We have carried out continuum and line polarisation observations of two Proto-planetary nebulae ( PPNe ) , CRL 618 and OH 231.8+4.2 , using the Submillimeter Array ( SMA ) in its compact configuration .
The frequency range of observations , 330–345 GHz , includes the CO ( J =3 \rightarrow 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 AGB phase .
We report a detection of dust continuum polarised emission in both PPNe above 4 \sigma but no molecular line polarisation detection above a 3 \sigma limit .
OH 231.8+4.2 is slightly more polarised on average than CRL 618 with a mean fractional polarisation of 4.3 and 0.3 per cent , respectively .
This agrees with the previous finding that silicate dust shows higher polarisation than carbonaceous dust .
In both objects , an anti-correlation between the fractional polarisation and the intensity is observed .
Neither PPNe show a well defined toroidal equatorial field , rather the field is generally well aligned and organised 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 organised magnetic field is parallel with the major axis of the ^ { 12 } CO 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 .