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Unveiling an Hourglass-Shaped Magnetic Field toward IRDC G351.77-0.53

O. R. Jadhav, L. K. Dewangan, I. I. Zinchenko, Thushara G. S. Pillai, Patricio Sanhueza, A. K. Maity, Ram K. Yadav, Saurabh Sharma

Abstract

We present the SOFIA/HAWC+ 214 $μ$m polarimetric observations toward the infrared dark cloud G351.77-0.53 (hereafter G351), complemented by existing multi-wavelength data sets. Infrared excess from the embedded sources indicate ongoing star formation activity in the cloud. The G351 cloud hosts two prominent star-forming clumps, i.e., c1 and c2. The plane-of-the-sky magnetic field lines from Planck observations are predominantly oriented perpendicular to the filament's major axis. Magnetic field orientations from SOFIA/HAWC+ 214 $μ$m observations reveal distinct hourglass-shaped field configuration toward c1, while the field lines remain perpendicular to the rest of the filament. Using the Davis-Chandrasekhar-Fermi method, we estimate a mean plane-of-the-sky magnetic field strength of $\sim$147 $\pm$ 60 $μ$G in the G351 filament, with values reaching $\sim$0.8 mG toward c1. The mass-to-flux ratio analysis indicates that the filament is magnetically transcritical, where the gravitational and magnetic field energies are comparable. The hourglass-shaped magnetic field observed toward c1 could result from magnetically regulated gravitational collapse, the alignment of converging sub-filaments with the magnetic field, or a combination of both processes. The energy budget analysis further indicates that magnetic fields play an important role in governing the cloud's gas dynamics, followed by contributions from turbulence and gravity.

Unveiling an Hourglass-Shaped Magnetic Field toward IRDC G351.77-0.53

Abstract

We present the SOFIA/HAWC+ 214 m polarimetric observations toward the infrared dark cloud G351.77-0.53 (hereafter G351), complemented by existing multi-wavelength data sets. Infrared excess from the embedded sources indicate ongoing star formation activity in the cloud. The G351 cloud hosts two prominent star-forming clumps, i.e., c1 and c2. The plane-of-the-sky magnetic field lines from Planck observations are predominantly oriented perpendicular to the filament's major axis. Magnetic field orientations from SOFIA/HAWC+ 214 m observations reveal distinct hourglass-shaped field configuration toward c1, while the field lines remain perpendicular to the rest of the filament. Using the Davis-Chandrasekhar-Fermi method, we estimate a mean plane-of-the-sky magnetic field strength of 147 60 G in the G351 filament, with values reaching 0.8 mG toward c1. The mass-to-flux ratio analysis indicates that the filament is magnetically transcritical, where the gravitational and magnetic field energies are comparable. The hourglass-shaped magnetic field observed toward c1 could result from magnetically regulated gravitational collapse, the alignment of converging sub-filaments with the magnetic field, or a combination of both processes. The energy budget analysis further indicates that magnetic fields play an important role in governing the cloud's gas dynamics, followed by contributions from turbulence and gravity.
Paper Structure (24 sections, 14 equations, 6 figures)

This paper contains 24 sections, 14 equations, 6 figures.

Table of Contents

  1. Introduction
  2. Datasets used
  3. NIR and MIR data
  4. FIR data
  5. Sub-millimeter data
  6. Millimeter data
  7. Radio data
  8. Results
  9. Physical environment toward the G351 IRDC
  10. POS B-field orientations from the SOFIA/HAWC+ and Planck
  11. Molecular gas kinematics
  12. Moment maps and PV diagram
  13. Mass estimation using C$^{18}$O(2--1)
  14. Estimation of magnetic field strength
  15. Volume density
  16. ...and 9 more sections

Figures (6)

  • Figure 1: a) unWISE 12.0 $\mu$m image of the G351 cloud overlaid with the positions of YSOs (red circles) identified using [5.8]-[4.5] vs [3.6]-[4.5] CCD (see text for more details). The white dashed box highlights the area shown in Figure \ref{['fg3']}a-- \ref{['fg3']}d. b) An overlay of SMGPS 1.3 GHz continuum emission contours (in blue) on ATLASGAL 870 $\mu$m contours (in red). The red contour levels are 0.14, 0.24, 0.38, 0.48, 0.94, 1.41, 2.35, 3.30, 4.71, 9.42, 14.12, 18.83, 23.54, 37.66, and 42.37 mJy beam$^{-1}$. The blue contour levels are 0.44, 0.88, 2.6, 4.4, 6.2, 8.8, 18, 26, 35, 44, 70, and 79 mJy beam$^{-1}$. c) Herschel 250 $\mu$m image of the G351 cloud. d) Spitzer 4.5 $\mu$m/3.6 $\mu$m ratio map of the G351 cloud overlaid with the 870 $\mu$m emission contour level at 0.14 mJy beam $^{-1}$. The cyan arrows are used to highlight the bright regions where star formation activity is prominent. The diamonds show the positions of ATLASGAL clumps. The scale bar corresponds to 3 pc at a distance of 2.0 kpc.
  • Figure 2: a) SOFIA/HAWC+ 214 $\mu$m image of the G351 overlaid with the red segments showing the direction of the B-field derived using the SOFIA/HAWC+ 214 $\mu$m polarization observations. b) The LIC map showing the B-field same as Figure \ref{['fg2']}a, overlaid on the SOFIA/HAWC+ 214 $\mu$m intensity map. The magenta curves highlight the hourglass-shaped B-field. The red and blue arrows show the direction of outflow toward IRAS 17233--3606. The black dashed line shows the direction of velocity gradient Klaassen_2015. c) Same as Figure \ref{['fg2']}a, with length of the segments proportional to the polarization fraction. A reference segment corresponding to a 5 % polarization fraction is shown in the bottom-right corner of the panel. d) Same as Figure \ref{['fg2']}a, overlaid with B-field segments (in green) inferred using the Planck 353 GHz data. The diamonds show the positions of ATLASGAL clumps. The beam size of the SOFIA/HAWC+ 214 $\mu$m observations ($\sim$18.$"$2) is indicated by a filled orange circle. The scale bar corresponds to 2 pc at a distance of 2.0 kpc.
  • Figure 3: a) C$^{18}$O(2--1) moment-0 map of the G351 cloud integrated over a range of [$-$10, 5] km s $^{-1}$. b) C$^{18}$O(2--1) moment-1 map. c) C$^{18}$O(2--1) moment-2 map. d) $N(\rm H_2)$ map of the G351 cloud. The contour level is 1.6 $\times$ 10$^{22}$ cm$^{-2}$. The dot-dashed line show the spine of the filament identified using the RadFil algorithm. The scale bar and symbols are same as Figure \ref{['fg2']}.
  • Figure 4: a) PV diagram along the spine of the filamentary cloud shown in Figure \ref{['fg3']}d. b) The panel presents the structure function ($D^{1/2}_\theta(\ell)$) for the G351 cloud. The red dashed line shows the linear fit of Equation \ref{['eq:quadratic_func']}, while the gray vertical dashed lines mark the lower and upper limits used for the fit.
  • Figure 5: a) B-field strength map of the G351 cloud. b) Mass-to-flux ratio map of the G351 cloud. The scalebar in each panel is same as Figure \ref{['fg2']}.
  • ...and 1 more figures