A Survey of Magnetic Field Properties in Bok Globules

TL;DR

This study provides a comprehensive statistical analysis of magnetic field properties in Bok globules using optical polarimetry, revealing their magnetic fields are dynamically significant and structures are oriented at various angles relative to the magnetic field.

Contribution

It introduces the first broad statistical survey of magnetic fields in Bok globules, combining optical polarimetry with Gaia data to analyze magnetic orientation and strength.

Findings

Magnetic fields in Bok globules are statistically significant and dynamically important.

Structures show a bimodal orientation distribution relative to magnetic fields.

Magnetic field strengths are consistent with a dynamically important regime.

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 Alfv\'en 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.