The e-MERLIN Legacy Cyg OB2 Radio Survey: Massive star feedback and evolution

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Stars born with several tens times more mass than the Sun have a very significant influence in galaxies. The energetics and loss of mass through the prolific outflows of these massive stars has pivotal consequences in several fundamentally important areas of astrophysics. Unlike stars like the Sun, the evolution of massive stars is guided by how much mass they lose throughout their lives. Predictions for the relative production rates of neutron stars and black holes and the nature and properties of the preceding SNe explosion or Gamma Ray Burst are entirely impossible without a predictive understanding of the mass-loss process. And given that the first stars are thought to be even more extreme than the current populace, this uncertainty stretches back to the chemical enrichment and evolution of the Early Universe. We aim to advance our understanding of several (inter-related) processes in the physics of massive stars, by fully exploiting the powerful new UK radio facility available from 2010 onward by e-MERLIN. Due to significant upgrades that include optical fibre links and broad bandwidths, e-MERLIN is set to become the premier international facility for the study of outflows of massive stars and the interactions with their environments. Our research programme is founded on, and branches from, the substantial allocation of a e-MERLIN Legacy project ('COBRaS'; P.I. - Prof. Prinja). Almost 300 hours of already awarded time to this project will used to assemble a uniquely sensitive dataset of genuine lasting value, that will allow us to deliver important science results in three major inter-related fields: (i) The COBRaS project will deliver highly sensitive datasets to resolve serious current uncertainties of the mass-loss process due the fact that the outflows are not smooth, but highly clumped and porous. We will rigorously test the current theory of how stellar winds from hot stars are driven. Specifically, utilising the properties of the uniquely rich and accessible population of massive stars in Cyg OB2 we will be able to systematically investigate the role of evolution in stellar mass loss. The population of Cyg OB2 is rich enough that it samples stars at every stage of their evolution, thus enabling us to directly probe the corresponding evolution of mass loss rate. To understand stellar evolution at early epochs of the Universe we must first ensure that mass loss at the current epoch and metalicity is understood before extrapolating to lower metalicity, high redshift environs. (ii) The high spatial resolution of e-MERLIN will be uniquely exploited in the COBRaS project, using data over multiple epochs between 2010 and 2013, to determine the proper motions of the Cyg OB2 constituents. Combining the radio data with already secured optical spectroscopy, will permit us to map a full 3-D picture of the kinematics of Cyg OB2; this would be the first such map for any massive stellar cluster. The basic parameters and dynamics of Cyg OB2 will provide new perspectives on the formation, substructure and dissolution of massive stellar clusters in galaxies. (iii) Massive stars commonly occur in binary systems with other massive stars leading to the phenomenon of colliding stellar winds, resulting in substantial X-ray emission and also non-thermal radio emission, which is an important high energy process that is not fully understood. The very rich radio survey of Cyg OB2 assembled in this project will permit a probing and unbiased study of the incidence of massive binary stars and statistically study the colliding-wind phenomenon. Understanding the binary frequency is pivotal for computing the evolution of substantial populations of stars in extragalactic astrophysics. The binary properties are also very important to constrain models of massive star formation.