A Radio-Polarisation and Rotation Measure Study of the Gum Nebula and Its Environment

Purcell, C. R.C. R.PurcellGaensler, B. M.B. M.GaenslerSun, X. H.X. H.SunCARRETTI, ETTOREETTORECARRETTI0000-0002-3973-8403BERNARDI, GIANNIGIANNIBERNARDI0000-0002-0916-7443Haverkorn, M.M.HaverkornKesteven, M. J.M. J.KestevenPOPPI, SergioSergioPOPPI0000-0002-4698-2607Schnitzeler, D. H. F. M.D. H. F. M.SchnitzelerStaveley-Smith, L.L.Staveley-Smith2020-03-252020-03-2520150004-637Xhttp://hdl.handle.net/20.500.12386/23548We would like to thank the anonymous referee, whose thorough comments significantly improved this paper. We are very grateful to Roland Kothes and James Allison for useful discussions on the physics of bubbles and MCMC analysis, respectively. We thank Tom Landecker for his careful reading of the manuscript and for his valuable comments. We are also indebted to Rainer Beck, Marijke Haverkorn, Wolfgang Reich, and Julian Pittard for detailed suggestions. CRP, BMG and XHS were supported by the Australian Research Council through grant FL100100114. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020. This work has been carried out in the framework of the S-band Polarisation All Sky Survey (S-PASS) collaboration. The Parkes Radio Telescope is part of the Australia Telescope National Facility, which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. The Southern H-Alpha Sky Survey Atlas (SHASSA) is supported by the National Science Foundation. This research has made use of NASA’s Astrophysics Data System. This research also made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013 ).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 H α 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 H α data to constrain average electron density in the shell ne. Assuming a latitudinal background gradient in rotation measure, we find {{n}e}=1.3-0.4+0.4 c{{m}-3}, angular radius {{φ }outer}=22\buildrel{\circ}\over{.} 7-0.1+0.1, shell thickness dr=18.5-1.4+1.5 pc, ambient magnetic field strength {{B}0}=3.9-2.2+4.9 μ G, 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{}^\circ ≲ \wp ≲ +44{}^\circ , 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 H α 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.STAMPAenA Radio-Polarisation and Rotation Measure Study of the Gum Nebula and Its EnvironmentArticle10.1088/0004-637X/804/1/222-s2.0-84946189175000354189500022https://iopscience.iop.org/article/10.1088/0004-637X/804/1/22/2015ApJ...804...22PFIS/05 - ASTRONOMIA E ASTROFISICAScienze Fisiche Settori ERC (ERC) di riferimento::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation