Magnetic fields in Bok globules: multi-wavelength polarimetry as tracer across large spatial scales

Abstract

Context. The role of magnetic fields in the process of star formation is a matter of continuous debate. Clear observational proof of the general influence of magnetic fields on the early phase of cloud collapse is still pending. In an earlier study on Bok globules with simple structures, we find strong indications of dominant magnetic fields across large spatial scales. Aims. The aim of this study is to test the magnetic field influence across Bok globules with more complex density structures. Methods. We apply near-infrared polarimetry to trace the magnetic field structure on scales of 104–105 au (~0.05–0.5pc) in selected Bok globules. The combination of these measurements with archival data in the optical and sub-mm wavelength range allows us to characterize the magnetic field on scales of 103–106 au (~0.005–5pc). Results. We present polarimetric data in the near-infrared wavelength range for the three Bok globules CB34, CB56, and [OMK2002]18, combined with archival polarimetric data in the optical wavelength range for CB34 and CB56, and in the submillimeter wavelength range for CB34 and [OMK2002]18. We find a strong polarization signal (P ≥ 2%) in the near-infrared for all three globules. For CB34, we detect a connection between the structure on scales of 104–105 au (~0.05–0.5pc) to 105–106 au (~0.5–5pc). For CB56, we trace aligned polarization segments in both the near-infrared and optical data, suggesting a connection of the magnetic field structure across the whole globule. In the case of [OMK2002]18, we find ordered polarization structures on scales of 104–105 au (~0.05–0.5pc). Conclusions. We find strongly aligned polarization segments on large scales which indicate dominant magnetic fields across Bok globules with complex density structures. To reconcile our findings in globules, the lowest mass clouds known, and the results on intermediate (e.g. Taurus) and more massive (e.g. Orion) clouds, we postulate a mass-dependent role of magnetic fields, whereby magnetic fields appear to be dominant on low and high mass but rather subdominant on intermediate mass clouds.