2014MNRAS.438.1794S

We have carried out continuum and line polarization observations of two protoplanetary nebulae (PPNe), CRL 618 and OH 231.8+4.2, using the Submillimeter Array in its compact configuration. The frequency range of observations, 330-345 GHz, includes the CO(J = 3⟶2) line emission. CRL 618 and OH 231.8+4.2 show quadrupolar and bipolar optical lobes, respectively, surrounded by a dusty envelope reminiscent of their asymptotic giant branch phase. We report a detection of dust continuum polarized emission in both PPNe above 4σ but no molecular line polarization detection above a 3σ limit. OH 231.8+4.2 is slightly more polarized on average than CRL 618 with a mean fractional polarization of 4.3 and 0.3 percent, respectively. This agrees with the previous finding that silicate dust shows higher polarization than carbonaceous dust. In both objects, an anticorrelation between the fractional polarization and the intensity is observed. Neither PPNe shows a well-defined toroidal equatorial field, rather the field is generally well aligned and organized along the polar direction. This is clearly seen in CRL 618 while in the case of OH 231.8+4.2, the geometry indicates an X-shaped structure coinciding overall with a dipole/polar configuration. However in the later case, the presence of a fragmented and weak toroidal field should not be discarded. Finally, in both PPNe, we observed that the well-organized magnetic field is parallel with the major axis of the 12CO outflow. This alignment could indicate the presence of a magnetic outflow launching mechanism. Based on our new high-resolution data we propose two scenarios to explain the evolution of the magnetic field in evolved stars.