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

TL;DR

This study uses polarization imaging to analyze the magnetic field structure around planetary nebula Sh 2-216, revealing a magnetic field of about 5 microGauss in the interaction region, and discusses its implications for probing the interstellar medium.

Contribution

The paper provides the first polarization-based measurement of the magnetic field in the interaction region of Sh 2-216, demonstrating the potential of old planetary nebulae as probes of the magnetized interstellar medium.

Findings

Detected a low polarized intensity arc coincident with the nebula's interaction region.

Estimated the line-of-sight magnetic field to be approximately 5 microGauss.

Suggested the magnetic field is predominantly interstellar rather than stellar.

Abstract

We present 1420 MHz polarization images of a 2.5 X 2.5 degree 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 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 arcminute 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 microG. 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.