A&A 481, 465-480 (2008)
DOI: 10.1051/0004-6361:20078884

Searching for links between magnetic fields and stellar evolution

III. Measurement of magnetic fields in open cluster Ap stars with ESPaDOnS

J. D. Landstreet¹, J. Silaj¹, V. Andretta², S. Bagnulo³, S. V. Berdyugina^{4, 5}, J.-F. Donati⁶, L. Fossati⁷, P. Petit⁶, J. Silvester⁸, and G. A. Wade<sup>8

1</sup>  Physics & Astronomy Department, The University of Western Ontario, London, Ontario, N6A 3K7, Canada
    e-mail: [jlandstr;jsilaj]@astro.uwo.ca
²  INAF - Osservatorio Astronomico di Capodimonte, salita Moiariello 16, 80131 Napoli, Italy
    e-mail: andretta@na.astro.it
³  Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
    e-mail: sba@arm.ac.uk
⁴  Institute of Astronomy, ETH, 8092 Zurich, Switzerland
    e-mail: sveta@astro.phys.ethz.ch
⁵  Tuorla Observatory, University of Turku, 21500, Piikkiö, Finland
⁶  Observatoire Midi-Pyrénées, 14 Av. Édouard Belin, 31400 Toulouse, France
    e-mail: [donati;petit]@ast.obs-mip.fr
⁷  Institut für Astronomie, Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
    e-mail: fossati@astro.univie.ac.at
⁸  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 20 October 2007 / Accepted 31 January 2008)

Abstract
Context. A small fraction of upper main sequence stars have strong, highly structured magnetic fields. The origin and evolution of these fields are not adequately understood.
Aims. We are carrying out a survey of magnetic fields in Ap stars in open clusters in order to obtain the first sample of magnetic upper main sequence stars with precisely known ages. These data will constrain theories of field evolution in these stars.
Methods. A survey of candidate open cluster magnetic Ap stars was carried out using the new ESPaDOnS spectropolarimeter at the CFHT. This instrument provides an alternative to the FORS1 spectropolarimeter used up to now for this survey.
Results. We have obtained 44 measurements of the mean longitudinal fields of 23 B6-A2 stars that have been identified as possible Ap stars and that are possible members of open clusters, with a median uncertainty of about 45 G. Of these stars, 10 have definite field detections. Nine stars of our sample are found not to be magnetic Ap stars. These observations significantly increase the information available about low-mass stars near the TAMS compared to our previous sample.
Conclusions. We find that ESPaDOnS provides field measurements comparable to those that we have previously obtained with FORS1, and that these data also contain a large amount of useful information not readily obtained from lower resolution spectropolarimetry. With the new data we are able to expand the available data on low-mass, relatively evolved Ap stars, and identify more robustly which observed stars are actually magnetic Ap stars and cluster members. Re-analysis of the enlarged data set of cluster Ap stars indicates that such stars with masses in the range of 2-5  show rms fields larger than about 1 kG only when they are near the ZAMS. The time scale on which these large fields disappear varies strongly with mass, ranging from about 250 Myr for stars of 2-3  to 15 Myr for stars of 4-5 . Our data are consistent either with emergent flux conservation for most (but not all) Ap stars, or with modest decline in flux with age.

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

© ESO 2008

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