The Hourglass-shaped Magnetic Fields and Dust Filaments in the HH 211 Protostellar Envelope

TL;DR

This study uses ALMA Band 3/6 polarization and JCMT 850 μm data to map magnetic fields and dust filaments in the HH 211 protostellar envelope, linking large-scale core magnetic fields to inner-envelope dynamics. Band 3 reveals three ~4000 au filaments, two of which align with core-scale magnetic fields, while Band 6 unveils an hourglass-shaped field near the protostar and toroidal components along the outflow, signaling field dragging by infall and rotation. Temperature and column-density analyses show east–west and south–west asymmetries, with the southern dense region potentially tracing an infalling stream in the disk midplane; CMU ballistic infall modeling supports this interpretation, suggesting magnetic fields guide or are dragged by accretion flows. The inferred mass infall along the southern filament (~(0.32–1.22)×10⁻⁶ M⊙ yr⁻¹) could represent a substantial portion of the central accretion rate ((0.72–4.30)×10⁻⁶ M⊙ yr⁻¹), indicating magnetic fields and filamentary accretion play a crucial role in early protostellar evolution and require non-ideal MHD considerations to explain scale-dependent flux evolution.

Abstract

Magnetic fields influence the structure and evolution of protostellar systems, thus understanding their role is essential for probing the earliest stages of star formation. We present ALMA Band 3 and 6 polarized continuum observations at $\sim$0.5$^{\prime \prime}$ resolution toward the Class 0 protostellar system HH 211. Three dust filaments ($\sim$4000 au in length) are found in the HH 211 protostellar envelope, two of which are aligned with core-scale ($\sim$10,000 au) magnetic fields detected by previous JCMT observations. This result suggests that the formation of the dust filaments may be influenced by magnetic fields. In the inner envelope ($\sim$1000 au), we detect a clear hourglass-shaped magnetic field morphology near the protostar and toroidal fields along the outflow directions. We also estimate the line-of-sight-averaged temperature and column density distributions in the inner envelope and find that the temperature is higher in the east, while the column density is enhanced in the southern and western regions. The southern dense regions of the inner envelope may trace either outflow cavity walls, due to their alignment with the outflow, or possible infalling channels in the midplane, given the close correspondence between the observed magnetic fields and the predicted infall trajectories.

Figures (18)

• Figure 1: Dust continuum around the HH 211 in ALMA Band 3 (97.5 GHz) with a robust parameter of 2.0. The synthesized beam and spatial scale bar are shown in the bottom left and right corners, respectively. The central coordinates are RA = 03$^{\mathrm{h}}$43$^{\mathrm{m}}$56.800$^{\mathrm{s}}$, Dec = +32$^\circ$00$'$50.00$"$.
• Figure 2: Left: The core-scale ($\sim$10,000 au) magnetic field orientations around the HH 211 observed with the JCMT. The greyscale shows the 850 $\mu$m intensity, with contour levels at 0.09, 0.3, and 0.8 Jy beam$^{-1}$. The beam size of the JCMT observation is shown in the bottom left of the panel. The cross marker indicates the position of the central protostar Lee_2018bLee_2023. Right: A zoomed-in view of the left panel overlaid with ALMA Band 3 observations. The green lines represent 0.025 mJy beam$^{-1}$ and 0.08 mJy beam$^{-1}$ levels in the ALMA Band 3 observations with a robust parameter of 2.0. Spatial scale bars are shown in the top right corner of both panels.
• Figure 3: Dust continuum around the HH 211 in Band 3 (Left) and Band 6 (Right) with a robust parameter of 0.5. The contour levels are 5, 10, 20, 50, 100, 200, and 500 times the rms noise levels, where the rms noises are 0.009 mJy beam$^{-1}$ in Band 3 and 0.045 mJy beam$^{-1}$ in Band 6. The synthesized beam sizes are shown in the bottom left of both panels. The red and blue arrows indicate the red-shifted and blue-shifted jet directions, respectively McCaughrean_1994. Spatial scale bars are shown in the top right corner of both panels.
• Figure 4: Left: Inferred magnetic field orientations around the HH 211 from Band 3 polarized continuum with a robust parameter of 2.0. The line segments represent plane-of-sky magnetic field directions, with green for $PI > 2\sigma_{PI}$, and red for $1.5\sigma_{PI}< PI < 2\sigma_{PI}$. The black lines are the Band 3 dust continuum with a robust parameter of 2, showing 3, 10, 20, 50, 100, 200, and 500 times the rms noise level. Right: Inferred magnetic field orientations around HH 211 from Band 6 polarized continuum with a robust parameter of 0.5. Green segments represent plane-of-sky magnetic field directions with $PI > 3\sigma_{PI}$. The blue and red contours indicate CO (2-1) integrated intensities in velocity ranges from 0 to 8 and from 10 to 18 km s$^{-1}$, respectively Jhan_2021. The blue and red contour levels are 0.03, 0.035, 0.05, 0.1, and 0.2 Jy beam$^{-1}$ km s$^{-1}$. The black lines are the Band 6 dust continuum with a robust parameter of 0.5, showing 5, 10, 20, 50, 100, 200, and 500 times the rms noise level. The synthesized beam size and spatial scale bar are shown in the bottom left and right of both panels, respectively.
• Figure 5: Polarized intensity around the HH 211 envelope in Band 3 with a robust parameter of 2.0 (Left) and in Band 6 with a robust parameter of 0.5 (Right). The black lines represent Stokes $I$ intensity levels with 3 (Left) / 5 (Right), 10, 20, 50, 100, 200, and 500 times the rms noise level in each observation. The synthesized beam size and spatial scale bar are shown in the bottom left and right of both panels, respectively.