Issue |
A&A
Volume 511, February 2010
|
|
---|---|---|
Article Number | A58 | |
Number of page(s) | 15 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200913505 | |
Published online | 10 March 2010 |
Radio emission from the massive stars in the galactic super star cluster Westerlund 1
1
National Research Council of Canada, Herzberg Institute for
Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248,
Penticton, British Columbia V2A 6J9, Canada e-mail: sean.dougherty@nrc.ca
2
Institute for Space Imaging Science, University of Calgary, 2500
University Dr. NW., Calgary, Alberta, T2N 1N4, Canada
3
Department of Physics and Astronomy, The Open
University, Walton Hall, Milton Keynes, MK7 6AA, UK
4
Dpto. de Física, Ingeniería de Sistemas y Teoría de la
Señal, Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain
5
Department of Physics and Astronomy, University of Victoria,
3800 Finnerty Rd, Victoria, B.C., V8P 5C2, Canada
6
Australia National Telescope Facility, PO Box 76, Epping, NSW 2121,
Australia
Received:
20
October
2009
Accepted:
16
November
2009
Aims. Current mass-loss rate estimates imply that main sequence line-driven winds are not sufficient to strip away the H-rich envelope to yield Wolf-Rayet (WR) stars. The rich transitional population of the young massive cluster Westerlund 1 (Wd 1) provides an ideal laboratory to observe and constrain mass-loss processes throughout the transitional phase of stellar evolution.
Methods. We present an analysis of deep radio continuum observations of Wd 1 obtained with the Australia Telescope Compact Array at four frequency bands that permit investigation of the intrinsic characteristics of the radio emission.
Results. We detect 18 cluster members, a sample dominated by the
cool hypergiants, with additional detections amongst the hotter OB supergiants and WR stars. The radio properties of the sample are
diverse, with thermal, non-thermal and composite thermal/non-thermal
sources present. Mass-loss rates determined for stars with partially
optically thick stellar winds are
~10-5 yr-1 across all spectral types,
insufficient to enable the formation of WRs during a massive star
lifetime, and the stars must undergo a period of greatly enhanced mass
loss. The sgB[e] star W9, the brightest radio source in Wd 1, may
provide an example, with a current mass-loss rate an order of
magnitude higher than the other cluster members, and an extended
nebula interpreted as a wind from an earlier epoch with a density
~3
the current wind. Such an envelope structure in W9 is
reminiscent of luminous blue variables, and one that shows evidence of
two eras of high, possibly eruptive mass loss. Surprisingly, three of
the OB supergiants are detected, implying unusually dense winds,
though they are embedded in more extended emission regions that may
influence the derived parameters. They also may have composite
spectra, suggesting binarity, which can lead to a higher flux than
expected from a stellar wind. Spatially resolved nebulae are
associated with three of the four RSGs and three of the six YHGs in
the cluster, which are due to quiescent mass loss rather than
outbursts. The extended nebulae of W20 and W26 have a cometary
morphology, implying significant interaction with either the
intracluster medium or cluster wind. For some of the cool star winds,
the ionizing source may be a companion star though the cluster
radiation density is sufficiently high to provide the necessary
ionizing radiation. Five WR stars are detected with composite spectra,
interpreted as arising in colliding-wind binaries.
Key words: stars: evolution / H ii regions / open clusters and associations: individual: Westerlund 1
© ESO, 2010
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