Polarization properties of OH emission in planetary nebulae

We present interferometric, full-polarization observations of the fourground-state transitions of OH, toward five confirmed and one candidateOH-emitting planetary nebulae (OHPNe). OHPNe are believed to be very young PNe,and information on their magnetic fields (provided by their polarization) couldbe key to understand the early evolution of PNe. We detect significant circularand linear polarization in four and two objects, respectively. Possible Zeemanpairs are seen in JaSt 23 and IRAS 17393-2727, resulting in estimates ofmagnetic field strengths between 0.8 and 24 mG. We also report the newdetection of OH emission at 1720 MHz toward Vy 2-2, making it the third knownPN with this type of emission. We suggest that younger PNe have spectradominated by narrow maser features and higher degrees of polarization.Shock-excited emission at 1720 MHz seems to be more common in PNe than in earlyevolutionary phases, and could be related to equatorial ejections during theearly PN phase.Comment: Published in Monthly Notices of the Royal Astronomical Society. Updated wrongly labeled Fig. 7, as shown in the Erratum published in the same journal. 18 pages (15 pages of main text + 3 pages of appendix A), 13 figure