A radio-polarization and rotation measure study of the gum nebula and its environment

The Gum Nebula is 36°-wide shell-like emission nebula at a distance of only ∼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 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 data to constrain average electron density in the shell n_e. Assuming a latitudinal background gradient in rotation measure, we find , angular radius , shell thickness , ambient magnetic field strength , and warm gas filling factor . We constrain the local, small-scale (∼260 pc) pitch-angle of the ordered Galactic magnetic field to , which represents a significant deviation from the median field orientation on kiloparsec scales (∼-7.°2). The moderate compression factor at the edge of the 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.