Distortion of Magnetic Fields in the Dense Core CB81 (L1774, Pipe 42) in the Pipe Nebula (1912.10029v1)

Abstract: The detailed magnetic field structure of the starless dense core CB81 (L1774, Pipe 42) in the Pipe Nebula was determined based on near-infrared polarimetric observations of background stars to measure dichroically polarized light produced by magnetically aligned dust grains in the core. The magnetic fields pervading CB81 were mapped using 147 stars and axisymmetrically distorted hourglass-like fields were identified. On the basis of simple 2D and 3D magnetic field modeling, the magnetic inclination angles in the plane-of-sky and line-of-sight directions were determined to be $4^{\circ} \pm 8^{\circ}$ and $20^{\circ} \pm 20^{\circ}$, respectively. The total magnetic field strength of CB81 was found to be $7.2 \pm 2.3$ $\mu{\rm G}$. Taking into account the effects of thermal/turbulent pressure and magnetic fields, the critical mass of CB81 was calculated to be $M_{\rm cr}=4.03 \pm 0.40$ M${\odot}$, which is close to the observed core mass of $M{\rm core}=3.37 \pm 0.51$ M$_{\odot}$. We thus conclude that CB81 is in a condition close to the critical state. In addition, a spatial offset of $92''$ was found between the center of magnetic field geometry and the dust extinction distribution; this offset structure could not have been produced by self-gravity. The data also indicate a linear relationship between polarization and extinction up to $A_V \sim 30$ mag going toward the core center. This result confirms that near-infrared polarization can accurately trace the overall magnetic field structure of the core.

• The paper mapped hourglass-like magnetic fields in molecular core CB81 using near-infrared polarimetric observations of 147 stars to determine field orientations.
• It estimated a magnetic field strength of approximately 7.2 ± 2.3 μG using the Davis-Chandrasekhar-Fermi method, indicating CB81's near-critical state.
• The study identified a 92'' offset between magnetic and mass centers, suggesting external turbulence or shocks influence core formation.

Magnetic Field Distortion in CB81 of the Pipe Nebula

The study of magnetic fields in dense molecular cores is crucial for understanding the processes underlying star formation. The paper entitled "Distortion of Magnetic Fields in the Dense Core CB81 (L1774, Pipe 42) in the Pipe Nebula" by Kandori et al. presents comprehensive observations and analyses of the magnetic field structure within the starless dense core CB81 using near-infrared polarimetric data. The observations reveal important insights into the alignment and strengths of magnetic fields within such environments.

Key Findings and Methodology

The paper's primary goal was to map the detailed magnetic field configuration within CB81, a dense core in the Pipe Nebula. Using polarimetric observations, the research identifies axisymmetrically distorted hourglass-like magnetic fields based on the polarization of light by magnetically aligned dust grains. This was accomplished by an extensive analysis involving 147 stars that provided the necessary data points for mapping.

1. Magnetic Field Structure: The researchers utilized simple 2D and 3D magnetic field models to delineate the shape and orientation of the magnetic fields. The modeling revealed inclination angles of approximately $4^{\circ} \pm 8^{\circ}$ and $20^{\circ} \pm 20^{\circ}$ in the plane-of-sky and line-of-sight directions, respectively.
2. Magnetic Field Strength: Employing the Davis-Chandrasekhar-Fermi method, the total magnetic field strength was estimated at $7.2 \pm 2.3$ $\mu$G for CB81. This measurement places CB81 near a critical state where magnetic and thermal/turbulent pressures are nearly in balance.
3. Core Stability and Mass: An important aspect of this study was determining the critical mass of CB81, which was found to be $M_{\rm cr}=4.03 \pm 0.40$ M$_{\odot}$. This value closely aligns with the observed core mass of $M_{\rm core}=3.37 \pm 0.51$ M$_{\odot}$, indicating that CB81 is close to its critical state, teetering between stability and collapse.
4. Offset Between Magnetic Field and Mass Center: The study identifies a $92''$ spatial offset between the centroid of the magnetic field geometry and the dust extinction mass distribution center. This observed phenomenon suggests external influences, likely turbulence or shocks, affecting the core during its formation.
5. Polarization-Extinction Relationship: The relationship between polarization and extinction reveals a linear correlation up to $A_V \sim 30$ mag, validating that near-infrared polarimetry effectively traces the magnetic field structure throughout the core.

Theoretical Implications and Future Directions

These findings have several implications for the field of astrophysics and star formation theories. The study suggests that even weak magnetic fields in dense cores can maintain alignment, providing crucial insight into the early stages of star core evolution. The presence of hourglass-shaped magnetic fields hints at the influence of magnetic pressure counteracting gravity, an aspect that significantly impacts the initial conditions for star formation.

Moreover, the spatial offset between mass and magnetic field centers suggests the necessity to account for non-uniform initial conditions potentially induced by external shocks or turbulence. Such insights are pivotal in refining theoretical models of molecular cloud collapse and fragmentation.

For future research, enhancing the statistical sample size with similar detailed polarimetric studies across various dense cores could lead to a more robust understanding of magnetic forces in star formation. Additionally, incorporating more sophisticated 3D modeling techniques that account for non-uniform initial conditions would be valuable for dissecting and predicting core dynamics further.

Conclusion

The work by Kandori et al. provides significant progress in understanding the role of magnetic fields in starless dense cores. By uncovering the fine details of magnetic field structures and their implications for core stability and star formation, the study sets a precedent for future astrophysical research in magnetic field characterization and modeling. The theoretical and practical outcomes from this paper are poised to advance the comprehension of cosmic magnetic field processes.