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We have carried out continuum and line polarisation observations of twoProto-planetary nebulae (PPNe), CRL 618 and OH 231.8+4.2, using theSubmillimeter Array (SMA) in its compact configuration. The frequency range ofobservations, 330-345 GHz, includes the CO(J=3-2) line emission. CRL 618 and OH231.8+4.2 show quadrupolar and bipolar optical lobes, respectively, surroundedby a dusty envelope reminiscent of their AGB phase. We report a detection ofdust continuum polarised emission in both PPNe above 4 sigma but no molecularline polarisation detection above a 3 sigma limit. OH 231.8+4.2 is slightlymore polarised on average than CRL 618 with a mean fractional polarisation of4.3 and 0.3 per cent, respectively. This agrees with the previous finding thatsilicate dust shows higher polarisation than carbonaceous dust. In bothobjects, an anti-correlation between the fractional polarisation and theintensity is observed. Neither PPNe show a well defined toroidal equatorialfield, rather the field is generally well aligned and organised along the polardirection. 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 adipole/polar configuration. However in the later case, the presence of afragmented and weak toroidal field should not be discarded. Finally, in bothPPNe, we observed that the well organised magnetic field is parallel with themajor axis of the 12CO outflow. This alignment could indicate the presence of amagnetic outflow launching mechanism. Based on our new high resolution data wepropose two scenarios to explain the evolution of the magnetic field in evolvedstars.

Submillimeter polarisation and magnetic field properties in the envelopes of proto-planetary nebulae CRL 618 and OH 231.8+4.2