Astrophys. J., 804, 22 (2015/May-1)
A radio-polarisation and rotation measure study of the Gum nebula and its environment.
PURCELL C.R., GAENSLER B.M., SUN X.H., CARRETTI E., BERNARDI G., HAVERKORN M., KESTEVEN M.J., POPPI S., SCHNITZELER D.H.F.M. and STAVELEY-SMITH L.
Abstract (from CDS):
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 ne. Assuming a latitudinal background gradient in rotation measure, we find ne = 1.3 ± –0.4+0.4, angular radius Φouter = 22.7°–0.1+0.1, 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° ≲ 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.
ISM: individual: Gum Nebula - magnetic fields - radio continuum: general - radio continuum: ISM - surveys - techniques: polarimetric
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