Magnetic Fields in Massive Star-forming Regions (MagMaR). VI. Magnetic Field Dragging in the Filamentary High-mass Star-forming Region G35.20--0.74N due to Gravity

TL;DR

This study addresses how magnetic fields influence filament formation and core fragmentation in a massive star-forming region by combining high-resolution ALMA dust-polarization data with gas kinematics from H$^{13}$CO$^+$. Using the Davis–Chandrasekhar–Fermi framework, the authors map magnetic-field strengths from $0.2$ to $4.4$ mG (mean $0.8\pm0.4$ mG) and mass-to-flux ratios from $0.1$ to $6.0$ (mean $1.1\pm0.8$), revealing subcritical envelopes and locally supercritical cores. A large-scale, nearly perpendicular magnetic field seen by SOFIA contrasts with the ALMA-scale field aligned along the filament, suggesting gravity-driven dragging of field lines as gas flows along the filament move cores together. The observed core spacings ($0.02$--$0.04$ pc) are smaller than the classical isothermal-cylinder prediction ($0.06$ pc) but consistent with fragmentation modified by magnetic effects and subsequent core migration on timescales of a few $\times10^{4}$ yr, highlighting a dynamic interplay between gravity, magnetic fields, and gas flow in high-mass star formation.

Abstract

We investigate the magnetic field orientation and strength in the massive star-forming region G35.20-0.74N (G35), using polarized dust emission data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the Magnetic fields in Massive star-forming Regions (MagMaR) survey. The G35 region shows a filamentary structure (a length of $\sim$0.1 pc) with six bright cores located along the filament's long axis. Magnetic field strengths across the G35 region range from 0.2 to 4.4 mG with a mean value of 0.8 $\pm$ 0.4 mG. The mass-to-flux ratio ($λ$) varies from 0.1 to 6.0 the critical value. The highest values are found locally around cores, whereas the remains of the filament are subcritical. A H$^{13}$CO$^+$ (3--2) velocity gradient of 29 km s$^{-1}$ pc$^{-1}$ is evident along the filament's long axis, aligned with the magnetic field direction. At larger scales ($\sim$0.1 pc), the magnetic field lines appear roughly perpendicular to the filament's long axis, in contrast to the smaller-scale structure ($\sim$0.003 pc) traced by ALMA. The magnetic field lines could be dragged along the filament as a result of the gas motion induced by the gravitational potential of the filament. Six cores in the filament have similar spacings between 0.02--0.04 pc. The initial filament fragmentation could have produced a core spacing of 0.06 pc, following filament fragmentation theory, and the current core spacing is the result of cores comoving with the gas along the filament. This core migration could occur in a few 10$^4$ years, consistent with high-mass star formation time scales.

Figures (10)

• Figure 1: Map of the magnetic field orientation obtained from dust polarization observations using ALMA in G35. The background image is the intensity (Stokes $I$) at Band 6 frequency, $\sim250$ GHz. The white segments show the magnetic field orientatio,n which is the polarization angle rotated by 90 degrees, and is plotted roughly per independent beam. The triangles are the positions of cores found in previous ALMA dust continuum observations with a higher angular resolution dthan ours Zhang2022 The core names are labeled based on previous studies Zhang2022Sanchez2013. The physical scale and beam size are shown in the bottom left and right corners, respectively.
• Figure 2: (Left) The skeleton of the filament in G35. Background image is the same as in Figure \ref{['fig:magori']}. White contours show the flux density of G35 at 3, 10, 20, 30, and 40 $\times$$\delta I$ levels. (Right) Angle difference between the main skeleton and magnetic field orientations.
• Figure 3: The maps of polarization angle dispersion (Left), volume density (Middle), and non-thermal velocity dispersion of the non-thermal component of H$^{13}$CO$^+$ (Right) in G35. Black contours show the flux density of G35 at 10, 20, 30, and 40 $\times$$\delta I$ levels. The region within 10 $\times$$\delta I$ level is comparable to the filament with the width of 0.015 estimated in Section \ref{['sec:vol']} along the black skeleton shown in Figure \ref{['fig:fila']}.
• Figure 4: The map of magnetic field strength in G35. Black contours are the same as shown in Figure \ref{['fig:poldisp']}. Blue segments represent magnetic field orientations obtained using ALMA, which are the same as those shown in Figure \ref{['fig:magori']}.
• Figure 5: (Left) The map of mass-to-flux ratio in G35. Black contours are the same as shown in Figure \ref{['fig:poldisp']}. The green triangles are the same as shown in Figure \ref{['fig:magori']}. (Right) Mean mass-to-flux ratio, $\overline{\lambda}$, as function of number density cut. The mean mass-to-flux ratio is calculated using mean column density and magnetic field strength within the number density cut. When $\overline{\lambda}$ is 1, the number density is 1.2 $\times$ 10$^6$ cm$^{-3}$, which is shown as the magenta contour in the left panel.