2015ApJ...804...22P

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 Hα 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 Hα 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, angular radius Φ_outer = 22.7°_–0.1^+0.1, shell thickness d_r = 18.5_–1.4^+1.5pc, ambient magnetic field strength B₀ = 3.9_–2.2^+4.9µG, and warm gas filling factor f = 0.3_–0.1^+0.3. We constrain the local, small-scale (∼260 pc) pitch-angle of the ordered Galactic magnetic field to +7° ≲ P≲ +44°, which represents a significant deviation from the median field orientation on kiloparsec scales (~-7.°2). The moderate compression factor X = 6.0_–2.5^+5.1 at the edge of the Hα 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.