The Gum Nebula is 36 degree wide shell-like emission nebula at a distance of
only 450 pc. It has been hypothesised to be an old supernova remnant, fossil
HII 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 ionised 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-halpha data to constrain
average electron density in the shell ne. Assuming a latitudinal background
gradient in RM we find ne=1.3−0.4+0.4cm−3, angular radius
ϕouter=22.7−0.1+0.1deg, shell thickness
dr=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∘≲℘≲+44∘, which represents a significant
deviation from the median field orientation on kiloparsec scales
(~-7.2∘). The moderate compression factor X=6.0\,^{+5.1}_{-2.5} 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 HII region around a
wind-blown bubble. Analysis of depolarisation 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.Comment: 33 pages, 16 figures. Accepted by The Astrophysical Journa