope toward a reflection nebula, NGC 7023. The nebula is powered by the Herbig Ae Be star HD 200775 and also forms a hub in the hub-filament cloud, LDN 1172/1174. We detect submillimeter emission to the north and east of the location of the central star. The magnetic field (B-field) morphology is found to be curved and follows the clump morphology. The comparison of the B-field morphology at the clump scales (∼0.02 pc) derived using POL-2 data with that of the envelope scale (∼0.5 pc) derived using archival Planck/optical polarimetric observations suggests that the field lines are not preserved from envelope to clump scales. We suggest that this reordering of the magnetic field lines could be due to the interaction with the already evolved high-velocity outflow gas around the central star, which hints at the presence of outflow feedback. We estimated a B-field strength of 179 ± 50 μ G in the starless core, and 121 ± 34 μ G and 150 ± 42 μ G in the protostellar cores, using the N ₂ H ⁺ (1–0) line observed with the 13.7 m single-dish telescope at the Taeduk Radio Astronomy Observatory (TRAO). The stability analysis gives magnetically subcritical values, while the magnetic, gravitational, and outflow kinetic energies are roughly balanced. Our study unveils the first possible evidence for magnetic field lines being shaped by outflow feedback in the vicinity of a photodissociation region driven by an intermediate-mass, pre-main-sequence star.

We present polarization maps of dust emission at 340 GHz in the luminous high-mass protocluster W3 IRS5, observed with the Submillimeter Array. The projected magnetic fields appear fairly organized, with a pinched morphology in the northern part and a concave shape in the southern part. We fit the polarization maps with a two-component magnetic field model: an hourglass model centered at the continuum peak, SMM2, and an empirical sphere centered at the O-type star, IRS7. Using the Davis–Chandrasekhar–Fermi method, we calculate a projected field strength of B _pos = 1.4 mG. Along with the Zeeman measurement, a total magnetic field strength of B _tot = 1.6 mG is obtained. We find that gravitational energy is the most dominant, followed by magnetic energy and then turbulent energy. Small values of the virial parameter, α _vir = 0.8, and the ratio of timescales, t _ff/t _cross = 0.6, suggest an ongoing collapse. We also show collimated molecular outflows in the CO (3−2) and SiO (8−7) transitions. The morphology of the magnetic fields and the surrounding H ii regions put forward a scenario for W3 IRS5. A gravitationally unstable dense core formed within a neutral gas ridge plowed by the expansions of W3 A and W3 B. The core began to contract, causing gravity to pull the magnetic field lines inward, which resulted in a pinched field morphology. The subsequent expansion of W3 F, ionized by IRS7, perturbed the magnetic field, creating the concave patterns. The dynamical interactions among protostars led to misalignment of their outflows.

We present a joint analysis of H I absorption Zeeman measurements and the morphology of filamentary H I emission to investigate the 3D structure of the magnetic field in the diffuse neutral interstellar medium. Our analysis is based on the Arecibo Millennium Survey and new data from the Five-hundred-meter Aperture Spherical radio Telescope toward radio sources 3C 75, 3C 207, and 3C 409. Toward 3C 409, we make a 4σ Zeeman detection and infer B _LOS = 9.1 ± 1.9 μG, in agreement with Arecibo results. We quantify the dispersion of H I filaments at the locations and velocities of Zeeman components using GALFA-H I narrow-channel emission maps. Focusing on a subsample of 42 spectrally distinct components, we find a weak but statistically significant positive correlation (Spearman ρ = 0.3, p = 0.01) between ∣B _LOS∣ and the circular variance of H I filament orientation angles. To examine its origin, we characterize the environments probed by H I absorption using dust emission, 3D dust maps, OH absorption, and CO emission. We find evidence that existing H I absorption Zeeman measurements trace magnetic fields that are coherent on parsec scales, probe primarily local gas (100–500 pc, often at distances consistent with the Local Bubble wall), and exhibit systematic differences in the magnitude of B _LOS. We attribute the correlation between Zeeman measurements and filamentary H I morphology to large-scale variations in magnetic field strength and/or inclination angle across different Galactic environments, which could arise due to the Local Bubble geometry or enhanced total field strength in denser regions.

The magnetic field is integral to our understanding of the formation and dynamical evolution of molecular clouds and star formation within. We present a polarimetric survey of 17 massive protostellar cluster forming clumps, covered in 34 pointings in the 230 GHz window using the Atacama Large Millimeter/submillimeter Array. The two array configurations, C43-1 and C43-4, probe linearly polarized dust emission, hence the plane-of-the-sky orientation of magnetic fields, at resolutions of 1″ and 0 . ″ 4 that correspond to approximately 0.01 pc core and 10³ au envelope scales, respectively. The relative orientations (ROs) of the magnetic field probed at two spatial scales are analyzed for the entire protostellar cluster sample and for a subset of objects in NGC 6334. We found a bimodal distribution of ROs with peaks at 0° (parallel) and 90° (orthogonal) for the entire sample combined, as well as for NGC 6334. We investigate the physical origin of this bimodal distribution through a projected Rayleigh statistical analysis in relation to column densities and local gravity in NGC 6334. We found an excess of parallel magnetic fields at column densities >10²³ cm⁻². The underlying cause of the RO distribution of the magnetic field is gravitational collapse at higher gas densities, which drags and reorients the magnetic field as shown in the alignment between the magnetic field and the direction of gravitational forces. The distribution of ROs observed here is consistent with the evolution of ROs of an initially sub-Alvénic cloud that becomes magnetically supercritical and super-Alvénic as the cloud collapses to form stars.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has been fully operational since 2020 January 11. We present a comprehensive analysis of the beam structure for each of the 19 feed horns on FAST's L-band receiver across the Stokes I, Q, U, and V parameters. Using an on-the-fly mapping pattern, we conducted simultaneous sky mapping using all 19 beams directed toward polarization calibrators J1407+2827 and J0854+2006 from 2020 to 2022. Electromagnetic simulations were also performed to model the telescope's beam patterns in all Stokes parameters. Our findings reveal a symmetrical Gaussian pattern in the Stokes I parameter of the central beam without strong sidelobes, while the off-center beams exhibit significant asymmetrical shapes that can be fitted using a combination of log-normal and Gaussian distributions. The inner beams have higher relative beam efficiencies and smaller beam sizes compared to those of the outer beams. The sidelobes of the inner beams contribute approximately 2% of the total flux in the main lobe, increasing to 5% for outer beams, with a peak at 6.8%. In Stokes U, a distinct four-lobed cloverleaf beam squash structure is observed, with similar intensity levels in both inner and outer beams. In Stokes V, a two-lobed beam squint structure is observed in the central beam, along with a secondary eight-lobed structure. The highest squint peak in Stokes V is about 0.3% of the Stokes I in the outer beams. These results align closely with the simulations, providing valuable insights for the design of radio multibeam observations.