Probing the Magnetized Interstellar Medium Surrounding the Planetary Nebula Sh 2‐216

Abstract

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 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 ~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 -->μ 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.