  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, massive stellar clusters shape the structure and energetics of
galaxies. We aim to conduct high-resolution, deep field mapping at 21cm of the
core of the massive Cygnus OB2 association and to characterise the properties
of the massive stars and colliding winds at this waveband. We used seven
stations of the e-MERLIN radio facility, with its upgraded bandwidth and
enhanced sensitivity to conduct a 21cm census of Cygnus OB2. Based on 42 hours
of observations, seven overlapping pointings were employed over multiple epochs
during 2014 resulting in 1 sigma sensitivities down to ~21microJy and a
resolution of ~180mas. A total of 61 sources are detected at 21cm over a
~0.48deg x 0.48deg 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 (BHG) star of Cyg OB2
#12. The 21cm 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 21cm data
are of lasting value and will serve in combination with other key surveys of
Cyg OB2.
