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

Lead Research Organisation: University College London
Department Name: Physics and Astronomy

Abstract

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.

Publications

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Blomme R (2013) The 2.35 year itch of Cygnus OB2 #9 II. Radio monitoring??? in Astronomy & Astrophysics

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Gendre M (2013) Flux density variations of radio sources in M82 over the last three decades in Monthly Notices of the Royal Astronomical Society

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Prinja R (2011) The e-MERLIN Cyg OB2 radio survey (COBRaS) in Proceedings of the International Astronomical Union

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Prinja R (2012) PN fast winds: temporal structure and stellar rotation Structure in PN fast winds in Monthly Notices of the Royal Astronomical Society

 
Description The goals of this project were to establish data analysis software and to perform
the scientific exploitation of the UCL-led e-MERLIN legacy project on massive stars
in globular clusters: 'The e-MERLIN Cyg OB2 Radio Survey' (COBRaS). As such, the
main objectives of this research included the preparation of data reduction packages
for direct application to the e-MERLIN COBRaS legacy project as well as other
e-MERLIN projects (both legacy and PATT). They also included the scientific
dissemination of the ~300 hours of observations of the massive OB association,
Cygnus OB2, to study the mass-loss and clumping of the massive stellar population
within this association.

As part of her role as the technical lead for the COBRaS project, Dr. Fenech
(RA appointed on this grant) has produced a number of data-reduction modules required to perform
the relevant processing of the COBRaS data. These include, for example on the smaller scale,
tasks to re-weight data as a function of frequency according to antenna
sensitivity, which is necessary to achieve the full sensitivity available. On the
larger scale very important developments such as the RFI mitigation software
written by Mr. Luke Peck (a STFC PhD student assisted by this funding) and Dr.
Fenech. This will become a vital e-MERLIN reduction procedure and is in fact
already in use processing data from e-MERLIN as well as other radio instruments.
This has been a marked step forward in handling e-MERLIN data, particularly at
L-band where RFI is significant. Its use will reduce the amount of 'good-data' that
would have otherwise been discarded during processing using less sophisticated RFI
mitigation techniques. In addition, the computing resources required for the full
processing of the legacy project data have now been put in place. This has included
optimisation of both hardware and software to enable efficient reduction of the
large volumes of data. To this end, we now have an established high-specification
processing computer (Dell R5100 with 32 cores) with network access storage for
approximately 60TB of data. As such we are one of (if not the only) legacy projects
with dedicated computing resources in place and available for full legacy processing.
In May 2012, a significant amount of L-band COBRaS commissioning data was provided
for this project. This consisted of around 36 hours of observations including two
pointings of Cygnus OB2 and relevant calibration sources. These data will enable
testing of our reduction procedures, as well as providing an opportunity to assess
the observational methodology before the full mosaiced legacy observations begin.
These commissioning data are currently being processed and in particular are being
used to bench-mark the RFI mitigation software. It is expected these observations
will provide important early scientific results for this legacy project.
We now anticipate, based on information from the e-MERLIN project, that the L- and
C-band Legacy datasets for the COBRaS project will start to be acquired during
then 2nd half of 2012 and into 2013. The technical innovations outlined above will
permit us to make substantial and timely progress in the extraction and analyses
of the Legacy data during next few years (up 2016).
Exploitation Route As pathfinders for the Square Kilometre Array (SKA)
Sectors Education

URL https://www.ucl.ac.uk/star/research/stars_galaxies/cobras/technical