Constraining the magnetic field properties of Bok globule B335 using SOFIA/HAWC+

TL;DR

This study uses SOFIA/HAWC+ far-infrared polarization observations combined with multi-wavelength data and modeling to analyze the magnetic field structure of Bok globule B335, revealing polarization holes and constraining magnetic field strength.

Contribution

First spatially resolved far-infrared polarization map of B335, combined with multi-wavelength data and modeling, to study magnetic fields and polarization holes in Bok globules.

Findings

Polarization holes occur at far-infrared wavelengths in B335.

Magnetic field strength is approximately 142 microGauss.

Model suggests dust alignment and radiation sources explain outer polarization, but not inner regions.

Abstract

Thanks to their well-defined shape and mostly isolated locations, Bok globules are suitable objects for studying the physics of low-mass star formation. To study the magnetic field of the prototypical Bok globule B335, we obtained a spatially resolved polarization map with SOFIA/HAWC+ at a wavelength of 214$\,\mu$m. For the first time, these observations reveal that polarization holes in Bok globules, that is, the decrease in polarization degree towards their dense centers, also occur in the far-infrared wavelength regime. The observed polarization pattern is uniform with a mean polarization angle of 48$^\circ\pm $26$^\circ$ and a magnetic field strength of $\sim$ 142$\,\mu$G. Moreover, we use complementary polarimetic data for B335 obtained at near-infrared to millimeter wavelengths to analyze and constrain the magnetic field across different scales. By applying the 3D Monte-Carlo radiative transfer code POLARIS (Reissl et al. 2016), we developed a model for the density and magnetic field structure as well as for the dust properties of this globule. We conclude that the column density towards the center of B335 is too low to cause the observed polarization hole in B335 via dichroic absorption (Brauer et al. 2016). Furthermore, we conclude that the effect of self-scattering has no significant impact on the observed polarization. Adopting dust-grain alignment via the radiative torque mechanism, a combination of the interstellar radiation field and the central star as radiation sources is consistent with the decrease in polarization degree at the outer regions of B335 ($\approx\,$10$^4\,$au from the core). However, the model fails to explain the low polarization degree within the inner 5000 au.