Probing the magnetized interstellar medium surrounding the planetary nebula Sh 2-216

We present 1420 MHz polarization images of a 2.5 degrees x 2.5 degrees region around the planetary nebula (PN) Sh 2-216. The images are taken from the Canadian Galactic Plane Survey (CGPS). An arc of low polarized intensity ( size 0.2 degrees x 0.7 degrees) appears prominently in the northeast portion of the visible disk of Sh 2-216, coincident with the optically identified interaction region between the PN and the interstellar medium ( ISM). The arc contains structural variations down to the similar to 1' resolution limit in both polarized intensity and polarization angle. Several polarization-angle knots'' appear along the arc. By comparison of the polarization angles at the centers of the knots and the mean polarization angle outside Sh 2-216, we estimate the rotation measure (RM) through the knots to be -43 +/- 10 rad m(-2). Using this estimate for the RM and an estimate of the electron density in the shell of Sh 2-216, we derive a line-of-sight magnetic field in the interaction region of 5.0 +/- 2.0 mu G. We believe it more likely the observed magnetic field is interstellar than stellar, although we cannot completely dismiss the latter possibility. We interpret our observations via a simple model which describes the ISM magnetic field around Sh 2-216, and comment on the potential use of old PNe as probes of the magnetized ISM.