1051 erg less: the Galactic H II region OA 184

A&A 454, 517-526 (2006)
DOI: 10.1051/0004-6361:20064881

10 $\mathsf{^{51}}$ erg less: the Galactic H II region OA 184

T. Foster¹, R. Kothes^{1, 2}, X. H. Sun³, W. Reich⁴ and J. L. Han³

¹ National Research Council of Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248, Penticton, BC, V2A 6J9, Canada
e-mail: Tyler.Foster@nrc-cnrc.gc.ca
² Department of Physics and Astronomy, University of Calgary, 2500 University Drive N.W., Calgary, AB, Canada
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

(Received 19 January 2006 / Accepted 13 March 2006)

Abstract
Aims.The identification of the object OA 184 as a Galactic Supernova Remnant (SNR) is re-examined, and evidence to the contrary is presented. The aim is to show definitively that OA 184 is actually a Galactic $\ion{H}{ii}$ region, and to estimate some physical parameters for the nebula (e.g. temperature, density, and magnetic field in the ionized shell).
Methods.To determine the broad spectral properties of OA 184 a multiwavelength approach is used, with historical and new radio data, and existing X-ray and IR data on OA 184 considered. Radio continuum 408 and 1420 MHz Canadian Galactic Plane Survey (CGPS) data, Effelsberg 2695 MHz data, and new Urumqi 5 GHz radio observations in Stokes I, Q, and U are presented. As well, we present CGPS $\ion{H}{i}$ line data, and eight radio recombination lines (RRL, H103-110 $\alpha$ ) observed for this study with the Green Bank Telescope.
Results.An integrated radio spectrum of index $\alpha=-$ 0.14 to -0.2 ( $S(\textrm{Jy}) \propto \nu^{\alpha}$ ) is determined from four radio frequencies. RRL emission appears at $v_{\rm LSR}=-25.8$ km s^-1 and at the intensity predicted for free-free thermal emission. Q and U maps at $\lambda~$ 6 cm show OA 184 as a depolarizing source, affecting a background filament of polarized non-thermal Galactic emission. An $\ion{H}{ii}$ shell of thickness less than 0.2 $\degr$ is indicated by this depolarization. CGPS ~1-arcminute resolution $\ion{H}{i}$ line maps are presented, and the systemic velocity of the ISM immediately surrounding OA 184 is -26.8 km s^-1. The distance of OA 184 is $2.2\pm0.4$ kpc, obtained from a new distance method based on a Galactic $\ion{H}{i}$ modelling procedure. A simple model as a shell of ionized hydrogen 4 pc thick is considered, and the RRL observations are used to estimate (under conditions of non-thermodynamic equilibrium) the electron temperature $T_{\rm e}=8300$ K and density $n_{\rm e}=5.3$ cm^-3. The line of sight magnetic field in the ionized shell is found to be $\langle B_{\parallel}\rangle\simeq-2.5~\mu$ G (directed away from the Sun).
Conclusions.We conclude that OA 184 is a Galactic $\ion{H}{ii}$ region energized by the lone O7.5V star BD+41 $\degr$ 1144.

Key words: polarization -- ISM: supernova remnants -- $\ion{H}{ii}$ regions

10 erg less: the Galactic H II region OA 184

10 $\mathsf{^{51}}$ erg less: the Galactic H II region OA 184