ALMA resolves the hourglass magnetic field in G31.41+0.31

Abstract: Context. Submillimeter Array (SMA) 870 micron polarization observations of the hot molecular core G31.41+0.31 revealed one of the clearest examples up to date of an hourglass-shaped magnetic field morphology in a high-mass star-forming region. Aims. To better establish the role that the magnetic field plays in the collapse of G31.41+0.31, we carried out Atacama Large Millimeter/submillimeter Array (ALMA) observations of the polarized dust continuum emission at 1.3 mm with an angular resolution four times higher than that of the previous (sub)millimeter observations to achieve an unprecedented image of the magnetic field morphology. Methods. We used ALMA to perform full polarization observations at 233 GHz (Band 6). The resulting synthesized beam is 0.28"x0"20 which, at the distance of the source, corresponds to a spatial resolution of ~875 au. Results. The observations resolve the structure of the magnetic field in G31.41+0.31 and allow us to study the field in detail. The polarized emission in the Main core of G31.41+0.41is successfully fit with a semi-analytical magnetostatic model of a toroid supported by magnetic fields. The best fit model suggests that the magnetic field is well represented by a poloidal field with a possible contribution of a toroidal component of ~10% of the poloidal component, oriented southeast to northwest at ~ -44 deg and with an inclination of ~-45 degr. The magnetic field is oriented perpendicular to the northeast to southwest velocity gradient detected in this core on scales from 1E3-1E4 au. This supports the hypothesis that the velocity gradient is due to rotation and suggests that such a rotation has little effect on the magnetic field. The strength of the magnetic field estimated in the central region of the core with the Davis-Chandrasekhar-Fermi method is ~8-13 mG and implies that the mass-to-flux ratio in this region is slightly supercritical ...

• The paper leverages ALMA's 1.3 mm data to resolve the hourglass magnetic field with an angular resolution down to approximately 875 au.
• It employs a semi-analytical magnetostatic model to reveal a dominant poloidal field, with magnetic strength measured between 8 and 13 mG and a slightly supercritical core.
• The findings underscore the vital role of magnetic fields in regulating high-mass star formation processes, even amidst core fragmentation.

Analysis of Magnetic Field Morphology in G31.41+0.31 Using ALMA Data

The authors of the paper have conducted a detailed study of the magnetic field morphology in the high-mass star-forming region G31.41+0.31 utilizing high-resolution ALMA observations. This paper builds on previous work done with the SMA, where an hourglass-shaped magnetic field configuration was identified, a hallmark indicative of magnetically dominated star-forming environments.

Key Findings

1. Enhanced Resolution and Observations:
   • The ALMA observations at 1.3 mm achieve an angular resolution approximately four times better than prior (sub)millimeter observations, allowing for a more granular analysis of the magnetic field configuration in G31.41+0.31, specifically down to scales of about 875 au.
2. Magnetic Field Configuration:
   • The study validates the hourglass-shaped magnetic field morphology, a characteristic structure formed when an initially uniform magnetic field is distorted by gravitational collapse, found in the earlier SMA observations.
   • The observational data were well fitted by a semi-analytical magnetostatic model of a toroid supported by a poloidal magnetic field, with a minor toroidal contribution. The magnetic field lines show a notable orientation perpendicular to the detected NE-SW velocity gradient attributable to core rotation, yet the rotation seems to exert minimal disruption to the magnetic field configuration.
3. Field Strength and Mass-to-Flux Ratio:
   • The strength of the magnetic field in the central region of the core was assessed through the Davis-Chandrasekhar-Fermi method, yielding values between approximately 8 and 13 mG. These measurements imply that the core in this region is slightly supercritical with mass-to-flux ratios in the range of 1.4 to 2.2, suggesting the field is strong yet insufficient to prevent fragmentation and collapse.
4. Comparison with Low-Mass Star-Forming Regions:
   • The magnetic field morphology in G31.41+0.31 shares similarities with the well-studied low-mass star-forming region NGC 1333 IRAS 4A, both displaying an hourglass configuration. However, key differences in size, mass, and luminosity highlight the nuances between high-mass and low-mass star formation.

Implications and Future Directions

• Magnetic Field's Role in Star Formation:

The findings suggest a predominant role for magnetic fields in regulating the formation and collapse dynamics within high-mass star-forming regions. The poloidal nature of the field, minimally affected by core rotation, provides insights into the physical conditions conducive to formation.

• Support for Magnetic Regulation Hypotheses:

These results contribute to ongoing discourse regarding the dominance of magnetic fields in the star-formation process as opposed to models proposing turbulence as a primary driving mechanism, particularly extending insights to high-mass cores.

• Fragmentation and Composition of Core Structures:

Despite the strong magnetic field, the presence of embedded sources indicates active fragmentation, aligning with observations that imply a complex interplay between gravitational forces and magnetic fields. This necessitates further investigation into the initial conditions and evolution pathways that lead to such fragmentation in both high-mass and low-mass environments.

Future research might explore the implications on multiple scales of star formation, potentially leveraging more sophisticated modeling techniques that consider both magnetohydrodynamic effects and the impacts of environmental conditions, such as turbulence and radiation, on observed polarization patterns and structure formation. Continued refinement in observational techniques and model robustness can be anticipated to crucially inform our understanding of stellar and planetary formation processes.