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arXiv · 1502.06296

A radio-polarisation and rotation measure study of the Gum Nebula and its environment

C. R. Purcell·B. M. Gaensler·X. H. Sun·E. Carretti·G. Bernardi·M. Haverkorn·M. J. Kesteven·S. Poppi·D. H. F. M. Schnitzeler·L. Staveley-Smith
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Abstract

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 $n_e$. Assuming a latitudinal background gradient in RM we find $n_e=1.3^{+0.4}_{-0.4} {\rm cm}^{-3}$, angular radius $ϕ_{\rm outer}=22.7^{+0.1}_{-0.1} {\rm deg}$, shell thickness $dr=18.5^{+1.5}_{-1.4} {\rm pc}$, ambient magnetic field strength $B_0=3.9^{+4.9}_{-2.2} μ{\rm G}$ and warm gas filling factor $f=0.3^{+0.3}_{-0.1}$. We constrain the local, small-scale (~260 pc) pitch-angle of the ordered Galactic magnetic field to $+7^{\circ}\lesssim\wp\lesssim+44^{\circ}$, which represents a significant deviation from the median field orientation on kiloparsec scales (~-7.2$^{\circ}$). 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.

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C. R. Purcell, B. M. Gaensler, X. H. Sun, E. Carretti, G. Bernardi, M. Haverkorn, M. J. Kesteven, S. Poppi, D. H. F. M. Schnitzeler, L. Staveley-Smith. 2015-02-23. A radio-polarisation and rotation measure study of the Gum Nebula and its environment. https://doi.org/10.1088/0004-637x%2F804%2F1%2F22

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