A RADIO-POLARIZATION AND ROTATION MEASURE STUDY OF THE GUM NEBULA AND ITS ENVIRONMENT

The Gum Nebula is 36 degrees-wide shell-like emission nebula at a distance of only similar to 450 pc. It has been hypothesized to be an old supernova remnant, fossil H II region, wind-blown bubble, or combination of multiple objects. Here we investigate the magneto-ionic properties of the nebula using data from recent surveys: radio-continuum data from the NRAO VLA and S-band Parkes All Sky Surveys, and H alpha data from the Southern H-Alpha Sky Survey Atlas. We model the upper part of the nebula as a spherical shell of ionized gas expanding into the ambient medium. We perform a maximum-likelihood Markov chain Monte Carlo fit to the NVSS rotation measure data, using the H alpha data to constrain average electron density in the shell n(e). Assuming a latitudinal background gradient in rotation measure, we find n(e) = 1.3(-0.4)(+0.4) cm(-3), angular radius phi(outer) = 22 degrees.7(-0.1)(+0.1), shell thickness dr = 18.5(-1.4)(+1.5) pc, ambient magnetic field strength B-0 = 3.9(-2.2)(+4.9) mu G, and warm gas filling factor f = 0.3(-0.1)(+0.3). We constrain the local, small-scale (similar to 260 pc) pitch-angle of the ordered Galactic magnetic field to +7 degrees less than or similar to p less than or similar to +44 degrees, which represents a significant deviation from the median field orientation on kiloparsec scales (similar to-7 degrees.2). The moderate compression factor X = 6.0(-2.5)(+5.1) at the edge of the H alpha shell implies that the old supernova remnant origin is unlikely. Our results support a model of the nebula as a H II region around a wind-blown bubble. Analysis of depolarization in 2.3 GHz S-PASS data is consistent with this hypothesis and our best-fitting values agree well with previous studies of interstellar bubbles.