10⁵¹ erg less: the Galactic H II region OA 184

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 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 H I line data, and eight radio recombination lines (RRL, H103-110α) observed for this study with the Green Bank Telescope.

Results.An integrated radio spectrum of index 0.14 to -0.2 () is determined from four radio frequencies. RRL emission appears at  km s^-1 and at the intensity predicted for free-free thermal emission. Q and U maps at 6 cm show OA 184 as a depolarizing source, affecting a background filament of polarized non-thermal Galactic emission. An H II shell of thickness less than 0.2° is indicated by this depolarization. CGPS ~1-arcminute resolution 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  kpc, obtained from a new distance method based on a Galactic 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  K and density  cm^-3. The line of sight magnetic field in the ionized shell is found to be G (directed away from the Sun).

Conclusions.We conclude that OA 184 is a Galactic H II region energized by the lone O7.5V star BD+41°1144.