Probing the Magnetized Interstellar Medium Surrounding the Planetary Nebula Sh2-216

We present 1420 MHz polarization images of a 2.5 × 2.5 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 0.2×0.7) appears prominently in the north-east 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 ∼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 RM through the knots to be −43± 10 radm. 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 μG. We believe it more likely the observed magnetic field is interstellar than stellar, though 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. Subject headings: planetary nebulae: individual (Sh 2-216) — ISM: structure — polarization — radio continuum: ISM National Research Council of Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, Box 248, Penticton, BC, V2A 6J9, Canada Present Address: 35-3737 Gellatly Road, Westbank, BC, V2T 2W8, Canada Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4, Canada