COBRaS: The e-MERLIN 21 cm Legacy survey of Cygnus OB2

¹ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
² Astrophysics Group, Cavendish Laboratory, University of Cambridge, Cambridge, UK
e-mail: dmfd2@cam.ac.uk
³ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁴ Harvard-Smithsonian Centre for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
⁶ Faculty of Computing, Engineering and Science, University of South Wales, Pontypridd CF37 1DL, UK
⁷ UK ARC Node, JBCA, Alan Turing Building, University of Manchester, M13 9PL Manchester, UK
⁸ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁹ Astrophysics Group, Leonard-Jones Building, Keele University, Staffordshire ST5 5BG, UK
¹⁰ Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹¹ Dominion Radio Astrophysical Observatory, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada
¹² ALMA, Alonso Cordoba 3107, 7630355 Vitacura, Chile
¹³ School of Physics and Astronomy, E. C. Stoner Building, The University of Leeds, Leeds LS2 9JT, UK
¹⁴ Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK

Received: 15 June 2017
Accepted: 25 December 2019

Abstract

Context. The role of massive stars is central to an understanding of galactic ecology. It is important to establish the details of how massive stars provide radiative, chemical, and mechanical feedback in galaxies. Central to these issues is an understanding of the evolution of massive stars, and the critical role of mass loss via strongly structured winds and stellar binarity. Ultimately, and acting collectively, massive stellar clusters shape the structure and energetics of galaxies.

Aims. We aim to conduct high-resolution, deep field mapping at 21 cm of the core of the massive Cygnus OB2 association and to characterise the properties of the massive stars and colliding winds at this waveband.

Methods. We used seven stations of the e-MERLIN radio facility, with its upgraded bandwidth and enhanced sensitivity to conduct a 21 cm census of Cygnus OB2. Based on 42 hours of observations, seven overlapping pointings were employed over multiple epochs during 2014 resulting in 1σ sensitivities down to ∼21 μJy and a resolution of ∼180 mas.

Results. A total of 61 sources are detected at 21 cm over a ∼0.48° × 0.48° region centred on the heart of the Cyg OB2 association. Of these 61 sources, 33 are detected for the first time. We detect a number of previously identified sources including four massive stellar binary systems, two YSOs, and several known X-ray and radio sources. We also detect the LBV candidate (possible binary system) and blue hypergiant star of Cyg OB2 #12.

Conclusions. The 21 cm observations secured in the COBRaS Legacy project provide data to constrain conditions in the outer wind regions of massive stars; determine the non-thermal properties of massive interacting binaries; examine evidence for transient sources, including those associated with young stellar objects; and provide unidentified sources that merit follow-up observations. The 21 cm data are of lasting value and will serve in combination with other key surveys of Cyg OB2, including Chandra and Spitzer.