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A&A 450, 777-791 (2006)
DOI: 10.1051/0004-6361:20054223

Searching for links between magnetic fields and stellar evolution

I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
S. Bagnulo1, J. D. Landstreet2, E. Mason1, V. Andretta3, J. Silaj2 and G. A. Wade4

1  European Southern Observatory, Casilla 19001, Santiago 19, Chile
    e-mail: sbagnulo@eso.org;emason@eso.org
2  Physics & Astronomy Department, The University of Western Ontario, London, Ontario, N6A 3K7, Canada
    e-mail: jlandstr@astro.uwo.ca;jsilaj@uwo.ca
3  INAF - Osservatorio Astronomico di Capodimonte, salita Moiariello 16, 80131 Napoli, Italy
    e-mail: andretta@na.astro.it
4  Department of Physics, Royal Military College of Canada, PO Box 17000, Station "Forces" Kingston, Ontario, K7K 7B4, Canada
    e-mail: Gregg.Wade@rmc.ca

(Received 19 September 2005 / Accepted 12 January 2006 )

Abstract
Context.About 5% of upper main sequence stars are permeated by a strong magnetic field, the origin of which is still matter of debate.
Aims. With this work we provide observational material to study how magnetic fields change with the evolution of stars on the main sequence, and to constrain theory explaining the presence of magnetic fields in A and B-type stars.
Methods. Using FORS1 in spectropolarimetric mode at the ESO VLT, we have carried out a survey of magnetic fields in early-type stars belonging to open clusters and associations of various ages.
Results. We have measured the magnetic field of 235 early-type stars with a typical uncertainty of ${\sim} 100$ G. In our sample, 97 stars are Ap or Bp stars. For these targets, the median error bar of our field measurements was ${\sim} 80$ G. A field has been detected in about 41 of these stars, 37 of which were not previously known as magnetic stars. For the 138 normal A and B-type stars, the median error bar was 136 G, and no field was detected in any of them.


Key words: stars: magnetic fields -- stars: chemically peculiar -- stars: evolution -- polarization -- techniques: polarimetric

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