Understanding the origin of the magnetic field morphology in the
  wide-binary protostellar system BHR 71
  We present 1.3 mm ALMA observations of polarized dust emission toward the
wide-binary protostellar system BHR 71 IRS1 and IRS2. IRS1 features what
appears to be a natal, hourglass-shaped magnetic field. In contrast, IRS2
exhibits a magnetic field that has been affected by its bipolar outflow. Toward
IRS2, the polarization is confined mainly to the outflow cavity walls. Along
the northern edge of the redshifted outflow cavity of IRS2, the polarized
emission is sandwiched between the outflow and a filament of cold, dense gas
traced by N$_2$D$^+$, toward which no dust polarization is detected. This
suggests that the origin of the enhanced polarization in IRS2 is the
irradiation of the outflow cavity walls, which enables the alignment of dust
grains with respect to the magnetic field -- but only to a depth of ~300 au,
beyond which the dust is cold and unpolarized. However, in order to align
grains deep enough in the cavity walls, and to produce the high polarization
fraction seen in IRS2, the aligning photons are likely to be in the mid- to
far-infrared range, which suggests a degree of grain growth beyond what is
typically expected in very young, Class 0 sources. Finally, toward IRS1 we see
a narrow, linear feature with a high (10-20%) polarization fraction and a well
ordered magnetic field that is not associated with the bipolar outflow cavity.
We speculate that this feature may be a magnetized accretion streamer; however,
this has yet to be confirmed by kinematic observations of dense-gas tracers.
