Magnetic Fields in Massive Star-Forming Regions (MagMaR) II. Tomography Through Dust and Molecular Line Polarization in NGC 6334I(N)

Abstract

Here, we report ALMA detections of polarized emission from dust, CS(J = 5 → 4), and C 33 S(J = 5 → 4) toward the high-mass star-forming region NGC6334I(N). A clear "hourglass" magnetic field morphology was inferred from the polarized dust emission which is also directly seen from the polarized CS emission across velocity, where the polarization appears to be parallel to the field. By considering previous findings, the field retains a pinched shape which can be traced to clump length-scales from the envelope scales traced by ALMA, suggesting that the field is dynamically important across multiple length-scales in this region. The CS total intensity emission is found to be optically thick (τ CS = 32 ± 12) while the C 33 S emission appears to be optically thin (τ C 33 S = 0.1 ± 0.01). This suggests that sources of anisotropy other than large velocity gradients, i.e. anisotropies in the radiation field are required to explain the polarized emission from CS seen by ALMA. By using four variants of the Davis-Chandrasekhar-Fermi technique and the angle dispersion function methods (ADF), we obtain an average of estimates for the magnetic field strength onto the plane of the sky of B pos = 16 mG from the dust and B pos ∼ 2 mG from the CS emission, where each emission traces different molecular hydrogen number densities. This effectively enables a tomographic view of the magnetic field within a single ALMA observation.

Magnetic fields in star formation: from clouds to cores

“…The GK effect, which can provide a velocity-resolved probe of magnetic field morphology, has been observed in outflows (e.g. Girart et al 1999;Ching et al 2017), and in the high-mass star-forming region NGC6334I(N) using ALMA (Cortes et al 2021a). However, the uncertainty on polarization direction complicates interpretation.…”

“…ALMA's ability to measure both line and continuum polarization in a single spectral set-up will be key to understanding under what conditions the GK effect produces parallel or perpendicular alignment (e.g. Cortes et al 2021a). A further complication is conversion of linear to circular line polarization by anisotropic resonant scattering by foreground material (Houde et al 2013;Chamma et al 2018).…”

“…How the magnetic field geometry of a dense core changes under external feedback may be important to core evolution in massive star-forming regions. There is limited evidence that linear fields are maintained in some cores under feedback (Cortes et al 2021b;Pattle et al 2021b;Könyves et al 2021), but more observations are needed. The toroidal magnetic field geometry is suggestive of dynamics dominated by gravitational collapse and rotation.…”

The Magnetic Field in the Milky Way Filamentary Bone G47

“…Moreover, the B-fields may provide some support against collapse. One much smaller scales (1000 -10000 au), YSOs themselves can have diverse magnetic field morphologies such as spiral-like, hourglass, and radial (e.g., as seen in the MAGMAR survey; Cortes et al 2021;Fernández-López et al 2021;Sanhueza et al 2021). Focusing on the large-scale observations of filaments, it is important to establish if fields are universally perpendicular to the spines of the main filament and whether the B-field strength is sufficient to help support the filament from collapse.…”