A Survey of Magnetic Field Properties in Bok Globules

TL;DR

This study conducts a uniform optical polarimetric survey of 21 Bok globules to quantify plane-of-sky magnetic fields, their strengths via the Davis-Chandrasekhar-Fermi method, and their relationship to cloud structure and extinction. By integrating Gaia-based structure, 2MASS extinction, and Planck cross-checks, the authors model grain alignment with RAT theory and analyze the relative orientation of clouds and magnetic fields using robust statistics. They find magnetic fields are typically strong, sub-Alfvénic, and sub-critical, indicating magnetic support against large-scale collapse, with a notable bimodal alignment between filaments and fields. The results emphasize magnetic fields as a key regulator of envelope stability and highlight the need for deeper infrared polarimetry to probe denser cores and transitions predicted by Planck studies.

Abstract

Bok globules are small, dense clouds that act as isolated precursors for the formation of single or binary stars. Although recent dust polarization surveys, primarily with Planck, have shown that molecular clouds are strongly magnetized, the significance of magnetic fields in Bok globules has largely been limited to individual case studies, lacking a broader statistical understanding. In this work, we introduce a comprehensive optical polarimetric survey of 21 Bok globules. Using Gaia and near-IR photometric data, we produce extinction maps for each target. Using the radiative torque alignment model customized to the physical properties of the Bok globule, we characterize the polarization efficiency of one representative globule as a function of its visual extinction. We thus find our optical polarimetric data to be a good probe of the globule's magnetic field. Our statistical analysis of the orientation of elongated extinction structures relative to the plane-of-sky magnetic field orientations shows they do not align strictly parallel or perpendicular. Instead, the data is best explained by a bimodal distribution, with structures oriented at projected angles that are either parallel or perpendicular. The plane-of-sky magnetic field strengths on the scales probed by optical polarimetric data are measured using the Davis-Chandrasekhar-Fermi technique. We then derive magnetic properties such as Alfvén Mach numbers and mass-to-magnetic flux ratios. Our findings statistically place the large-scale (Av < 7 mag) magnetic properties of Bok globules in a dynamically important domain.

Figures (19)

• Figure 1: Comparison between maps of BHR 140 obtained from various telescopes. Top left - 2MASS $(H-K)$; Top right - Herschel 250 $\mu$m, Bottom left - raw Gaia star density; Bottom right - smoothed (binned and convolved) Gaia star density.
• Figure 2: An outline of the procedure used to find the dominant orientation for each cloud upto filament extraction (for BHR 75 in this figure, images from left to right): creating Gaia stellar density maps, convolving it to smooth out boundaries into a roughly elliptical shape, masking at a threshold to extract the ellipse, running the fil-finder Python algorithm
• Figure 3: Application of our mass determination method to the Barnard 68 Bok globule. Colorbar denotes the visual extinction, and circular region marks the area used for mass estimation
• Figure 4: Histogram of the ratios of mean extinction $\overline{A_V}$ obtained from using 2MASS colours to Planck emission maps. The distribution is centred at $\sim 1$, with only one cloud lying outside the region where the ratio is $<0.5$ or $>2$ (corresponding to a difference > factor of 2)
• Figure 5: Visual representation of quantifying the error in cloud angle - the red sweep represents the angle from the major axis over which the length of the diameter is $\geq 95\%$ of the major axis length. The angle of the sweep is taken as a measure of the cloud angle error. The image is cropped to the central region where the cloud lies.