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Issue A&A
Volume 470, Number 3, August II 2007
Page(s) 1105 - 1109
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20066299



A&A 470, 1105-1109 (2007)
DOI: 10.1051/0004-6361:20066299

Radio observations of candidate magnetic O stars

R. S. Schnerr1, K. L. J. Rygl1, A. J. van der Horst1, T. A. Oosterloo2, J. C. A. Miller-Jones1, H. F. Henrichs1, T. A. Th. Spoelstra2, and A. R. Foley2

1  Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
    e-mail: rschnerr@science.uva.nl
2  ASTRON, 7991 PD, Dwingeloo, The Netherlands

(Received 25 August 2006 / Accepted 24 May 2007)

Abstract
Context.Some O stars are suspected to have to have (weak) magnetic fields because of the observed cyclical variability in their UV wind-lines. However, direct detections of these magnetic fields using optical spectropolarimetry have proven to be very difficult.
Aims.Non-thermal radio emission in these objects would most likely be due to synchrotron radiation. As a magnetic field is required for the production of synchrotron radiation, this would be strong evidence for the presence of a magnetic field. Such non-thermal emission has already been observed from the strongly magnetic Ap/Bp stars.
Methods.We have performed 6 and 21 cm observations using the WSRT and use these, in combination with archival VLA data at 3.6 cm and results from the literature, to study the radio emission of 5 selected candidate magnetic O stars.
Results.Out of our five targets, we have detected three: $\xi$ Per, which shows a non-thermal radio spectrum, and $\alpha$ Cam and $\lambda$ Cep, which show no evidence of a non-thermal spectrum. In general we find that the observed free-free (thermal) flux of the stellar wind is lower than expected. This is in agreement with recent findings that the mass-loss rates from O stars as derived from the H$\alpha$ line are overestimated because of clumping in the inner part of the stellar wind.


Key words: stars: magnetic fields -- stars: early-type -- stars: general -- stars: mass-loss -- radio continuum: stars -- radiation mechanisms: non-thermal



© ESO 2007

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