Polarization properties of OH emission in planetary nebulae
  We present interferometric, full-polarization observations of the four
ground-state transitions of OH, toward five confirmed and one candidate
OH-emitting planetary nebulae (OHPNe). OHPNe are believed to be very young PNe,
and information on their magnetic fields (provided by their polarization) could
be key to understand the early evolution of PNe. We detect significant circular
and linear polarization in four and two objects, respectively. Possible Zeeman
pairs are seen in JaSt 23 and IRAS 17393-2727, resulting in estimates of
magnetic field strengths between 0.8 and 24 mG. We also report the new
detection of OH emission at 1720 MHz toward Vy 2-2, making it the third known
PN with this type of emission. We suggest that younger PNe have spectra
dominated by narrow maser features and higher degrees of polarization.
Shock-excited emission at 1720 MHz seems to be more common in PNe than in early
evolutionary phases, and could be related to equatorial ejections during the
early PN phase.
